Dee Estuary Birding

What are you doing here when you should be in the steamy jungles of south-east Asia? And where will you go to next when you leave here?

Those two questions I was asking as I was looking at this tiny sprite, birders have been asking for many years - and the fact that they remain unanswered sums up how puzzling their autumn arrival each year is. I will briefly summarise here what the puzzle is, and I have already gone into some detail in my previous article in 2016 - Yellow-browed Warbler Influx 2016 (Dec 2016 Newsletter). There have also been many articles published in ornithological journals about the YBW puzzle and the one I found the most interesting and comprehensive is The Yellow-browed Warbler (Phylloscopus inornatus) as a model to understand vagrancy and its potential for the evolution of new migration routes by Paul Dufour Et al (Ref 2).

The Puzzle

1. YBWs breed right across Siberia from the Urals in the west to the Sea of Okhotsk in the east, and winter in southern Asia with Thailand being particularly favoured. A very different location and climate from western Europe.
2. The YBWs which reach western Europe appear to be flying in the opposite (i.e. 180°) direction to the rest of the population. This suggests some kind of Reverse migration, but nobody really understands what this is or what causes it.
3. Records of YBWs in the UK have increased greatly this century, yet there is no evidence of a westwards range expansion or of an increase in numbers genearally.
4. YBWs are seen in the UK in good numbers in September and October, but numbers over-wintering, either here or further south, are tiny in comparison. Also, numbers recorded on spring passage is tiny. Does that mean very few do over-winter and return to Siberia in spring, or is it just that we haven't found their wintering area yet?
5. There is therefore very little evidence that a new migration route is being established. Yet it seem incredible that the thousands, and increasing numbers, of individuals are just vagrants (i.e. do not return to breed and therefore are lost to the gene pool) rather than being true migrants.

I end this section with a quote by Steve White and Chris Kehoe from their Report on scarce migrant birds in Britain in 2016 (Ref 3): Something is clearly happening somewhere with Yellow-browed Warblers but a clear explanation for the exponential increase in British records is still lacking. Something of an understatement!

The 2024 Influx

2024 saw another massive influx of YBWs into north-west Europe, on a par with 2016 which Steve and Chris were commenting on above. The two maps below tell the story of their arrival into the British Isles.

Early September saw just a handful of records, then the birds arrived in good numbers on the 19th with 40 records reported to BirdGuides, followed by 99 the next day - and kept on coming. The left hand map (records for Sep 18th to 20th) clearly shows their arrival along the east coast and northern Isles with just a few birds seen further west, including Hilbre on the 20th. By mid-October (right hand map) they were everywhere in England and Wales, as well as Sotland and Ireland. A lot of records still from the east coast and Shetland presumably indicates birds were still arriving into the country. But there was obviously a movement from north-east to south-west with a lot of birds in south-east Ireland, the Scilly Isles and Cornwall. Whilst I was writing this I looked at the records for the middle week in November, there was just one record from Shetland and they were largely absent from the east coast of England with most being seen to the south-west of Ireland, England and Wales. Most had obviously already passed through, there were a total of just 77 records for the whole of that week in mid-November, compared to over 540 in the three days Oct 14th to 16th.

Here on the Dee Estuary, as the above graph shows, there was a sharp increase in numbers in the first five years of this century after which the rise in numbers has been much slower, until these past two years when there has been another big increase to 10,422 in 2023 and 10,565 in 2024.

As well as the rise in the population generally there has been a big shift in the monthly distribution on the Dee Estuary. In the 1990s they usually peaked in winter, but by 2004 the peak was in August and September. In particular counts in August showed a massive change with just 51 birds present in 2000 compared to 3,567 just five years later in 2004 - demonstrating the birds were now using the Dee Estuary as a post-breeding moulting site. The Welsh side of the estuary was particularly important for them at this time, and for the big count of 6,452 in September 2004, 94% were between Connah's Quay Nature Reserve and Bagillt Bank.

The Black-tailed Godwits which use the shore between Heswall and West Kirby, and the adjacent inland feeding and roosting sites, have been monitored almost on a daily basis for at least 25 years and we've seen some big changes both in numbers and in their behaviour over that time. Here the shift from a winter peak to a summer/autumn peak took place almost ten years after the same happened on the Welsh shore, and the following two graphs tell the story.

The shore between Heswall and West Kirby is very muddy, just what the 'Blackwits' love! As you can see from the above graph they were very much a winter visitor up to at least 2010 with counts being typically between one and two thousand from December to February. They were virtually unknown inland until the autumn of 2008 when we started to see a major change with a count of 773 on October 1st in a small horse field near West Kirby which we called the Gilroy Wader Scrape. This field was quite wet with a shallow flood in the middle, and it was in this flooded field that the godwits decided to roost.

In 2013 numbers at Gilroy started to significantly increase - and it became obvious, with birds arriving in July and August, that they were using the field as a safe post-breeding moulting site with little disturbance and access to freshwater. By 2016 Gilroy was recognised as one of the most important moulting sites in the country for birds returning from Iceland - so it was a huge shock when the field was drained in October that year. But it didn't seem to bother the godwits too much as they started using another small field just over one mile to the south, also largely undisturbed and with access to  fresh water. Numbers continued to increase at this field, the Caldy Wildfowl Collection, such that by 2023 I counted 6,350 on August 20th - which is still the site record. Amazingly, in this one small field there were more Black-tailed Godwits than in the whole of the Ribble Estuary or in Morecambe Bay! Watching them fly in was an incredible experience.

Typically, the godwits roost at the moulting site about two hours either side of high tide, but sometimes they stay all day arriving shortly after dawn and not leaving until late afternoon - in the summer they often lie down on the grass sun-bathing, living must be easy! When not roosting they feed on the adjacent estuary between Heswall and West Kirby, and at low tide they can sometimes be right out on a sand bank in the middle of the estuary and only visible with a telescope. The big rise in numbers between July and October has coincided with a big drop off in over-wintering numbers. We don't know why this is but maybe it's simply that the birds are depleting the available food so that there isn't enough to maintain wintering birds.

The demise of the Gilroy wader scrape demonstrates how vunerable these inland roosting/moulting sites are. and the Caldy Wildfowl Collection field is just a private site and is unlikely to outlive the current owner. However, the good news is that Wirral Council have been given a £314,000 flood management grant (Ref 2) which will be used to create a wetland on the Hoylake Langfields close to where the Gilroy Wader Scrape was. This will be managed by Cheshire Wildlife Trust with, I understand, the involvement of the RSPB, particularly at the creation stage. I'm proud that an article of mine seems to have put this on the agenda - see 'Rewiding Hoylake Langfields (a fantasy?)' http://www.deeestuary.co.uk/news0421.htm. Lets hope the Black-tailed godwits like it!

Colour Rings

Black-tailed Godwits are responsible for my obsession with colour rings. With their long legs the colour rings are relatively easy to spot and I was thrilled when I first saw them and realised I could track the movements of individual birds. The first one I saw, in 2005, had been ringed in Portugal, and since then I have logged a remarkable 3980 records, all on the Dee Estuary and its adjacent fields, thus making a significant contribution to the research into this species. Lots of other people have also read colour rings and the total number of records for colour-ringed Black-tailed Godwits on the Dee Estuary is 6,060 (as of mid-Oct 2024).

I have described the movements of many individual birds in my Colour ring Reports in my monthly newsletters, a good example is the report in the August 2024 edition - http://www.deeestuary.co.uk/news0824.htm#second. But for this article I've done something different and summarised the data in the form of two pie charts which show where these godwits go when they are not on the Dee Estuary.

The above pie chart is a fair representaion of how the Blackwits distribute themselves across the British Isles. The birds are constantly on the move, of course, and this distribution will be constantly changing. In the north-west the Mersey Estuary is a major site for them but because of industry and the Manchester Ship Canal access is often difficult and we get few colour ring records from there. So most north-west records come from Marshside RSPB by the Ribble Estuary, and various sites around Morecambe Bay, particularly Leighton Moss RSPB.

South-east England is a major area for them and a look at the map reveals why with a whole series of estuaries and creeks, from the Swale and Medway in the south, the Thames Esuary with its various tributaries, then  north to the Crouch, Blackwater Colne, Stour and Orwell - we have records from all these. Along the south coast is the complex of harbours - Portsmouth, Langstone and Chichester, but we have records all the way west to Cornwall. Going back up to the east coast the vast area of the Wash, plus the complex of reserves along the North Norfolk coast, is hugely important to them, as is the Humber estuary further north.

A lot of the Black-tailed Godwits which moult in the UK in late summer and autumn go on to winter further south. When I compiled the above pie chart of where the Blackwits are seen outside of the British Isles I was quite surprised just how important France was for them. Most are recorded along the Atlantic coast between St Nazaire in the north to Bordeaux in the south with birds stopping over here on their way to and from Portugal and Spain, as well as over-wintering there. A major site is the rice fields on the Tagus Estuary in Portugal where tens of thousands can sometimes be seen of both races. In spring there is an interesting movement north-east from Portugal and Spain to the Netherlands before they fly north-west in readiness for the breeding season. They breed in Iceland, of course, and many of the Icelandic records are from when the birds have been caught and ringed there, including many chicks.

References

1. Woodward, I.D., Calbrade, N.A., Birtles, A., Feather, G.A., Peck, K., Wotton, S.R., Shaw,  J.M., Balmer, D.E. and Frost, T.M. 2024. Waterbirds in the UK 2022/23: The Wetland Bird Survey, BTO.

2. Ed Barnes, "WIRRAL Council has been awarded more than £300,000 to develop a wetland nature reserve", Wirral Globe March 2024.

NOTE 1: Most of the data for the graphs and charts are from records which are on the Dee Estuary Birding Website (www.deeestuary.co.uk) and in my personal Colour Ring records database.

NOTE 2: Several articles on Black-tailed Godwits have been published in previous Newsletters, including 'Observations of Black-tailed Godwits'.

Richard Smith