4 May 2012

acredula Willow Warbler – Alba Marsh (Bahrain)

There are obviously at least two different types of Willow Warbler passing through Bahrain with the main one being the brightly coloured trochilus type. Many of these are very small in size with wing measurements below 64 indicating they are from this sub-species. We have, however, also caught birds with wing lengths of 73 mm meaning they are not trochilus and are either acredula or yakutensis, but are not very distinctive and most probably are acredula. At the weekend we caught a very brown/grey and white bird that was quite large but had yellow fringes to the secondaries indicating it was an acredula. This is the first distinctive bird we have caught since I have been ringing and is obviously different from the other Willow Warbler in the photo plumage wise.
acredula Willow Warbler (right)

acredula Willow Warbler (right)

acredula Willow Warbler

acredula Willow Warbler

There are three different sub-species of Willow Warbler

Phylloscopus trochilus trochilus is the nominate race and breeds in Ireland, Britain and Southern Sweden south to northern Spain, central and north-west France east to Southern Poland and northern Romania. They winter in sub-Saharan west Africa. This sub-species is smaller and more olive-green and yellow than the other sub-species. They are generally olive-green above and much yellower below, although also show whitish base colour with varying amounts of ill defined, sparse, faint yellow streaks on the throat and breast and often the under-tail coverts. The supercillium is moderately pronounced.

Phylloscopus trochilus acredula breeds from Scandinavia, except southern Sweden & central and east Europe to central Siberia. They winter in Africa south of Sudan. It is similar to nominate in west of range becoming greyer and then browner towards the east. They are generally paler and brighter olive brown above with a less prominent greenish tinge and whiter below. The fringes of the retrices and remiges are pale olive brown (much reduced amount of yellow) and the supercillium is longer and more distinct and less yellow in colour compared to trochilus. They look large, bulky and greyer all over than nominate.

Phylloscopus trochilus yakutensis breeds in central and east Siberia and winters in east & south Africa. They are the largest and palest race and are paler brown above and white with a greyish wash to breast. The rump and fringes to the retrices and remiges show only a trace of olive-green and the supercillium is almost white. They show virtually no yellow and very little green in their plumage. It is smaller billed and often shows some streaking on the throat and upper breast.

The problem with Willow Warblers is they are very variable birds with as much variation within races as there is between them and are clinal across their range with acredula being very variable and intergrading with yakutensis in Siberia. This makes it difficult to assign birds to certain races unless a clear distinctive example can be found. The best way to try to assign race is on size and colour, but only fresh plumaged birds should be considered.

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