Rock Thrush/ Blue Rock Thrush Questions

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.

Rode Rotslijster, Den Helder, 12042013, Co van der Wardt OO9A0411 (800x533)First summer male Rock Thrush…, Den Helder, Netherlands April 2013 by Co van der Ward.

Anomalies

•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).

2013-04-13_165524Reddish feathers on underparts seem to be fresher than blue feathers. So is blue moulting away…??

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?

Rode Rotslijster, Den Helder, 12042013, Co van der Wardt OO9A0456b (800x533)First summer male Rock Thrush…, Den Helder, Netherlands April 2013 by Co van der Ward.

 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…

8 thoughts on “Rock Thrush/ Blue Rock Thrush Questions

  1. Maarten Platteeuw

    But this Blue Rock Thrush philippensis type is much slimmer, longer-billed and longer-tailed than the Dutch bird…

    Reply
  2. nilsvanduivendijk

    Thanks! A further anomalous feature is the dull eye-ring (normally quite obvious white). On the other hand I guess nobody would argue the ID as Rufous-tailed Rock Thrush if this bird had just normal coloured underparts, new wing-coverts and uppertail-coverts…

    Reply
  3. Wouter Teunissen

    Hi Nils and Martin,

    I don’t think you can say that “Reddish feathers on underparts seem to be fresher than blue feathers. So blue is moulting away…”. This is because juvenile feathers are neither blue-based, neither orange based. If it would be true what you state, this would mean that (assuming the bird is a second calender year) this bird got a “mysterious” inbetween plumage between its juvenile and adult feathers. As far as I know plumages of thrush like birds move over from juvenile into adult plumage and there’s no extra phase.

    I do agree that the blue feathers look more abraded than the orange ones. I have also seen this in the field. Maybe the blue based feathers have a pigment that is less strong than the ones with orange pigment? (This is however a bit strange, since in general, dark feather tend to be of stronger quality than light coloured feathers).

    Interesting bird!

    Regards,
    Wouter Teunissen

    Reply
  4. Peter de Knijff

    More detailed pictures of this bird have been posted in the DB site and clearly show no substantial age differences between the orange an blue feathers.
    All seemingly pure orange feathers have remants of white tips, as do the similarly pure blue feathers.
    Other feathers have more complex patterns but all show the same pattern of damaged tips.
    From this I think that it is not possible to discern older and younger breast feathers, which certainly complicates the id of this remarkable bird.
    Of course, ageing this bird on the other age differences in wing feathers remains undisputed.

    Reply
  5. Martin Garner

    HI Wouter and Peter

    Thanks very much for helpful comments. Agree and I was a bit hasty putting this up. All part of the cut and thrust and asking questions about strange and interesting looking birds, I think Nils and others have asked some really good questions and pointed out the many odd aspects of this one

    Cheers Martin

    Reply

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