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 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 (Ref 1). There have also been many articles published in ornithological journals about the Yellow-brpwed Warbler (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. So what are they doing in the very different location and climate in 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/Mirror 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 are tiny. Does that mean very few do survive the winter and return to Siberia in spring, or is it just that we haven't found their wintering area yet?
5. There is very little evidence that a new migration route is being established. Yet it seem incredible that the thousands, and increasing numbers, of individuals arriving here in autumn 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 they 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 Scotland and Ireland. A lot of records were still from the east coast and Shetland and these presumably indicate 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-west 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 in 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 just three days from Oct 14th to 16th.

Dee Estuary and North Wirral 2024

We've had 32 records this year, slightly below the highest ever total of 35 in 2016. No doubt the actual number of YBWs passing through our area will have been far higher, as you would expect for a small bird which skulks hidden in shrubs and trees. When you get birds turning up in birder's gardens where they can be identified, you wonder about all the other gardens where people would have no idea what they were looking at.

The hotspots this year were undoubtedly Hilbre and gardens adjacent to the west end of Hoylake promenade. Hilbre had the first two birds seen, with singles on Sep 20th and Oct 3rd, ending up with a total of five. Jane Turner's wildlife garden at Hoylake had the majority of records with at least 10, and another three were recorded elsewhere in Hoylake. One caught by Jane on Oct 10th was carrying significant fat reserves, but it had increased it's bodyweight by 20% when it was re-caught on Oct 13th with a fat score of 50 - i.e. a very well fed bird. So, whatever happened to these birds when they left our area, they had obviously found plenty to eat.

History

The first YBW to be recorded on the Dee Estuary/North Wirral was on Hilbre on the relatively recent date of Oct 13th 1973. Another was at Hilbre the following September, and it was 1979 before the next one turned up, in a garden at Red Rocks. There was a gap of six years but then, reflecting the increase nationally, they became more or less annual from 1985 onwards (Refs 4 and 5). As can be seen in the bar chart above, annual numbers remained small with usually just two or three recorded for the first 10 years of this century*. Nationally, they started to significantly increase from around 2003, but here on the west coast it wasn't until 2012 before we saw a similar increase, peaking with the remarkable 35 records in 2016, and numbers have remained relatively high ever since.

References

1. Richard Smith, Yellow-browed Warbler Influx 2016, December 2016 Dee Estuary Newsletter.

2. Paul Dufour et al., The Yellow-browed Warbler (Phylloscopus inornatus) as a model to understand vagrancy and its potential for the evolution of new migration routes, Movement Ecology, 10, Dec 2022.

3. Steve White and Chris Kehoe, Report on scarce migrant birds in Britain in 2016 (Part 2. passerines), British Birds 111, September 2018.

4. Cheshire and Wirral Bird Reports 1973 to 2022, CAWOS.

5. Allan Conlin & Eddie Williams, Rare and Scarce Birds of Cheshire & Wirral, 2017.

* The number of Yellow-browed Warblers in the bar chart were those reported in various sources including this website, but not all records were submitted to the Rarity Committee so the 'official' number of records quoted in the Cheshire and Wirral Bird Reports will, for some years, be lower.

Richard Smith