Puzzling rock thrush in the Netherlands
by Nils van Duivendijk and Martin Garner
A rock thrush found by Andre Boven in Den Helder on Thursday 11 April 2013 keeps Dutch birders busy.
After the first views the ID seemed to be simple: a stocky rock thrush with a short reddish tail, bluish head and orange on the underparts; clinched: Rufous-tailed Rock Thrush. But with better views and pictures published on the internet (good set of photos here), more and more features seem at odds for Rufous-tailed… The easy part is ageing and sexing: the moult contrast between new lesser coverts and 2-3 inner greater coverts against the old rest of the wing is typical of a first summer. The blue head and orange on the underparts makes it a male.
•Extensive bluish/blue feathering on the underparts.
•Tail on the long end for RTRT resulting in relative large part of tail visible from below (undertail coverts not reaching close to tail-tip).
•Replaced lesser coverts and inner greater coverts with blue wash.
•Blue uppertail coverts with only buffish tips.
•Extensive dark barring on whole underparts.
•Dusky outer webs to reddish outer tail-feathers.
•Blue back feathers (though probably normal for first summer).
Most of these plumage anomalies are more or less normal for, or towards Blue Rock Thrush. Some have suggested ‘Eastern’ Blue Rock Thrush philippensis… and indeed especially the underparts of our bird are very similar to some first summer male philippensis. However the longer primaries/ shorter tail ratio (and short legs) of the Den Helder bird is much more similar to RTRT than the longer tail/ shorter wings ratio of all Blue Rock Thrush taxa. Furthermore, as far as we know, philippensis should not have any red in the tail. Also the wing formula of our bird is in line with Rufous-tailed i.e. very short p1 (much shorter than the primary coverts) and there is only one clear emargination (p3), and a very slight second on p4. In Blue Rock Thrush of all forms p1 is at least as long as the primary coverts and there should be three emarginations which are also closer to the feather bases (in our bird about halfway to the primary projection). So ‘just’ an aberrant Rufous-tailed Rock Thrush?
Or do we have a rare²-bird… Hybrids between RTRT and BRT are said to occur but up to now we have failed to find a picture or description of such hybrid. From hybrid Black x Common Redstart it is known that the wing formula is somewhere in-between in at least some known birds. As far as we can ascertain from the photos this seems not to be the case here.
Postscript: droppings have been collected, but as there are only sequences from mitochondrial DNA to compare this will only lead to the ID of the mother.
More to be discovered…