Friday, 10 February 2017

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.

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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 build-up on the neighbouring Swale estuary perhaps reflects the routine return to 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.

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