A thrush was photographed at Pui O on 7th November by Vivian Cheung (see link above) and XinTian Yu (from Shenzhen - see below). It was initially identified from photographs as Black-throated Thrush Turdus atrogularis, but it was soon noted that it showed some red colouration in the tail, suggesting that it might be a hybrid between Black-throated Thush and Red-throated Thrush T. ruficollis. Subsequently, John Allcock drew attention to Moores (2002) and suggested that the bird might be a hybrid between T. atrogularis and Naumann’s Thush T. naumanni due to the extensive rufous mottling/spotting on the flanks. Though this last feature is reasonably apparent in the last of the three photos in the original post (below), it is more apparent in a photo submitted by Xin Tian Yu:
The closely related group of thrushes comprising Black-throated, Red-throated, Dusky and Naumann’s Thrushes (Turdus atrogularis, T. ruficollis, T. eunomus and T. naumanni) has at various times been treated as one polytypic species (e.g. Portenko 1981); and as two species, Dark-throated Thrush (containing the taxa atrogularis and ruficollis) and Dusky Thrush (containing the taxa eunomus and naumanni) (e.g. Vaurie 1959); but is now treated as four species by the International Ornithological Congress (IOC) the authority followed for the Hong Kong List. Despite this taxonomic treatment, it is recognised that hybrids are relatively frequent: Clement and Hathaway (2000) illustrated hybrids between atrogularis and ruficollis and between eunomus and naumanni, and described (from Portenko 1981) hybrids between both atrogularis and ruficollis with naumanni.
The Pui O bird is clearly a hybrid of at least two of these species, as, of this group, only atrogularis has a black(ish) throat, and atrogularis has no rufous colouration in the tail. Thus, it must be a hybrid between atrogularis and either ruficollis or naumanni (or both, as it need not be a F1 hybrid). (A hybrid with eunomus is perhaps theoretically possible but unlikely as the bird shows no features of this taxon.)
Assuming that a hybrid would most likely exhibit features of a parent and not features of another taxon, the key issue is the difference in pattern and extent of flank and undertail covert streaking and spotting between ruficollis and naumanni, with this being much more extensive in the latter taxon (Moores 2002). On this basis, this bird must be an atrogularis/naumanni hybrid: as was shown by Moores, the spotting on the flanks is too heavy and extensive for ruficollis.
Accordingly, this record has been accepted by the Records Committee as ‘a bird showing the features of a hybrid between T. naumanni and T. atrogularis’.
Acknowledgements
The Records Committee wishes to thank the original observers and all those who contributed to the discussion of this record, in particular John Allcock who first suggested that the bird was a atrogularis/naumanni hybrid.
References
Clement, P. and Hathaway, R. 2000. Thrushes. Christopher Helm, London.
Thank you for publishing this.
As well as being interesting, it gives completeness to a discussion that was first started on the website.
Now that so many records originate as photographs on this website, I hope more RC decisions can be explained here also. Author: kmike Time: 18/07/2010 23:23
Agreed!
I too would welcome feedback on the more interesting decisions.
I would add that I do not see publication of decisions as an opportunity to open a debate on the RC's decisions (or there woudl be no point in having an RC), but as an opportunity for us all to learn
Cheers
Mike K Author: cgeoff Time: 19/07/2010 15:27
Thanks for your commments.
Certainly it is our intention to post reasons for decisions on the more worthwhile records.
The RC is happy to receive informed comments or criticism if anybody feels an error has been made.
GeoffC
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