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Friday, 23 September 2011

AZOREAN ATLANTIC-TYPE GULL still present

FRIDAY 23 SEPTEMBER

STEWARTBY LAKE AND ENVIRONS (BEDFORDSHIRE)

After obtaining reasonable views of the Atlantis-type gull in ploughed fields north of the new bypass this afternoon, I returned this evening to see it roosting.....

Just 14 observers turned out this evening - mostly Beds locals but also including Howard Joliffe from Essex and John Lees from Sussex. The bird was located by Allan Stewart at 1855 hours and roosted until dusk - part of an assembled group of about 1,700 large gulls, which included at least 22 YELLOW-LEGGED GULLS (13 adults, 4 2nd-winters and 4 first-winters), 27 Great Black-backed Gulls (mostly adults) and 56 Herring-types; just 2 Common Gulls were amongst the throng. There was also an adult Lesser Black-backed Gull with a very heavily streaked head.

The subject bird was almost certainly the bird that visited Oxfordshire two years ago, now in full adult plumage. It is as distinctive now as it was then, typified by its very noticeable hood (darkest in saturation on the ear-coverts and crown-sides), the prominent pearly-white iris, the distinct bluish cast to the grey upperparts, gleaming white underparts, thick bill and short washed-out creamy-yellow legs. Interestingly, despite now being adult, the bill pattern is largely the same - insepid greyish-green at the base and pale orange at the tip and blackish in the middle. It is also a particularly sturdy and large individual.

In every respect the bird appears to be an AZOREAN ATLANTIC GULL but recent correspondence I have received from Daniel Velasco, a good birding friend from Spain, has placed grave doubts about the authenticity of these type individuals. Daniel has spent a lot of time in recent years studying the large white-headed gulls that occur in the Cantabric and Northwest Atlantic coasts of Spain and his findings make very interesting reading. The presence of Yellow-legged Gulls with extensive dark grey hoods are not that unusual and begs the question of what is actually occurring in this region. Although there are several key features that do separate the Oxon/Beds individual from those that Dani has highlighted below, it really does raise the prospect of confident identification - a few links to images below

http://farm4.static.flickr.com/3114/2920168504_3c8191c010_o.jpg
http://es.groups.yahoo.com/group/galiciaves/attachments/folder/1192979208/item/1921112256/view
http://sgosgo.org/fotos.asp?id_fotos=1582
http://sgosgo.org/fotos.asp?id_fotos=1581
http://sgosgo.org/fotos.asp?id_fotos=1558
http://sgosgo.org/fotos.asp?id_fotos=1557
http://sgosgo.org/fotos.asp?id_fotos=2065
http://sgosgo.org/fotos.asp?id_fotos=2066
http://sgosgo.org/fotos.asp?id_fotos=1947
http://sgosgo.org/fotos.asp?id_fotos=1946
http://sgosgo.org/fotos.asp?id_fotos=2071
http://sgosgo.org/fotos.asp?id_fotos=2072
http://sgosgo.org/fotos.asp?id_fotos=2106
http://sgosgo.org/fotos.asp?id_fotos=2171
http://sgosgo.org/fotos.asp?id_fotos=2154
http://sgosgo.org/fotos.asp?id_fotos=2136

The propensity of so many birds of this appearance on the west coast of Iberia is truly perplexing, not least because winter-plumaged Yellow-legged Gulls on Madeira and the Canary Islands do not show such dark head streaking but white heads like most michahellis. The majority of Cantabric gulls relate to Yellow-leggeds of the form lusitanicus, which are darker, smaller and slightly more streaked on the head in winter and also importantly, frequently have a single white mirror on p10 (see montage at http://gaviotasyanillas.blogspot.com/)

Dani informs me that hybridisation between Yellow-legged Gull and Lesser Black-backed Gull in Galicia is commonplace, making him ponder about the appearance of these 'atlantis lookalikes', but it also seems likely that a cline exists between atlantis and lusitanicus and that perhaps explains the anomalies. Pure Azorean Atlantic Gull is essentially a saltwater gull and preliminary studies in the Azores of birds of all ages have shown little evidence of northward migration or displacement.