The Dun Sentinel of the marshes

This an odd one. A short post on a snail.

Assiminea grayana to be precise. Common name, the Dun Sentinel. A tiny salt marsh specialist extending to just above the high water mark, common on estuaries between the Humber and Pegwell. Found around the southern North Sea, also very recently (the 1990s) discovered in Ireland and the north-west of England.

Why mention on a birding blog? Studies have shown that where the Dun Sentinel is present, it can amount to up to 98% of the food resource for any estuary Rock Pipits. Dietary differences. The resident Rock Pipits on other parts of our coast get away with staying put in a maritime environment on a diet largely consisting of Melarhaphe neritoides- the Small Periwinkle. Driven out by the cold further north, our wintering visitors have found a different food resource to exploit

I admit to having never knowingly seen a Dun Sentinel, but I think I'll now include on my (bait-)bucket list and make an effort this spring once things have warmed up a bit and the winterers have departed. Rock Pipits are another favourite of mine after all.

The taxonomic fun over the years, separating Rock, Water and Buff-bellied Pipits (only given to full species status by the British Ornithologists Union some 19 years ago) is based only in part on visual differences. Habitat specialisation when breeding and wintering comes into the argument. The Dun Sentinel goes some way to explaining just why migrant Rock Pipits can get by on estuaries in winter- they've found a prey item out on the saltings they can do very well on. Some argue there's very little overlap between the species/subspecies in their wintering habitats but, some inland sites such as Stodmarsh/Grove in East Kent, are now throwing up headaches for Water/Rock aficionados.

Field i.d. is a blogpost for another day, but for now is always worth remembering the visual identification of each species is not as easy as some would have it. If you read the monographs rather than the sparsely worded field guides, you will find most distinctions use caveats of terms such 'generally' and avoid claiming many identification pointers to be as clear cut as we would like them to be. Small wonder it took us so many years to pigeonhole to species status...

The more we learn, the more there is to know.

I'm no conchologist, but I'll enjoy stringing a shell in a few weeks, all the time chuckling in the knowledge the snail taxonomists are still arguing about whether Dun Sentinel is actually a separate species, or if they need to lump it in with other Assiminea down around the Mediterranean.

Spot the Rock Pipit, hunting out the other two percent of his diet.
(Horrid Hill, March 2015)

Reed Buntings- court out in the cold

One might be forgiven for thinking I'm a waders and wildfowl fanatic. I'm lucky enough to live on an estuary, but I'd have to admit my favourites always were, and still are, passerines. So, I really should write a bit on one here.

Reed Buntings.

We're now at that time of year when the males start to stake a claim a territory. They leave their roost before dawn to make for their chosen territory, where they then sing for an hour or so. After that they head off to loiter around the flock's feeding areas. As the season progresses, they stay longer, and longer. The wintering groups break up slowly.

Sing out loud and show off your home in case a potential mate comes by, then go strut about a bit in their company. Male aggression starts to build now as well. It's tough trying to persuade a potential mate you're the one that's worth it.

I was lucky enough to live on a lowland farm in East Sussex for several years, where Reed Bunting was a very common bird at the garden feeders at this time of the winter- counts of twenty, thirty, were not uncommon, a hundred or could be ringed each winter. In February, their timing was always the same, matching the text books. None for the first hour or so, a steady build (females predominating at first) to a mid-day peak, and finally all disappear an hour or so before sunset. Late-winter males, such as the one below, coming into breeding plumage, were usually ringed late morning.

So, late February I'm always listening out for Reed Buntings in the mornings here along the Medway, and an insistent male at Eastcourt Meadows the other morning prompted me to write this note.


Winter roosting

Sadly, numbers along the southern Medway are nowhere near those of my old East Sussex stronghold. And nowhere near where they were when I started birding just over thirty years ago. The texts always talk about the collapse in numbers in the seventies (intensive farming), but here an almost imperceptible decline has continued. There used to be a regular roost at Horrid Hill, the scrub reaching climax (tall and closing off light to the herbage growth below) plus the Country Park policy of cutting the grass edges right up to the clumps put paid to the dense herbage they prefer to roost in some years ago. Increased human footfall has probably not helped.When I watched the areas in the 1980s I could manage counts of 20+ in the late autumn; now, lucky to manage a handful on any day.

A smaller roost on a private part of Motney Hill is now the most best site for double figure counts along the inner basin. Of course, Motney does has the most preferred roosting habitat- reedbed edge interspersed with rank herbage.

There are a couple of smaller gatherings around Ham Green and Lower Halstow. Small numbers can be found at the foot of Raspberry Hill and there is certainly a roost somewhere on the private Barksore; birds flitting around the Funton Creek saltings head off that way at day's end.

Most UK roosts are small in number- 40 is seen as a large roost, most are less than half that. Easy for birders to miss.

Winter roosting actually becomes spring roosting, in that birds continue to make nightly returns even when a territory is held and nest being built; only once the first egg is laid will a bird sleep there.

Throw in big variances in defended territory size during firstly establishment, then nesting and finally fledging, add in a mix of bigamous males, and you start to have a real problem with judging population sizes.


Migrants?

The Migration Atlas is unequivical: "...ringing data have been used to show conclusively that almost all breeding individuals from the British breeding population winter within Britain, at which time they are joined by very small numbers of predominantly Scandinavian breeding birds..."

Some ringing chums have been quite excited this year, by trapping a Scandi bird here and there around the UK; the strength of the easterlies last autumn may the have helped displace some movers but this does not detract from the bigger picture.

Taking a look at the Reed Bunting account in the excellent free download of the Nord Pas-de-Calais migration book the ringing recovery map shows Scandinavian birds will, for the main part, be taking the coastal European flyway I have mentioned in several previous posts in relation to waders and wildfowl; passerines may not 'use' the mudflats, but the majority will be hugging that coastline as inferred in the screengrab below:

So, any autumnal fluctuation in numbers here in the Medway is most likely due to local dispersal, moving to suitable wintering roost catchment areas; males on average move only a couple of kilometres, females maybe about six km (ensuring a local gene flow). Having one or two broods a year, fledging just over two young per brood, you might well encounter two, three times the number seen earlier in the year. And that's it.

The Reed Bunting is often overlooked by birders nowadays, but is well worth recording in the notebook. For those who say 'why should we bother, we know enough already', here's one take on how much we really don't know.


Tetrads- distribution, not density

For overseas readers, tetrads are particularly 'British' in their use on our Ordnance Survey maps and are well explained here. They are used routinely for mapping distributions in our avifaunas. Recording absence or presence is easy enough. Possible/probable/definite breeding have often been mapped. Thanks to the statistical wizardry and the size of data collated during their national surveys, BTO can go for a form of robust abundance mapping. Some counties still want to try to come up with a figure, a numeric, and Kent is one that tries hard to come up with such a quotable total.

How useful are these figures?

British Reed Buntings are largely sedentary and these small numbers seen recently reflect the small population along the southern Medway; numbers are nowhere near the estimated "30-50 pairs per tetrad for reed-beds" used by the County's latest breeding atlas- based on my own mapping singing males and ringing dispersing young, within my four local tetrads that contain reedbed/wet ditch/rank herbage habitat mixes we only have about 10-20% of that guesstimate. Perhaps because, on the ground, there are tetrads completely covered by optimum habitat. A tetrad is some 400 hectares- the main Motney reedbed/ditch complex covers 25 hectares, and might have five pairs in a good year. So, yes, 30-50 pairs per reed-covered tetrad, but in reality just 16% of that particular tetrad is 'suitable habitat'.

At least Kent had stayed faithful to their previous atlas, published in their 1996 Report, when they dismissed the old CBC methods (which produced a figure of some 4,800 pairs) 'since the county's marshland areas support high densities in reedbeds and dykes'.

That really should be 'some' marshland areas. Take Chetney. A paper published in 80s had shown Reed Bunting numbers to be way below that.

The truth is that estimating a common bird population is exceptionally hard, and if you want them from historic comparisons you need robust models that remain useable atlas after atlas.

At a time when 'professional' BTO survey results show there to be a continuing decline in the south-east, for any species author to state an optimum guesstimate of 'numbers broadly stable' for the county has to stand up to scrutiny. At least the new county Atlas acknowledges the BTO's BBS decline in the south-east before ignoring it. The following graph is reproduced from the BTO/JNCC/RSPB Breeding Bird Survey population trends webpage:

The 'changes in relative abundance' map in the latest national Atlas also showed a decline around Kent over a twenty year period. It would have been useful to read how that 'broadly stable' stance was taken by the county.

To this observer, the calculation is too simple. In statistical language, it is both 'imprecise and inaccurate'. Some comment is made- slight losses in western and central Kent, with slight gains in the same area. Looking at the maps, the losses are linked to watercourses (the Medway and its tributaries and the ignored east Kent losses along the Stour). Local 'improvements' with loss of rank herbage? A continued drying out, as rainfall continues to decrease, a whole host of questions not covered.

The tetrad losses have almost been balanced out by tetrad gains. But the county's latest (six year) Atlas period was longer than the previous, and made use of the social media to appeal for extra observer effort to fill gaps in that extra year. The 1996 Atlas made some attempt to quantify the number of observations received (a 20% drop on the '66-'73 Atlas), so the parameters changed. What consideration was there for the old chestnut of 'observer bias'?

Their 1988-94 county survey stated 8,000 to 13,000 territories. The 2008-13, 7,000 to 13,000 territories. Pessimistically a drop of just 12.5%, at the most optimistic a zero percent change. At least the Reed Bunting entry in this latest county Atlas ends by stating 'we need more evidence of current breeding densities, in reed-beds and on farmland'.

That's why birders still have a part to play. From published counts, very few sites in Kent have achieved any notable counts in recent years. True picture? Or extremely few attempts to find them?

All probably why several reviewers commented on a lack of relative abundancy maps in the latest county Atlas. Population estimates can be horribly inaccurate and imprecise. With just over a decade to the start of field work for the next Atlas, perhaps enough time to reflect on goals at the local level.

Has Kent really have bucked the trend of the rest of the south-east? Are populations holding up better here? I very much doubt it. But  hopefully local birders will rise to the book's challenge for more local studies. Go and count the songsters this spring to help them and, perhaps, prove me wrong.

Just as this published guesstimation is likely imprecise and as inaccurate, I could be hopelessly, hopelessly wrong myself. (Wouldn't be the first time !) It's why many records from many sources will always be needed.

Song of the South(-east)

If you need to brighten up a dull day, there is a great game you can play with the song when starting to record your Reed Buntings. A short burst of song, a long gap, then another short burst is most likely an unpaired male (what you hear around now). But a longer burst, shorter gap, longer burst is most often a paired male. Examples can of course be found on xeno-canto, but the one I'd like to link to is the oddest, and one I'd love to hear; an extreme version of the long song (described by the recorder as a 'marathon song') here. Going at full throttle. Now thought to be the sign of a successful Lothario going for as many extra-pair couplings as possible. When it comes to sexual shenanigans, Reed Buntings are up there with Dunnocks in my book (but that's for another blog post).

The 1996 county Atlas acknowledged the suggested population numbers better reflected territories than actual numbers, and while pair estimation worked well for some species, for the many species where polygamy, polyandry and the like are a norm, it is not that easy to say pairs. Count your singing males.

Here along the south Medway, continuing losses in suitable habitat (e.g. away from reedbeds, lot more saltings now go under on spring tides, making unsuitable for nesting), at the same time as a growing decline in habitat quality (the general tidying up) have been major drivers in the local decline in Reed Bunting numbers between the last national atlases. Together with a continued drying out in the south-east through climate change, and growing development pressures, numbers will have declined further by the time of the next national atlas.

Prove me wrong people.

Knot, Tellin: the full story

The other day I had the pleasure of showing someone around the southern Medway. Of course, the only day/time they could visit was when tides were poor for showing off the birds. An already nigh-fully uncovered dropping neap. Ouch. Close up views were going to be at a premium, so it was down to playing with numbers. I decided we could tell a story of inter-tidal movement through a game of 'chase the Knot'.

Kicked off with a  deliberate 'miss' at Funton; no Knot in view. (They would have roosted up Stangate, and more favoured muds were opening up as dawn broke.)

Followed this up with a jaunt to Shoregate and views over Ham Ooze for the win. The Knot were there. Impossible to see with the naked eye. No matter to start with, my visitor had the chance to soak up the atmosphere of bleak Dickensian marshes at their finest, framed by the distant curve of the new Sheppey bridge.

Of course, the distance these Knot were at made them hard to pick up with 'bins. I'd cheated, prior knowledge, I knew they'd start at the higher flats. To ensure they enjoyed the spectacle, the 'scope came out. I set up on the 'rear' of the flock tight by Slayhills saltings, and told my guest to keep scanning along the line towards the river. And keep scanning... keeeep scanning.... keeeeeeeep scanning...

Mission accomplished- suitably impressed with the spectacle (though I noted numbers were down), my visitor left with an impression of the importance of Ham Ooze within this complex estuary complex, not just for the Knot, but for all the other species present.

- - - - - - -


Racing Knots

"Immature Knots visit our Medway mud flats in very small numbers in the autumn; half a dozen is the most I have met with at this time together..." Prentis, "Notes on the Birds of Rainham including the district between Chatham and Sittingbourne", 1894

We see two races of Knot here in the U.K. Unfortunately these races are assigned mainly on geographical grounds- breeding continents apart, wintering continents apart, having different travel arrangements. And for the ringer the visual differences are, putting it mildly, highly subtle and subjective. Birders have little chance of seeing any difference. Knowing about the differences helps to appreciate them though.

Islandica breed in northern Greenland and the islands of high Arctic Canada. The UK and Ireland support about 70% of their total population in midwinter.

Siberian Knot (specifically, largely from around the Taimyr peninsula), canutus, pass through the Waddenzee en route for their wintering grounds in Africa.

Knot, Funton, February 2014

Migration routes- the flow comes in

"The Knot is one of the species most frequently heard during its migration as it passes over our sea-side towns, attracted by the glare of the lights during dark nights, and is easily recognised by its peculiar croaking cry amongst the babel of cries of the other wading birds..." Ticehurst, 'A History of the Birds of Kent', 1909.

Islandica leave late summer, short-stopping in Iceland before making directly for their chosen moulting grounds.

Canutus are known to occur, in North Kent, thanks to September ringed birds popping up in Africa in October. Chances are that most early autumn birds are canutus. Most of the canutus ringed in the UK are juveniles on their first migration, mirroring the age of the majority of late summer sightings on the Medway. Adult Knot in brick-coloured summer finery are a red letter day. I have yet to find any hints of islandica adults, birds staying for any length of time, showing signs of moult. There is no staging here.

There are also relatively few oversummering birds in the UK, with Knot returning to breeding grounds in their first summer. The ringing atlas considers this choice to be down to body condition, being individuals simply not fit enough to undertake the flight back.


Staging in the Waddenzee

The huge estuary complex between the Netherlands and Denmark is a safe moulting area. Moult takes about three months, between August and October. Once complete, birds move on to their wintering grounds. Many of 'our' Knot here in the south are presumed to be from the Waddenzee. The nearest U.K. staging post, the Wash, sees a similar redistribution but apparently mostly to the north


Winter adjustments

"Though a few arrive in imd-October, they are seldom abundant until until December, and, when severe conditions set in, individual flocks may total up to a thousand or very occasionally double this number..." Gillham and Homes, 'The Birds of the North Kent Marshes', 1950.

The Migration Atlas reports that arrival dates of ringed birds shows dispersal from moulting areas to wintering grounds taking place between October and December. Arrivals here are always at the latter end of that timescale.

It also suggests inter-estuary movements occur, as a response to food availability. (Both annual changes and intra-seasonal adjustments).

Ringing evidence now suggests the 'old conclusions' of hard weather adjustments, as supposed by the authors such as Gillham and Homes, are not that important in the scale of things, perhaps not even happening. This seems at odds with their apparent rational thinking, but drops in numbers counted during severe weather at sites like the Waddenzee are now known to be more often down to logistical problems for the counters themselves. (I sympathise. As a soft southerner I find it hard to force myself to stroll out in a drizzle.)

So, birds are known to then make for the UK through the last months of the year, then shuffle about mainly due not to weather but a change from a diet of their favoured bivalves (the Baltic Tellin) to snails (hydrobia)

A Baltic Tellin distribution map here shows the Greater Thames estuary is a stronghold for the favoured food. Medway, not so much. Having been a bit of a budding (bumbling) concologist when working at Sandwich, the shell is a familiar one, and one I do not encounter that often here.

Looking at the month-by-month peaks, the Medway December numbers follow a short arrival period, whilst the Thames overall numbers build from October to December. The Medway December influx happens at the time that food switch is suspected to happen.

Are 'my' birds arrivals from the Waddenzee, or early arrivals repositioning themselves from Essex? Hard to say.

The New Atlas confirms there is routine interchange between nearby sites (estuaries), which can lead to large fluctuations in a season. Checking the WeBS interface, it is interesting to note the Swale peaks in February, the Thames and Medway in December.

Ringed Knot, south Medway, September 2016

Numbers

"...the compact flocks of Knot... move from place to place and keep so much together, that they are usually seen in large numbers or not at all..." Gillham and Homes, 'The Birds of the North Kent Marshes', 1950.

Taylor quotes mean winter peaks of Knot for the Medway as
2,300 1953/54-1960/61
2,000 1961/62- 1968/69
   900 1969/70- 1976/77

A decade on, Holloway in the Birds of Gillingham stated "About 100 or so regularly winter in the estuary and can be seen feeding in very tight flocks on the mud-flats..." Certainly numbers were low until the 1990s when we benefited at the expense of the Waddenzee (the causes are explained later). A simple charting of peak counts from published Kent Bird Reports shows the upsurge in numbers:

The annual WeBS peaks in the winters since I came back to the Medway in late 2012 have been much lower than the true numbers on the estuary, being just:
   2012-13:   75
   2013-14: 200
   2014-15: 100
The WeBS five year average is just 1,306.

In reality, there have been triple that for the south shore alone. Two roost sites on the eastern side of Stangate have been favoured, with switches to Deadman's and, infrequently, to Bishop's.

So, despite the picture given by WeBS, there have been routine nationally important totals present (3,200 being the present threshold) and the international threshold (4,500) has been broken on several occasions (counts by other observers can be found on the county's database ).

Rainham Creek, January 2015

Migration routes- on the March

The routine return is via the Waddenzee. Spring numbers there are much higher than in autumn; many Knot that moulted in the U.K. in the autumn go there in preference to our shores. Most will not leave the Waddenzee until mid-May to stage in either Iceland or in just a couple of fjords in northern Norway prior to hastening to the breeding grounds late May/ early June and getting straight into breeding.

If you are lucky enough to find a Knot sporting a yellow leg flag then this will be a Norwegian stager; a few have been reported from the Medway in recent years.

Squint hard enough and that Knot is wearing a yellow flag; Twinney, February 2014

Population crashes- on the breeding grounds

Knot have some tremendous variances in numbers. They suffered a huge crash in the 1970s, from which there was only a slow recovery. It was thought to have been caused by a series of poor breeding seasons. By mid 80's they were down to a population of around 345,000, from 609,000. There have been good years since, but the species is red-listed by BirdLife International as 'near-threatened' in part because trends and numbers are so unclear.


Population crashes- on the wintering grounds

Between 1996 and 2005 more than 50% of the suitable feeding areas on the Waddenzee were lost, with monitoring confirming Knot numbers dropping by half during that period as well. This was all down to the cockle industry and a government policy which allowed large-scale damage by mechanical dredging. Ringing recoveries have shown some Knot did manage to relocate to sites in Britain and France. Same happened to some degree with the Wash. Woods, writing in 2007 in 'The Birds of Essex' reported that, nationally, numbers had recently decreased by some 15% but, at the same time his county had seen an increase of some 40%.


So, do the Medway birds arrive via Essex or direct from Waddenzee?

Although there will be a turnover of birds masking true numbers using the Thames, generally Essex numbers increase sharply in November to then peak in January. The Medway rise follows later, with the first large arrivals now usually seen in early December.

Sadly, the Migration Atlas highlights the Greater Thames is under-represented in ringing results. Not enough birds ringed, not enough retrapped. The simplest model is of birds arriving on the Essex flats, then dispersing. Usually Medway birds first turn up 'mid-river', out on the Ham Ooze then settling into routines around Twinney, Halstow and Funton. To buck that trend, in one autumn I witnessed first large numbers arriving via the Swale, cutting overland west over both Chetney and then Barksore.

Four winters in, it's fair to say I really don't yet know how the dispersal/displacement operates in this part of the world.

Knot crossing Barksore, December 2014

Taking the high ground

"Mr G.B.Rimes wrote me (31.v.1948) to the effect he had met with small parties at Motney Hill in the winter, but apparently these birds are not much given to straying far from the tidal basins and their immediate inlets and tributaries" Harrison, The Birds of Kent, 1953

Settling arrivals stick close to Funton Creek, close to Bedlams, on the northern edge of Twinney, close to Slayhills. All areas that, often up to this point in the wintering season, have not been favoured by other species in large numbers.

All are wide and open areas, better for accommodating a large flock. They are also what I think of as 'higher ground', and are what I now know to be exactly that being harder, sandier areas. Many other species have not been targetting any Tellin to be found in their sort of habitat. They may well be dining out on favourite food before switching to the hydrobia in the softer mud. All just a theory, but it keeps me guessing...


Taking to the roost

Though the birds can and do turn up all around the estuary, the normal pattern for a rising tide, from low water to covering (yellow), is for the Knot to start out on Ham Ooze. As the tide covers the Ooze from around the 2.5 metre mark, the birds move either to Funton (the edge of the Creek, or Bedlams Bottom, often split between the two) or to Twinney. Both flightpaths take Millfordhope Creek, and avoid cutting over the islands.

The main roost has been at Chetney Hill (red), but will also roost further north along Stangate Creek or along Deadman's island.

Less frequently (yellow circles) birds will use Bishop Ooze at low water and then sometimes feed around Rainham Creek. When remaining in this western basin for several days, the roost may move to Bishop Saltings (red square).


Taking cnout

"Deducing the overall migration system requires a flyway-scale analysis of Knot ring-recoveries." Davidson, in 'The Migration Atlas; movements of the birds of Britain and Ireland', 2002.

Similarly, efforts to find out more on Knot numbers in the greater Thames require a co-ordinated effort. The possibility is there for the wader counters of both Kent and Essex to get heads together and ensure the Thames counts are co-ordinated. A logistical nightmare for sure, but the birds would benefit.

One day...

Yes, I know some regular readers will now be banging their heads off of keyboards muttering "he's off on the hobby horse again". Perhaps I should now mention that person I was showing around the other day had already been looking at existing published counts in a professional capacity, and finding them nowhere near as useful as they could be if they were being carried out correctly.

One day...

In the meantime, if anyone feels the urge they can help by having a go at counting/estimating numbers ofany flocks they encounter.