Yellow-browed Warblers in the North West - Autumn 2005

by Steve Williams

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Introduction

The status of Yellow-browed Warbler Phylloscopus inornatus in the UK has previously been described as ‘a vagrant from Asia’. However, following an increase in annual occurrence over the last 30 years and particularly after this autumn’s nationwide influx it is perhaps best described as ‘a regular passage migrant that occasionally arrives in very large numbers’. Indeed this autumn produced one of the largest ever arrivals of this stunning little leaf warbler in Britain and Ireland.

The North West Region produced only a very tiny number of individuals (approximately two percent) compared to the English East Coast, the South West or Scotland, but the twenty five that did arrive in our region were still a large number for any autumn.

The purpose of this article is to review the influx of this species from a regional as well as national context, to detail the North West records this autumn and to speculate on the reasons behind the arrival of a species which breeds no closer than the northern Urals.

Brief Description

Yellow-browed Warbler is a very small leaf warbler, green on the upperparts contrasting with white underparts, as its name suggests it has a broad yellow supercilium (brow), and two conspicuous yellow-white wing bars. The only real confusion species in the UK are Pallas’s Warbler Phylloscopus proregulus and Hume’s Leaf Warbler Phylloscopus humei. Pallas’s has a much brighter and more distinct head pattern, with bright yellow central crown stripe (Yellow-browed occasionally has a pale central crown stripe but never as distinct as Yellow-browed) and Pallas’s has black stripes above each yellow supercilium and a bright lemon-yellow rump.

Previously considered a sub-species of Yellow-browed Warbler, Hume’s Leaf Warbler is very similar to Yellow-browed Warbler but is less green, more grey above, has less yellow supercilium not reaching the bill, ¬less yellow and fainter wing bars (particularly the second wing bar) and has a distinctive disyllabic call (for more discussion on the separation of Hume’s Leaf Warbler from Yellow-browed Warbler see BNW Vol 1:1 pp5-7).

Habitat and Distribution

In its breeding range Yellow-browed Warbler prefers open light woodlands especially with willow, birch and mixed species of trees. On passage in the UK it is often found in sycamores. Its behaviour is extremely active, constantly on the move and picking insects off leaves or even fly-catching – thus making it extremely difficult to photograph in the field.

Yellow-browed Warbler breeds in Siberia from the northern Urals to the sea of Okhotsk, south to the northern Sayan Mountains. In Siberia it is one of the commonest birds. On its autumn migration it passes through China in large numbers to its wintering grounds from central Nepal and Bangladesh east to south-east China and south-east Asia, the Malay Peninsula and southern China.

It is the most regular Siberian migrant occurring in the UK. However, in the ten year period between 1958 and 1967, only 275 individuals were recorded in Britain and Ireland (Sharrock 1972) and this included 128 birds seen in 1967 alone.

During 1968-1998 over 7,000 are said to have occurred in Britain, clearly there has been a remarkable rise in the occurrence of the species in Britain since 1958 and more than can be explained by simple increase in observer coverage.

Yellow-browed Warbler - This species is notoriously difficult to photograph (Garry Bellingham)

Autumn migration begins in August and September from the breeding grounds with birds initially heading eastwards via north-east China they then drift south or southwest through China to their wintering grounds, where they arrive from mid-October remaining to mid-April.

Its appearance in Western Europe has been attributed to reverse migration in a westward direction by part of the population. In basic terms this means that a small percentage of the population migrates in completely the opposite (or reverse) direction than that followed by the majority of the population. The great circle route theory then predicts that birds will turn south once hitting Western Europe.

Nationwide arrival – Autumn 2005

It is extremely difficult to analyse the exact number of individuals arriving in the UK during such an incredible influx and analysis and removal of probable duplicate records from the data is a fairly subjective exercise.

However, the extensive data kindly provided by Birdguides has been analysed and the figures provided are indicative of the overall patterns of arrival. The figures also represent only those birds reported to Birdguides so they are inevitably an underestimate. It is apparent that there were in excess of 1,000 birds involved during September and October 2005.

Apart from a remarkably early inland record at Draycote Water, Warwickshire, from 7th, the first bird to arrive on the east coast of the UK was on Blakeney Point, Nofolk on 10th, this was quickly followed by further birds in North Norfolk on 11th. These were followed by birds at Spurn (East Yorkshire), Flamborough Head (North Yorkshire) and Gibraltar Point (Lincolnshire) on 12th. The first bird to arrive in the South West was also early with Portland Bill’s (Dorset) earliest ever record on 17th September and the South East’s first bird at Dungeness on 19th.

The histogram below shows the number of new arrivals per day in Britain and Ireland during 10th September to the end of October 2005.

The initial wave resulted in a small peak around 25th September that included 15 birds on Fair Isle. The next large arrival occurred on 5th and 6th October; the latter being the first major peak arrival date with as many as 140 birds recorded. The main arrival areas for this period were the English east coast with over 50 in the North East and over 40 in East Anglia on 6th. These included 24 at Spurn and 18 at Flamborough Head both on 6th. However, Southern Ireland produced 25 new birds on 5th.

The second major arrival occurred between 14th and 16th October, with the peak arrival date for the autumn being 15th October with almost 150 birds arriving. Again sites such as Flamborough Head and Spurn had good numbers, as well as Wells Wood and Holkam Pines (Norfolk) and sites in Northumberland, such as Holy Island which had seven birds on 14th. During the whole period the North East had the largest share of records with 26% (Northumberland to East Yorkshire) followed by East Anglia (from Lincolnshire to Essex) with 22% and Scotland with 19%. The pie chart above indicates the percentage for each region. It has been postulated in the past that birds arrive in Scotland and Northern Britain and then filter south and that this explains the phenomenon of the peak arrival times in Scotland being a week or so earlier than North East England and East Anglia, which in turn is usually a week or so earlier than South West England and Southern Ireland.

However, as stated above the peak arrival date for Southern Ireland was 5th October. It is apparent from the data that the first major arrival during 5th and 6th October was on a very broad front which affected the whole country. Whereas the second wave did not have the same pronounced affect on numbers in the South West and Southern Ireland and was particularly concentrated in the North East as well as Scotland and East Anglia.


North West Records

Twenty five Yellow-browed Warblers were recorded in the North West this autumn (up to the end of October). No fewer than six different birds in the region on the peak day which was the 8th. Having travelled so far from east of the Urals to reach the ‘North West’, each individual deserves a mention:

Woolston Eyes NR on 2nd Bardsey on 6th (trapped) Carmel Head on 6th - 7th Rhiw on 7th - 8th New Laithe Farm, Newton, between 7th and 12th Crosby Marine Park on 8th Great Orme on 8th Bardsey on 8th Mount Park, Fleetwood, on 8th Soldiers' Point on 9th - 10th Hoylake on 9th Freeman's Wood, Aldcliffe Marsh on 10th

Snab Point, Walner, on 14th Breakwater CP on 15th Heyshan NR on 16th (trapped) and 17. Rhiw (two birds) on 17th Fluke Hall, Pilling on 17th Heysham NR between 18th and 20th (unringed) Hilbre (trapped) on 19th Breakwater CP on 29th Great Orme on 20th Fleetwood Cemetry on 23rd Breakwater CP on 23rd Mount Park, Fleetwood 29th

The histogram below indicates the arrival dates of these individuals in our region and this is interesting to compare with the arrival peaks nationwide as well as comparison to other regions

It is interesting to note that the two small peaks for the North West on 8th and 17th October respectively occurred exactly two days after the peak arrival for the rest of the country (mainly North East, East Anglia and the South West), being 6th and 15th respectively. This may be worth considering when looking for Yellow-browed Warblers in our region in future autumns!

Discussion

The arrival of Yellow-browed Warblers in the UK usually conform to the phenomenon of arriving first in Northern Scotland, then in the North East and East Anglia before later being found in the South West and Southern Ireland.

Sharrock (1972) provided three possible explanations for this shift in arrival dates of the species from Northern Scotland to South West England. The first being birds arriving further north in the UK then filter south and west. The second possibility was similar to the first but included for birds previously arriving in mainland Western Europe, the Netherlands and France in particular, and then drifting west to East Anglia and South West England respectively. The third explanation was that reverse migration occurs on a broad front and that the southern part of the Yellow-browed Warbler population lags behind that of the northern population as a result of starting autumn migration later and they have a greater distance to travel.

This last explanation has had some support from analysis of Swedish records in the past were records from a northerly observation station were a week earlier than a more southerly location. Sharrock concluded that all three of these possibilities probably contributed to the pattern of occurrence in the UK.

However, the first main arrival this year appeared to be on a much broader front with peaks in Southern Ireland and South West England coinciding with those in Scotland, the North East and East Anglia. Clearly, the birds in the South West and Southern Ireland were new birds rather than simply onward passage of the same individuals from the North or East. This does not fit in neatly with Sharrock’s theories, but may occur when a mass influx is concerned or may have involved a more southerly population migrating earlier this year as a result of weather conditions on their breeding grounds.

Yellow-browed Warbler - Hilbre, October 2005 (Val Burnett) One of at least three trapped in the region this autumn

The second arrival fitted the expected pattern with peak arrivals in Scotland, the North and East Anglia and then perhaps a filtering of birds westwards across and down the country. Albeit the peaks for Scotland 15th and 16th, North East 14th and 15th, East Anglia 14th and 15th and in the South East 15th and 16th again indicates an arrival on a broad front but this wave had a more northerly and easterly bias. It is significant that good numbers of Yellow-browed Warblers have been ringed this autumn (as in the past) particularly in the North East but recoveries (so far) of birds elsewhere in the Country are very few, if any. Considering the numbers involved one might expect that birds filtering through or across the country to the South and West would result in the odd ringed bird being seen or controlled in those regions (in this autumn or any other).  In the past a bird carrying a Norwegian ring was trapped on Fair Isle in October 1990 and another ringed at Portland on 21st October 1988 was re-trapped on Guernsey the very next day.

These records are consistent with the reverse migration and the great circle route theories but do not necessarily provide evidence of birds filtering from the North or East of Britain to the South and West. Obviously, Yellow-browed Warbler arrivals in the UK are to an extent weather dependent with easterly winds drifting birds particularly across the North Sea, but the fact these birds have already travelled vast distances from the east can only be explained (by current understanding) by reverse migration. The increase in numbers following this westward migration route and therefore occurring in the UK has been tentatively explained (Wernham et al 2002) by the fact that they may be contributing genetically significantly to the population and might be pioneering new wintering areas, possibly in Africa, for the species. If this was the case then presumably some of the birds which arrive in the UK must be making it back to their breeding grounds the following year. Only by ringing recoveries will the answers to many of these questions be obtained.

Yellow-browed Warbler - Hilbre, Wirral October 2003 (Steve Williams)

However, for now the specific origins and final destinations of these birds arriving in the UK still remain unknown!

Steve Williams – November 2005

References:

Cramp, S (Editor) ‘Handbook of the Birds of the Western Palaearctic – Vol 6’, 1992 (OUP)
Sharrock J T R, ‘Scarce Migrant Birds in Britain and Ireland’, 1972 (T & AD Poyser)
Simms E, ‘British Warblers’, 1985 (Collins New Naturalist Series)
Wernham et al, ‘The Migration Atlas’ 2002 (T & AD Poyser)

Acknowledgements:

Many thanks to Russell Slack at Birdguides (www.birdguides.com) for the extensive data provided for Yellow-browed Warblers nationally and to Ted Abraham of Birdline North West for this autumn’s North West records.

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