Thursday, 16 August 2018

An interlude: up by the bootstraps

And we're back. Wonder if anyone'll notice before Christmas?

First couple of posts are going to relate to an old matter, and the first really is a bit of a scene setter. You'll see why in the next post.

Today we're going to play with some statistics.

Standard error- a thickie's guide

If you only have a small dataset, then the 'standard error' within that dataset could well be large. In English? A birder who goes once a month to the nearby reserve and says it's his 'local patch' would produce a pretty wobbly write up for common species.

If you have a large dataset, then the 'standard error' within that dataset is much more likely to be small. Another birder on the same site, out 300+ days of the year, would have the better grasp on the site.

If you only count 'your' site on three dates in three months, then there's a really high chance you will have missed the more meaningful days. But a dozen once a month-ers could combine their data. In the olden days, that was the local grapevine and the Ornithological Society. Now it's taken to higher levels by the stats gurus.

Confidence interval- a thickie's guide

A confidence interval is a posh way of saying the correct value is likely to be between 'x' and 'y'.

If your dataset has a couple of stupid counts in it, say an average of a 1,000 but one morning when there were just 6 seen (fog) or 2,000 (migration fallout) then you might have no confidence in those outlying results. You can throw 'em out by simply ignoring. Or you can throw 'em out by having a confidence interval. By only using, say, the middle 95% of your data. Ignore those best and the worst. (WeBS helps their counters with this by asking them to note if their area coverage was complete; their counters know low won't make the national stats cauldron when they work magic on the numbers.)

Now 95% is a pretty common CI in statistics. You can hold a pretty good confidence level in it.

Three observers count a flock. 2,000, 3,000, 4,000. Can't do too much with that. And just throwing out a number means birders getting antsy about high/low counts and individual birders pee'd that they're not believed.

Now if 10 observers had submitted a count (p'raps there was a rare that day), 1,000, 1,750, 1,800, 1,850, 1,875, 1,900, 2,000, 2,125, 2,140, 4,000. You can start to have some confidence the actual flock size is somewhere between 1,750-2,140. And work towards publishing a meaningful figure in the annual report.

The magic tricks used by Statisticians aren't really tricks; within their framework they mirror common sense reality. There's a whole field, a flavour of the month right now, called Bayesian Statistics, that works this way with probabilities. Like a stage magician's audience, us mere mortals can but marvel at how the results they produce are somehow 'right'. True magic!

But they still need raw data. What I've been musing on is how they cope when a dataset is small.


So you've got a small dataset. Say just a hundred entries. You find your standard errors and confidence intervals, but you still don't trust them- they might be weak, but you can't go out and collect any more data.

Simples! Do the magic trick over and over. Just re-use your original data, over and over again. Witchcraft?! It certainly is- one method, despite many in the audience doubting, that works.

(The Magician's secret revealed)

Okay, the forgettable bit. If you really wanted to, how do you bootstrap?

You only have a 100 pieces of data.

Print out all your data on little individual slips, and put all 100 in a bag.

Draw one out, and write it down. That's your new data.

Put it back in the bag.

Draw again, from all 100 individual slips.

You now have two new pieces of data to work with.

And repeat.

And repeat.

And repeat until you have thousands and thousands and thousands of new pieces of data..

Now use this huge dataset to calculate your stats. (Thank the Lord for computer programmes that can do all that drawing out, eh!?)

Anyone with a stats O level or similar, please don't sue me. I'm fick and this is an imprecise guide. I only want the average birder to have a rough idea of how and why their BBSs, Birdtracks, WeBS, get played with. We often think the figures we've provided do the hard work. Well, without them the magic couldn't happen. But the magic has to happen.

Challenging results that are challenging

The reason I'm saying all this? To explain when the stat wizards say the results of one methodology are compatible to another, they've probably used wizardry like bootstrapping. There are ways and means to compare results gained from one methodology against results from another. A bird survey that needs only three visits has been compared to a daily survey, and shown to have, once tinkered with, results are are comparable to another. Of course, neither might be actually precise! If the confidence levels overlap, we're getting there.

For a practical survey methodology the statisticians cut data collection to the bare, it ensure it is robust enough to produce a result comparable to the actual.

Win win. The observer, the citizen scientist, has a figure they have absolute faith in. They will tell you there were 4,000 of 'x' last week absolute certainty. And they're happy. Of course, they could still be well out. It doesn't really matter when the stat wizards might well receive 999 different figures from 999 observers that they can play with.. Win win. Observer happy, statisticians happy. All the worrying about whether there's 500, 700, 900 present. Doesn't really matter.

No, they just guide us by giving us, well, guidelines. In your BBS, trying not going out with a team of 3 or 4 counters- better if all the counters have just one set of eyes. In your Garden Birdwatch, sticking to the time limits. On WeBS, coordinating counts. They're guidelines. They know we're human and some go out with a mate, some will go way beyond the time limits to get the extra species and some will count on the covering tide on a weekday, rather than high at the weekend, because that's when the most birds are there.

And it's why you should never cheat. I knew one site where one excellent birder was out every day, and got some great peak monthly counts. And then that area's WeBS counter went through the birder's figure (without their permission/knowledge) and instead claimed every peak monthly count as the number seen on WeBS day. Done with best intent, but hyper-inflating the site's value against others.

If a national survey, they are looking for national comparables. You're not doing it to get 'high counts'.

Regional trends can be pulled out by a statistician once they play with confidences, once they vet the data to remove those who admit to relaxing the guidelines. Locally though, they might not have enough confidence in your dataset to pull out a meaningful result. Why they appeal for as many records as possible.


Now, you're probably wondering why I've rambled on. Well, what if you have different surveys running in tandem? Comparability and compatibility. That's the background for the next blogpost.

Monday, 30 April 2018

The procure of Bramblings

Second half of April I gambled, and won. I had worried I wouldn't turn up any Brambling on my final Magpie roost count for the 2017-18 season, but about ten minutes before sunset a small group bounded into view. That view was through the 'scope, naturally; distant white rumps flashing among the branches and, unlike in true 'winter mode', when they would sit for long periods up in the tops to act as attractants to others, this late in the season, pre-migration, it was once around the block then down into cover.

And that's the birdy news over for this post. You can leave now if you want. The blog post is more about Berengrave LNR itself, and changes since the boardwalk closed.

Imagine the quarry paths as a (very rough) triangle. In the good old days (when I moved back in 2013), coming in from the estuary, you had two routes down either side to choose from to reach the third southern edge. To the west, an undulating walk along the quarry edge, with very few views down into the pit. To the east, a wonderful roller coaster of a boardwalk, rising and falling into the quarry depths until a final steep climb out of what one birder described as the nearest thing on planet earth to Dagobah. Colour me green and call me Yoda. Easy to guess which way I always chose.

In those days, the boardwalk was in good condition, but time was running out. The construction and repair work had, over the years, been carried out mainly by a support group, the Friends of Berengrave. Long story short, politics crept in and the rot set in; repairs were not allowed and the boardwalk was eventually closed off to the public by the Council.

What this meant was that any circular route around the reserve was no longer available. The number of visitors plummeted. Thankfully I could still get to my Magpie count spot and while I couldn't explore as much as in the past I was now keen to see what happened with the birds. Ever since the Foot and Mouth outbreak in 2001 and the 'closure of the countryside' I'd supported what was found by many back then; you stop the public from wandering the countryside and birds do better.

So, first thought had been 'this is going to be good'. The main users of this tiny local nature reserve were recreational dog walkers. A known disturbance. Here though, dogs had always been mainly on leads (there are some steep drops around the quarry), so not as much uncontrolled disturbance as elsewhere. Noise disturbance? Sure the odd yappy (and by that I mean the shouty owner), but usually fairly quiet. The main effect noticed in the past had been flighting. A noisy visitor would put up early roosters, with Woodpigeon and Magpie the most noticeable. About two thirds of the time, they would circle and resettle, otherwise they would move off, normally towards cover nearby (beyond the Berengrave Nurseries and towards the Bloors Community Woodland). The Magpies would then often work back, the Woodpigeon not so much. Harder to note the effects on smaller passerines, but small groups of finches would put up, and again, some species more prone to leaving Berengrave and not returning that evening; Goldfinches often appeared the most reluctant to stay, and they never roosted deep in the quarry anyway, preferring the edgelands; roost quality obviously played a part in any flight decision.

There were sometimes noisier human visitors of course. The reserve appealed to the younger element. The rising boardwalk was a good climbing frame, the paths, despite the no cycling signs, a nice little off-road workout.

I think during those first few years I met fewer than a dozen naturalists. And that would have been one botanist and eleven birders- we were never great users. A hard to work site, with minimal facilities? Birders were always likely to amble elsewhere.


Following the boardwalk closure local dog walkers still take the eastern route to and from the main Country Park, but in much smaller numbers. Many dogs don't get long walks in Medway. Fewer still dog walkers bother with the dead end of a  southern edge now it leads nowhere. During the 90 minutes of a count I see less than a quarter of the number of park walkers I had before. This southern edge is where my viewpoint is and it is certainly true contacts are up. Though for most common species my roost numbers are fairly similar, birds seem more likely to loiter along this top edge rather than hurry down in.

Fewer adventurous youth now as well. The boardwalk still does appeal though, being a big ol' scary climbing frame now. But the routine flushings of the pre-roost gatherings of Magpies and Woodpigeons are a thing of the past now.

All in all, I'd say the birds are enjoying the lower disturbance levels.

But there has been a growth in another area of human activity. Fewer recreational public, more recreational drug users.

Good and bad birders. Good and bad dog walkers. Good and bad potheads. The nice ones are really pleasant. They come to sit on one of the three benches along the southern edge, smoke their weed, take in the view and chill out. And they're pretty chilled about me if I turn up at the first bench to count. I explain what I'm about, and they go 'whoooah', have a look through the 'scope and then wander off to one of the other benches. I'm sure one of their number was behind the 'save the bees' graffiti that appeared on the barrier.

Good 'uns usually arrive in pairs. Stumble in upon a hazy cloud made up of more than half-a-dozen, and you know to tread carefully. Usually no more than a bit of verbal if you set up shop, normally never more than a loud verbal, especially if they're at the second bench just about five metres away.
I just think of the abuse being rather like adolescent chimps flinging faeces. Same old sh*t we've all had to put up with over the years. And they never actually threaten actual violence. T'was ever so.

But you know on the evenings you don't show, they're the ones setting the small fires, breaking the barriers. Throwing their beer cans down the bank. T'was ever so.

The third bench, the far bench? Well, that's another few hundred metres on, and only the most adventurous dog walkers venture that far. You have to want to be going there. So this spot appeals to those who like tablets a little stronger, the ones that come in teeny plastic bags with cheery little logos printed on them.


What the heck do the owners, the local council, do with an 'asset' like Berengrave? We nature lovers don't show the love enough enough, yet we expect our Councils to uphold our LNRs. Should they spend loads on replacing the boardwalk when it is so little used? Should they pull it down as it is no longer fit for purpose? Or do they close off all access as they did for a short time prior to my move back?

Never mind the human politics, what would the birds say?

Well, it is one of the few overgrown areas left locally. Roosts opportunities abound. For common 'town' species, the noisy humans never really had an effect on numbers in the past, and neither have their roosting numbers increased since it has become quieter. Too soon to say if the more 'sensitive' species have increased. Their numbers, like these Brambling I've watched this past winter, are up and down for other reasons. A lot of these species are usually the last into roost anyway, chances are they've always missed the noisier people by dropping in at last light.

Of course, summer and breeding are a whole different study but Berengrave remains a safe winter haven. Long may it stay so. With the amount of housing going up close by, the importance grows.

Me, personally? Well, I hope they never feel the need to completely close off again because of purely selfish reasons; I've loved my five winters of Magpie roost counting.

Who would want to be a Council employee/ Councillor responsible for making best decisions for all users of a site like this?

There's a price to pay for hanging on to sites like these, one that fewer birders seem willing to pay. We don't shop for our sightings at the Mom'n'Pop old style sites. We don't even go and browse at the out-of-town birding megastores so much. We browse the internet for good stuff, and buy in to what we're told are the must-haves this season.

Me? I've just resolved to make a few more visits than usual this breeding season, before the 'everything must go' signs appear..


For more information on this LNR do check out the now sadly dormant 'Friends of Berengrave' website.

Berengrave present:
Sections of the boardwalk are closed off by simple planks..

..the adventurous just climb beside them..

..viewpoint barriers tend to disappear during school hols..

Bench three

Berengrave past: Views from the boardwalk

Monday, 23 April 2018

Points of views

The morning had started out as a Shelduck chase. Following a few twitter exchanges about the territories they use (which is a blog in itself) I decided a good morning to count some of the non-breeding flocks in the eastern basin.

Working back from the Swale station, I soon found myself up on Raspberry hill. Here, and Tiptree hill have long been known as the best viewpoints for the estuary islands by both birders and non-birders alike. In Helm's 'Where to watch birds in Kent, Surrey and Sussex', Don Taylor had long sung the praises of the viewpoints for 'scoping the distant creeks. They provide some of the best birding on the estuary (probably why I chose Chetney for my first blogpost here). Both viewpoints even have seats of sorts nowadays; the owners have put in a superb huge beam of a bench up on Raspberry hill, while broken chunks of concrete from one of the old wartime buildings have been piled up around the base of the telegraph pole on Tiptree. This is the busier of the two viewpoints, as the path here is one of the highest points of the Saxon Shore Way long distance footpath, although I am usually joined by walkers rather than birders. With my caveat that general birding remains much better on reserves, I've promoted both often enough in the past, but the distant birds views only appeal to obsessive birders (with my cakky old Bridge camera I never get a decent image, but they're fine for record shots of flock sizes, behaviour, etc., besides, heat haze for both 'scope and camera can be a pig). Walking from either Lower Halstow or Iwade appeals to few (leaving a vehicle anywhere at Funton can be problematic; break-ins are, sadly, routine). But they are worth the effort.

Taylor, Wheatley, James, 4th ed., 2003 Don't trust the 'P',
some footpaths are missing, but that Tiptree star is spot on
While I've tweeted and blogged plenty of pics from up there these past five years, another reason I haven't banged on about them these past couple of years because of the ongoing Coast Path discussions. Some stretches of new footpath are likely to arrive in the area, and during this time I'd been asked for information, mainly concerning on both roosts and general disturbance levels. Just a couple of months back I was contacted about one roost near here, when I heard that publication of the route through the estuary is looking likely to be in September (which will raise a few blogposts at that time). So, we're getting close enough not to have to worry about any impact from publicising the viewpoints to birders a little more on here (heck, the other month I was even tweeting the county Recorder saying it'd be worth while getting up and having a scan over Chetney). To date I've only published the first part of the shoreline site guide for the same reason; the remaining sections will come online shortly.

Today that Chetney viewpoint did me proud, and Shelduck numbers were high, higher than expected. (Numbers along the whole of south shore this spring are par; somewhere around the levels that used to get picked up on WeBS in the 90s, nothing that would excite anyone other than a local.)

Rather than take footpath along the ridge I followed the road west down to Bedlam's Bottom and then along to Funton Creek. I was soon distracted by Rooks. They were feeding manically among the saltings and cord-grass beds.

I cursed myself- another pet subject was playing out, and I had nearly missed it. The mini-heatwave we had just enjoyed meant the ground surface was solid in many fields nearby, making difficult feeding for corvids. You can often see three figure counts of Crows on the flats during the year, they have adapted to estuary feeding, but Rooks (and, to a lesser extent, Jackdaws) only join them in big numbers when feeding is tough. Only the day before I'd been impressed by how much like concrete my allotment beds had already become, but I hadn't sussed it was Rook time. Nearly a Rookie mistake.

Some local landowners control corvids, just as some North Kent nature reserves do. You can have a big debate on the merits of that, but what interests me is why some landowners spare Rooks, others cull. The texts are clear; Rooks will take both game bird eggs and wader eggs. But to what extent? 'Birds of the Western Palearctic' states the young are fed almost exclusively on insects, but that these are hard to obtain when fields are dry. So, perfect conditions for looking at a question I'd been interested in for years.

I took the bridlepath up Tiptree. I went to count my Shelduck flocks first, and found they were nearly all missing (explaining in part the raised Chetney numbers). Then I noticed the local Lapwings were agitated. Not just territorial interaction going on, but real agitation, birds even gathering in a 'mini-flock' sitting out in the middle of one of the pasture fields. Others were flight-distracting. These behaviour continued during all the time I was up there, and it was not going unnoticed by others; numbers of Rooks, as well as Crows, were coming off of the saltings and into the fields.

Now the land below Tiptree is no nature reserve. The land is worked, and workers are often out and about. But I then saw someone strolling slowly down the central counter wall and wandering through the centre of the small peninsula below me. Paperwork and behaviour gave away as a bird surveyor, and clearly noting the Lapwings.

Now of course there's an appeal to wandering fields, but any good surveyor should know better than most just how disturbance can happen during a survey. Whilst the published survey methods are clear about checking every field, at the same time they also stress disturbance must be kept to a minimum:

"Extensive open areas should be checked from roads tracks and footpaths without disturbing nesting birds. If birds are flushed, retreat as quickly as possible.. counts are most accurate when birds are undisturbed.. an observer walking through a field, or a crow flying over, will usually cause all the lapwings to fly up..'

All probably why the methodology states a telescope is the essential piece of kit for Lapwing surveys. This surveyor was diligently noting each bird that overflew them, or that they picked up in their binoculars. They just didn't appear to be noting what was happening behind them. From nearly a kilometre and a half off. I just had to sit and watch it happen. Easy for Lapwings to keep one or two Crows away, much harder to target a couple of dozen Crows among a hundred Rooks. And corvids are just as good as us at mapping out territories. They were watching.

That person could well try to justify, and you can justify any action. Justifications hold more weight when countering criticism, but the feedback was pure. In this instance the Lapwings were clearly disturbed, known predators were clearly gathering and published methodologies were not being employed. Why it is best to try to appreciate why the RSPB's Bird Monitoring Methods ask you to do certain things, and then apply.

Especially if in plain sight of any observer who happens to be up on what is acknowledged as one of the county's premier birding viewpoints, on one of the county's busier footpaths. Without fieldcraft, you will be seen. And local birders are already on the lookout for problems.

A couple of years back, after discussions with RSPB Investigations, I highlighted the plight of our gull colonies; reports of unauthorised landings were on the rise, and some satellite colonies within the island complex had deserted, some witnesses allegeding watching a gang of egg thieves out on the islands (taking the eggs for either for the restaurant trade, or for selling locally, private or otherwise. Sadly, with so much wildlife crime still happening, priorities have to be made and gull colonies come lower than birds of prey. So it has to be local birders who 'recognise, record, report' (the tagline of BAWC). The county ornithological society even kindly published some links to my pleas in here and in other county birding facebook pages.

As it is coming up for that time of year again, here's the link to my original 2016 appeal.

Of course, viewpoints out to these colonies are Godsends. 'Scopes and cameras necessary tools.

I'm told by some there are mobile phones with better cameras than I own. Don't doubt it (but still don't ever want to own a mobile). No-one should be naive enough to think they aren't able to be viewed on the marshes, and their actions, whether disturbance or criminal, recognised, recorded and reported. I know I'm often photographed or filmed by some landowners, and I'm more than happy with that.

We birders are being asked to set high standards to the general public when alongside our SPAs nowadays, as legally required initiatives to reduce disturbance, such as Birdwise, come into play. Studies have already proven birders to be one of the main disturbances to wintering wildfowl and waders. Often inadvertently, but disturbance all the same. Birders' egos bruise easily when called out on such things, but it really is time to put such sensitivities aside and for us all, birders and birdwatchers, professionals and 'citizen scientists', to acknowledge such feedback and keep raising standards in the field- because we know additional big pressures on our estuaries are coming, and we need to get the public, not just ourselves, 'birdwise'.

Finally, whether we like it or not, pics of poor birding behaviour appear nigh daily in our birding timelines, often with calls for outright 'naming and shaming'. My harsh tweet that day was deliberately general in details, and non-illustrated. The chair of the county's Conservation and Surveys Committee did reply pretty quickly asking for details, so I did provide clues needed by giving him enough to work out details and appreciate problem (which he did, even though I'd ensured the cropped pic I'd attached for positioning the incident had a nice anonymising blob added over much of the highly pixellated person).

Being on the receiving end of feedback can be painful, but we all have to face up to it from time to time. If we don't want to face up to it, we fall back on logical fallacies; appeal to emotion, claim false cause, personal incredulity or 'the strawman'. Or we can take on board, and improve. And that 'we' is why I've posted this; if a few birders think more about such things, then worth it.

A final thought. I normally veer away talking about Schedule one birds during the breeding season, but as the county's rep on the Rare Breeding Bird Panel recently talked about the estuary in an open Facebook group, I'll make the following point. If you were surveying an area with Schedule One nesting birds, are you committing 'reckless disturbance' (the legalese) if you fail to act in accordance with that species' proscribed methodologies once those methodologies have been pointed out to you? Tricky; why best to try to stick to the published methods in the first place.


I'll say it again; if you can put up with a lot of the birds on show being specks, and you haven't yet been to either of these viewpoints, do go. On a rising tide, or over a full tide (especially a spring) at this time of year. On a winter's afternoon, any time of tide. You really won't be disappointed. Just remember to take your 'scope.

From Tiptree hill:

From Raspberry hill:

Monday, 9 April 2018

The lure of Bramblings

In 1951/52, attempts were made to count a winter roost of Bramblings in Switzerland. One stream into the roost was estimated to hold around 36 million birds. To quote from Newton's 'NN 'Finches', "..however, only about half the birds could be seen from the observation points, so the total number was probably at least 70 million.."

That is a lot of Bramblings. To a soft Kent birder, a three figure count would be impressive. This winter, I have been more than happy with a number going to roost somewhat under that. Of course, I knew, like the Swiss, I probably wasn't counting the whole flock from my viewpoint, but it was fun to dream it could have been millions. Or a hundred. I'd have settled for counting a hundred.

Berengrave Chalk Pit

Go back to the 1980s, a regular spot for Brambling in the run of hard winters back then. I still have fond memories of trudging through the snow to see them, standing under the ivy covered trunks watching birds sneaking in.

Since I moved back from East Sussex, I have been counting the Magpie roost at Berengrave. As mega-exciting as that sounds, I decided from the start to count everything that roosts (a series of links at the end of the blog will take you to past summaries for most of the more common species including Brambling). This year autumn passage on the estuary hinted at a small but slightly higher than usual passage and, sure enough, after a run of nigh-blank winters, I finally had some Brambling to look at. Not 70 million, not even 70, but it would do.

I must be one of the few birders who routinely takes a telescope into an overgrown quarry. Simple fact of the matter, the birds were always at the northern end, the only decent viewpoint at the southern.

Pre-roost assembly and roosting

The Magpie count period has always been a set timed count from 60 minutes pre-sunset to 30 after. The finches often start arriving before that so, on arrival, a scan of the distant tree line would often find a mix of species up on display. When I say mix, there tended to be a general clumping of one species over another. Chaffinch, in the largest numbers, were all through the higher trees but the Goldfinches and Bramblings would be in tens, twenties, both having favourite trees and both able to flight without taking any of the other flocks with them.

Unlike the Magpies, the finch show would be over by sunset. Usually roughly twenty minutes before when the various groups would drop into lower branches. Mainly without any fuss, only sometimes, especially later in the season, making a few circuits of the Pit before piling in. Another interesting difference was that during the coldest part of the year perching in the tree tops would still happen, but for much, much shorter periods. Any urge to advertise the roost to newbies was overridden by an urge to secure a good spot.

Arrival was almost always from the north, and mainly the north-east. This put the majority on a flightpath back to what I discovered to be their favourite local food source.

Bramblings along the Medway

Taking winter as December 1st to February 28th, sightings were clustered around the main early season food source before spreading to a later winter support system of gardens. The initial attraction was not, as the field guides would have it, beech mast, but brassica, in game cover crops. Looking at 'BWP', brassica is a source many switch to when the mast runs out. There was clearly enough good feeding on these local farms, with safe roosting nearby, to keep a few birds present all winter.

Sadly, I was hampered by my early December operation and did not get to search the eastern end of the shore as often as I would have liked; Brambling arriving at the other more easterly roost I did visit tended to fly from the north-west and north-east, so were also no doubt also taking advantage of local cover crops; another local farmer more to the east always has them in his cover crops when they are in the county. Another site for me to check up on in the next good winter.

The second coming

Many texts hint at late winter hard weather movements. My set counts might very well hint at such a thing, but I can't say for sure. I found myself asking another question instead.

Go back to that monster Swiss roost. The actual count was never the number of birds present. That had to be estimated. In no way do I think my counts were the true number present. Just a nice comparison of relative numbers throughout the winter. But because my site is a b*gg*r to view, and with birds dropping down much more quickly during the coldest periods, could it be I missed more birds in the cold than the mild? My primary reason for counting here is, after all, the Magpies, so I never kept my eyes glued to the finches' favoured pre-roost perches. Did more sneak in past me at this time? Was the drop as strong as suggested by the graph?

Which had me thinking on other aspects of Brambling wintering behaviour and movement where the texts tend to differ a little.

Unlike other finches, Brambling are not, in the main, diurnal migrants; many birds, some texts suggest the vast majority, actually move at night. (BWP makes an intriguing statement that most diurnal movement takes place overland, while more nocturnal movements are over large water bodies. Years later the Migration Atlas states '..Brambling movements are often nocturnal and may leave exhausted stragglers grounded at the coast during the day..') The best bet is that we often just pick up on early morning stragglers or daytime adjusting movements; why, on  a good autumn morning, we never have more than a handful in comparison to Chaffinches.

Those late winter second pushes into the U.K. from the continent; how much evidence actually exists? Why couldn't late winter peaks be birds adjusting from, say, already within the U.K.? Again, the Migration Atlas " conditions may bring new arrivals from the Continent in mid-winter, often in substantial numbers.." referencing the Birds of Norfolk, but intra-seasonal ringing controls that back this up are lacking somewhat, plus, on reading that referenced species account the mid-winter flight directions are mentioned, but none suggest an 'in-off'; movements south through that county could just as easily be birds already in the U.K.

The probability is good, but there is still much to confirm.

Spring departure and some real experiments rather than my guessing games

A direct comparison of Chaffinch vs. Brambling shows a typical Chaffinch curve, albeit steeper than usual thanks to higher numbers. A continued rise into December as the migrants find the better roosts/feeding areas, a decrease after New Year as winter bites, then a drop from March onwards as the locals move out to set up territories and the winterers start to feel migratory restlessness setting in; Chaffinch move a few weeks before Brambling. Makes sense, Brambling being the more northerly breeder, no need to move off quite as quickly; those breeding in, say, northern Finland, can't really get started there until warm enough in mid-, late- May. Their diet becomes more insectivorous, why risk getting back there and finding everything still frozen if there's still food available further south? Why some texts hint at their peak departures from here being mid-April, a fortnight or so after the Chaffinch peaks.

Having a trawl around the internet, I found a couple of interesting papers on work on migratory cues for Bramblings.

In one experiment, birds were kept under artificial light set to short (winter) daylengths. These lights were switched abruptly to 14.5 hours. It was like turning the dial up to eleven. Within three days, the birds had developed migratory restlessness. Zugunruhe. Fat deposits grew. So, effectively by mid- to late-April, migratory condition in Bramblings should be well advanced. Doesn't mean they all move at once, the texts say they stay in some number throughout the month.So why do we see less and less? Migration Atlas hints at an answer. Large flocks may have short-hopped to the continent, or they may have dispersed. Diet again. Stalling an early arrival on snowy breeding grounds again.

A second, more technical experiment, supported the hypothesis that "changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by either allowing or inhibiting nocturnal activity.." "Nocturnal activity"- now that made me sit up. Despite those more recent texts hinting diurnal migration might well be as strong as nocturnal, this species has true nocturnal urges. Puts me in the camp standing by the theory we only ever see a fraction of the true numbers of Brambling in our viz mig efforts. All the guesses that there's probably a lot more Brambling mixed in a high-flying Chaffinch flock than we can pick out? Nah, there's probably just a similar percentage to those we pick out in the lower flocks; a handful in the hundreds. The majority arrived earlier. (Be interesting to see if I eat these words in the coming years as more birders take up 'noc mig'..)

So, we might not be able to see them go. Do we know when they really do leave?

Borderline differences

Newton, in New Nats, stated British winterers move early March to April. What have our county avifaunas said?

Norfolk (Taylor): "..April records are now not at all unusual and large flocks can occur.. can still be present in good numbers into early April.. most tend to have slipped away by the middle of April.. coastal passage in the spring is much less noticeable than in the autumn.."

Suffolk (Piotrowski): "..pre-emigrating flocks.. ..feeding flocks are frequently reported until early May.."

Essex (Wood): "..the status of those birds occurring in the county from March onwards is unclear. Many are probably wintering birds, but it is likely that from the end of March there is an increasing number of returning migrants involved..

London (Self): "..several influxes during the middle of winter.."

Surrey (Wheatley): "..some evidence of passage March and larger arrivals in January.."

Kent (Taylor) "..small, but often noticeable spring passage, usually indicated by the presence of temporary flocks, during March-April.."

Sussex (James): "..a small spring passage is evident from mid-February onwards.."
(Thomas): "..spring passage is much less noticeable than in autumn.."

Where specific, not exactly any consensus among the local counties then.

Nationally, the Migration Atlas goes for "..large wintering flocks have typically dispersed or returned to the continent by mid-April.." so, they may or may not have left. It also asks if late April/early May birds on the coast are late leavers or passage birds heading up from south-west Europe.

We really still don't know as much as we like to think we do.

Among the county avifaunas, as always, for me, Wood stands out (fnarr). It makes plain we don't really know, and hints at what might be going on. Why when anyone asks me what's the best text on birds for Kent, I always say 'Birds of Essex'. I've always stated outstanding for esturine birds, but it also really works for a lot more species as well.

Not much left for me to do now this season. Just the one Magpie count left, in second half of April, but I'm already geared up for a nil return on Brambling, if only because my allotment backs onto the northern edge of their assembly area, and this past week I've not been hearing any in the late afternoons- or will they prove me wrong? Who would guess forecasting negative data for a common species could be so much fun?


Links to other Berengrave roost posts:
- Introduction
- Finches
- Thrushes
- Non-passerines
- Other passerines

Friday, 16 March 2018

A Cuckoo by any mean

Rarities. Always get the blood coursing. Excitement levels high. Even when you know the biology of the Coccyzus family of Cuckoos from North America is such that they will not survive an Atlantic crossing you might still get so beside yourself you start tweeting 'Boom mega lifer!' at seeing a moribund bird in the autumn. Such is birding.

Survival. Birds need everything to be just right.

So when I received a Direct Message on twitter last night, with what I knew to be a bit of a loaded question, I deflected a tad.

"Do you think a Cuckoo is possible in Kent on March 11th."

My reply: "What do Cuckoos eat?"


Birding is full of statistics., the question was just the sort to tempt me. Even if I am a bear of little brain. As I joke with other ringers, I've tried reading Fowler and Cohen's 'Statistics for Ornithologists' several times, but crash and burn every time. Some stuff has stuck, not enough.

Birding is full of statistics. Not just mathematical stuff. Statistics is about finding ways of organising, sumarising and describing quantifiable data. Statistics is also about making inferences and generalising that data. What works for one question isn't necessarily the best system for another. And it seems we're into that wonderful mindset of there only being one way to describe the average. Which there isn't.


There are three main averages.

Average mean. The one we are all familiar with. Easy to do. The arithmetical average of your dataset. Give something a numerical value, total them, and divide by the number of items. Boom.

But that figure is not necessarily the same as the middle value of the dataset.

And it may not be the most likely value.

Average median. The middle value. The best way to describe this is to steal the example in the book. You have a colony of Manx Shearwaters on a small island, and you are looking for the average distance the young travel. If you did it purely by, say, ringing recovery, and you have 50 birds recovered within ten miles of your island, and another one recovered waaaaay down in the South Atlantic then your average mean is going to be south of the equator- which of course it isn't. Sort them by value, you have 51 records, count through to number 26, and that's your modal value, right there.

Average modal. The value that you see the most often. For those Manxies, it could be there were twenty distances of 15 miles. Because that's how far it is to the next island colony, and birds are swapping; context is a wonderful thing.

Now, with us simple birders and first dates, we nearly always play with average means. Especially when it comes to first arrival dates of spring migrants. Our dataset is never always simple. What if we have an overwintering Lesser Whitethroat? Some would say you clearly need to leave it out the figures then, d'uh. We run into real problems with overwintering Blackcaps and Chiffchaffs, and we might have to choose a date of the first multiple coastal (presumed) arrival, or similar; we manipulate our data.

Now, you might think something as simple as Cuckoo could be easy to play with. Well, yes and no. BWP quotes a ringing recovery of a second-calendar year bird found in France March 5th. This was presumed to have been one of those wonderful exceptions, a bird that wintered in Europe (young Cuckoos in their first summer generally return a little later than adult birds). You wouldn't really want that messing up your averages, would you?

So, this Cuckoo debate I was being asked to comment on. Seems that there's been a multi-observer heard-only record for 11th of March this year. Good for them I say. I try never to say someone hasn't seen something.

Instead I had asked my correspondent 'what do Cuckoos eat?'


Banging a mathematical value on a date is easy. January 1st =1, February 1st, 32, and so on. You can play with dates. Many birders are fascinated by first dates.

The full debate on this early sighting had focused on an average mean published in the County report, based on the last 15 years. The average mean was quoted as being 27th March, and was being queried by birders from other counties.

Now, for my blog, I went to the species descriptions in the annual bird reports for the dates. This is because the data had usually been reviewed by both the editor and the author for that species.

One big problem. The earliest date listed in that 15 year period under review in the debate was 13th March 2004. The Report actually had 3rd April in the body; the 13th appears in a 'first and last' table. 21 days out. That would throw things.

Your data has to be robust.

I'm not worried about which one is right at the moment, I want to focus on other things. I'll let others look into and confirm. I'll just take the species accounts dates for now.

With just 18 pieces of datum (datum = singular, data = plural) hard to get much more than a mean and a median. They match nicely at this point. Statistical analysis works better with more data, so I then went back a further ten years:

Now we had all three averages showing. Going back another ten years so as to have an even bigger dataset..

you still see a clustering of averages in early April. Note I've highlighted dates for this century; there is a pattern of earlier arrivals, which we'll come back to later.

So, I was mean, and 'threw out' that 13th March record. Because, often in statistical analysis you need to throw out the highest and lowest. It's not about the truth of the records, it's to help focus on the trend; 'probability', 'variation', 'confidence', 'inferrentials' 'standard errors', 'normal distribution curves' all make my head spin.

But, to a thickie like me, 'standard deviation' sticks out, and that's all about quantifying the amount of variation, focusing in close the furthest data are from the mean. If do do something that will make statisticians wince, and ignore the first and last dates from my chart, this happens:

The modal, median and mean all come together for this dataset. Neat! Well, neat trick. You can use any figures to support an argument. Any argument. Heck, the last bird report I bought before leaving Sussex had an average mean of April 4th, it would be nice to think neighbouring counties would be close..

No, we need a more sensible line of thought: what do Cuckoos eat?


Looking at the species accounts in the annual reports I have, I tried another statistical analysis. I grabbed the first date, the second date, and then the date for the first multiple arrival. Of course, this is not robust: I have no access to the database to check the authors have included multiples, I'm just trying to show a trend:

Often the first date is well before the second. And well before the main. Our fixation on first dates is well known. Just as last dates, just as highest counts. But they are the simple things. The devil is in the detail. The average first date may well be the 4th (or earlier if that 13th March is correct) but it doesn't move the main arrival. Start banging medians and modals on that lot, and see what happens. Otherwise you just have birders hinting at not believing records. You don't have to not believe something; you have to consider if robust enough enough for the correct analysis (a really early date throws the median arrival and you can easily see the modal for arrivals).

Early arrivals. We might think we can say the run of early dates means something, but if we're not seeing early multiple arrivals, they're pretty much statistically insignificant (in the nicest sense).

And there's the beauty of BTO's Birdtrack; a big dataset focusing in on these larger trends. Why every birder should consider providing data to them.

I fear I might now have to have yet another go at reading 'Statistics for Ornithologists'. I think I'd find Klingon easier to pick up, I just hope this dummies guide might have helped some readers to think about data a little more.

Meanwhile, in the ornithological world, any early March Common Cuckoos, like their cousins from over the pond, those Yellow-billed and Black-billed Cuckoos turning up here in the autumn, are pretty much beggared. Because of what they eat.


Caterpillars. They love 'em. They adore 'em. They turn up and hoover 'em up. No point in getting here early. Not unless laying dates for host species and food sources move their dates dramatically. And this month, so far, hasn't been that good for hatchings. They turn to beetles in their absence. Now, how many of them have been active of late? How easy it would it be for a Cuckoo to find enough food? (Why that early March ringing record in BWP was described as exceptional. How had it survived?) No self-respecting Cuckoo is going to want to turn up early. The timing impulse to leave Africa is inbuilt;  they can't tell what the weather is like here when they set off. They slow up when they near if they hit bad weather but, seriously, why would a Cuckoo want to race him just to die?

Why I don't focus on first dates any more, but try to learn more about behaviour and ecology. If you haven't learnt anything about statistics from these ramblings, you might not forget what a Cuckoo eats.

Friday, 9 March 2018

Inlets and Yantlets

An inlet. A word most know.

A yantlet. One that few outside of Kent, and then mainly only those with a nautical bent, might have ever heard of. Both mean the same thing.

Most Kent birders, on hearing the word, think of Grain and Allhallows,where the once navigable channel, now dammed, separates the Isle from the Hoo Peninsula. Yantlet Creek. Just north of there the main deep water channel of the Thames, from Mucking out to just west of Southend pier, is the Yantlet Channel. (An invisible line running north from the mouth of the Yantlet at Allhallows to Essex is the political end of the Thames Estuary.)

But here, nearby on the Medway, is a another Yantlet- South Yantlet Creek. A channel that divides Nor/Friars from Bishop, navigable only on a high tide. (on a low, water remains only as far as no. 3 buoy) petering out just before Yantlet Spit. These three bodies are now badly fragmented (Bishop so much so that, at the western end, Darnet Ness is now a separate island on all tides), but all, thankfully, still have high ground remaining for breeders and roosts.

Screengrab from the excellent

Running east-west, the South Yantlet has never been viewable from shore so, as the old saying goes, out of sight, out of mind. But it does as important a job for the estuary birds as do the more well-known eastern basin creeks.

The very best time to appreciate this importance from shore is on the ebb and to a lesser extent, the flow. Over high tide the waters are too deep for effective feeding for many species, and the 'SYC' is an obvious refuge.

From the east, birds drift in and out via Half Acre. 'Scope from Horrid or Bloors. From the west, travelling against the flow, most chose to flight, best viewed from the northern stretch of Sharps Green. Sad fact, the more interesting species for a birder are more often found around the distant eastern end.

Grebes: a fair number of Great Crested Grebes head into 'SYC', meaning counts from shore over the tide are often under-representative of actual numbers present. If Slavonian or Black-necked Grebes are about, they are often hidden in among their cousins here.

Diving duck: if a Pochard or Tufted Duck is loitering, they will also move to the SYC, swapping between there are mainly Nor. On the drop, they will flight to Ham Green/ Otterham. They are most noticeable early in the breeding season, often birds not doing well in the early morning pairing up displays- singles that steer well clear of the fleets being claimed by potential breeding pairs. Their scarcer cousin, Scaup, when present also use the creek, but stay closeby longer, preferring shallow water for feeding and happier to loaf on the flats from time to time. 'Nor' is one of the most recorded sites for Scaup in old records, but the SYC is why birds would often be hit-and-miss for birders.

Sawbills: Red-breasted Mergansers often ride the tide out in and around Friars. On low tide they will have drifted out to Half Acre. Usually at this time of year they group to display, but numbers have been extremely low in the western basin for several weeks now, the birds having taken a liking to Shepherds and Stangate creeks to the west, both having more protection from westerlies/easterlies..

Seaduck: If there are any small numbers of Eider and Common Scoter present, they will happily remain in Half Acre/Bartlett in most conditions except for strong northerlies, when they might shelter in the SYC over the tide. (With all these generalisations, there is a percentage game needing to be played; on some tides, perhaps due to wind, swell, disturbance, any of the estuary birds might switch to east-southeast of Nor in the sheltered bay formed by the arm of Friars, or hole up close-in north-east of Bishop; reading the conditions help increase your chances of catching up with them.)

Cormorant and the odd Shag often feed here, as easy to pick up on a falling as a rising tide as they follow the shoals in and out of this relatively narrow mid-estuary channel. It is one of the best spots to see Cormorants working together.

If there are any Spoonbills wintering on the north shore in their favoured Damhead creek area, then the SYC is often their first choice of refuge when disturbed.

The island complex defining the SYC is a vital roosting area for the eastern basin, and waders transit and roost in number, the main roosts being on the more 'natural' Bishop and Friars. Both are shot by wildfowlers (dabbling duck love the islands here), but usually not both are shot on the same day, so roosts chop and change.

Human disturbance is greater in the eastern basin, mainly because these creeks and islands are closer to the more populated areas of the estuary. Jet-skis, especially on weekends, use the SYC as their favoured route. Why weekend birders fare worse for any 'target species' here than the weekday visitors.

There are numerous unauthorised landings on the islands, sometimes culminating in overnight camping. Canoes venture to the forts. Once, even a shore fisherman over a spring tide on the Darnet saltings.

Most breeding numbers are on Bishop. Canoeists and small craft can disturb on the high tide. The increasingly popular recreational activity of 'mudlarking', searching flats for old 'treasures' lead the more adventurous out onto the creek from time to time.

Disturbed roosts may sometimes feel safe enough to just shift from one of the other surrounds of the SYC. Darnet is popular if the threat level is only slightly elevated, but a full scare will see birds off to Hoo, or the western basin, or even aerial roosting. And the chance to clamber over the old fort on Darnet is a magnet for the more adventurous river user.

A new initiative, BirdwiseNK (which will be getting a blogpost of its own shortly) has a code of conduct for those out on the water;

- At high tide, stay away from roosting birds.
- Avoid landing on the islands; they are used for breeding in the summer and roosting in the winter.
- View wildlife from at least 100m away and move away if they become agitated.

All aspirational of course; a lot of work to do but it will be interesting to monitor disturbance now Birdwise is 'live' and word spreads.

Friday, 2 March 2018

Get the facultative movement outta here

“It is well for us to recollect that even in our own law-abiding, not to say virtuous cases, the only barrier between us and anarchy is the last nine meals we've had”
A.H.Lewis, 1896

The arrival of the Beast

Obligate movements. Obliged to do it; inbuilt. The urge, for some, to migrate, at the correct season, at the correct time.

Facultative movements. There is a choice. Stay or flee.

As I plot this blog out, it's 06:00 and the Queenborough weather site is showing minus one, with a minus ten windchill. North-easterly wind, averaging 35 m.p.h., gusting to 46 m.p.h. The 'Beast from the East' continues to hit. Do I go out tat dawn, or stay in? My choice. I wanna bird, but I also want to stay warm. If this blog gets posted before nine o'clock, you'll know what I chose. That's based more on the weather, because I've got a kitchen full of food to keep me going and I've already had my porridge.

But we're all only nine meals from anarchy. Birds? Some are nine meals away. Some, just a couple. Individuals of different species take decisions to move at slightly different times.


Someone told me the Lapwings are migrating away at the moment. Well, strictly, no, they're not. The annual obligatory migration route is established. Within that range, there can be movements during the non-breeding part of the year. Radar studies have shown movements can happen almost daily throughout a winter, in varying numbers.There is a continual readjustment, a refinement, as birds make choices.

It is a movement, not a migration.

In the past I've heard it said this is a cold weather movement. That would be nice if it was, because temperature is measurable, and we could soon say what isotherm triggers the movement, just as we have an understanding of how temperature correlates with migration, but cold isn't the main drive.

All-too often you hear hard weather movement. Nice term, but what does it mean? Define hard weather. I've yet to hear someone call a drought 'hard weather', but in that circumstance, waterbirds often make a choice to move, often northwards. Our influxes of, say, Glossy Ibises, have yet to be called hard weather movements. Or autumn pressure systems bringing 'Yanks' to Europe being hard weather- but if that's not hard weather, I don't know what is. You could say 'weather-related' I guess, but many scientific texts avoid the phrase.

They tend to call it an escape movement.


The hard weather we're having on the Medway at the moment is mainly deep snow cover, freezing temperatures, chilling winds, puts pressure on a number of the birds here.

Take sheltering. There is a limited amount of decent shelter from the chilling winds.Watch a clump of cord grass and you might not immediately realise how many birds are in there; scanning a birdless edge of saltings a couple of days ago, a cord grass clump callled like an alarming Redshank. The next clump over answered, and the next, then the next.. Yesterday one flighting Redshank crashed into a clump and put out twelve. They moved to a nearby clump where, finding no room at the inn, they gathered on the sheltered side and huddled.

Take food. Their preferred meals haven't migrated, but they're moving. Downwards. the colder the mud surface, the deeper the critters burrow. And the surface has been cold. Two nights ago, snow arrived during the low tide cycle. Usually snow on mud melts, but it was cold enough for the snow to form a thin sheet of ice over much of the estuary. The birds could not get at their food. Some could. Often the edges of a creek or tideway do not ice up; gravity plays a part in keeping water flowing, so the mud remains, well, muddy,and there is some feeding available. Squabbles ensue. Some birds chose to move. They chose to try to escape starvation.

Yesterday, the only birds sitting out on the more open flats were the Brent Geese and the Shelduck. The bigger waterbirds. They can play hunger games for a longer period than their smaller cousins, the ducks. These geese may not move for three or four days, the ducks might move after a couple, but never usually all of them while some feeding remains available. Weighing up when to go is a balancing act.


Take Redshank. Like quite a few wader species, once they have chosen an estuary in their first winter, stick with it in future winters. They know it well. For most of the winter, they will not carry big food reserves. Carrying additional fuel means slower day-to-day flighting from ever-present predators. Better to be lean. And hope any poor feeding only lasts a short while, because they won't have the reserves to move a big distance without having an impact on their survival odds. If the extreme weather lasts too long, many will die. They run the risk. The strongest, the fittest, those that can hang on to a feeding spot, survive.


Birders are reporting the 'usual suspects'.

Plovers on the move. Plovers rely more on eyesight to locate prey. The species that feed more in fields than mud flats, Lapwing and Golden Plover, have real problems with snow. They escape early.  Haven't been in huge flocks, birds make the decision individually; twos, threes overflying aren't that exciting but add up the day total and the movement is seen.

Similarly, the field-feeding thrushes, Fieldfare and Redwing, are escaping in a similar manner. Numbers break off from one gathering and wander. Garden birders are reporting one or two in the back yard one morning becoming a half-dozen later in the day, then a dozen, then two.. Snow cover in south-east England is patchy, more are trying to avoid fleeing, but birds in the snow need to move. The total involved is fluid.

Transfer that thought process to small calidrid waders and you begin to understand why inland patch workers are getting one or two Dunlin or an odd Knot. Some are choosing to make a short switch to another estuary (perhaps the Thames and Swale for Medway birds; our sister estuaries are that little bit different, with fewer sheltered waters that are freezing out from the shore), others chancing finding a new estuary by fleeing inland.

The county avifaunas often refer to the largest movements in the worst winters, and we birders sometimes hope to see hundreds and thousands of birders racing past us, to experience the movement. More likely we'll only pick up on a few dozen, or perhaps break three figures for a couple of species. Escape movements are nowhere near as predictable as obligate migration.


Some are waiting to see wildfowl on the move. Probably won't happen, because most will still choose to move at night. We'll see changes in day-to-day local totals. But these might be very short movements, flights to nearest open areas (these birds are now starting to respond to their obligate drives and wanting to move toward breeding areas) rather than huge jumps. Looking at a satellite image for yesterday, it was clear that most of the low countries were free of snow. We've only got so much thanks to the North Sea providing moisture. Is there any reason for any duck there to escape? It might be bloomin' cold, but if they can feed..


Escape movements. Yes, they correlate to the weather, but they come down to food. We humans are nine meals from anarchy. Some birds are one or two from death. Calling it a hard weather movement helps hide the life-or-death decisions being taken. At the moment, all too often we birders will get excited by the rare garden bird, the patch tick, the hard weather movement day total and fail to appreciate they've appeared because they're having troubles..

Hang on, it's 07:30 already. Time to stop. Think I'll leave going out for an hour or so, tide's not until 10:00. Ooo, I can have a second breakfast, lucky ol' me (!)