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. Our 'common' Cuckoo needs to do the same in the spring.

Many excited when they first make landfall here in the spring. Getting the first for the year used to be a 'boom! initials in the annual bird report moment.' Part of why we remember first dates more than main passage periods I guess. 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?"

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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, summarising 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?"

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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. But hey, 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..

So why we also need a more sensible line of thought: "What do Cuckoos eat?"

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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. Why? "What do Cuckoos eat?".

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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 here 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 (and half-decent mathematicians will probably be groaning at my interpretations) then more likely you may not forget what a Cuckoo eats. So if you hear an early, or mid-, March bird well done and worth trying to get a visual...

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 http://fishing-app.gpsnauticalcharts.com

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.

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 at 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.

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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.

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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.

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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.

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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.

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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..

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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 (!)