First proof that it's the nominate subspecies of Hume's Leaf Warbler that reaches Europe.
Aerobics, the final countdown and warbler taxonomy
Back in the 80s, Hume's Leaf Warbler was still considered a ssp. of Yellow-browed Warbler. Since then not only our hair fashion and taste in music has improved, but also our insights in leaf warbler taxonomy. There are significant genetic, morphological and acoustic differences between the two and acoustic experiments in overlapping breeding ranges show that these species basically ignore each other (Irwin et al 2001). So: different species. No discussion.
But what about Hume's Warbler ssp?
But Hume's Leaf Warbler has two ssp: humei and mandellii. These two ssp. also differ genetically, morphologically and acoustically. Whether you call them species or subspecies, depends a bit on the species concept you prefer (Irwin 2001). The nominate breeds in mountains in the northern part of the Indian Subcontinent, Central Asia, S Russia and Mongolia, while Mandelli's Leaf Warbler has an isolated range in C China, 1500 km away from the nearest breeding Hume's. Hume's winters around the Indian Subcontinent, Mandelli's in SE Asia. Mandelli's is somewhat browner, with a greyer cap, it usually has no upper wingbar and it often has some yellow in the eyebrow and on the underparts (Clement 2017). The genetic distance between the two ssp. is much smaller than between Hume's and Yellow-browed. Hume's and Mandelli's have two song types: a "double note" and something vaguely reminiscent of a squeaky V-belt. The double note slightly differs since it's a bit higher pitched in Mandelli's. Mandelli's als has a higher pitched call. Acoustic experiments however show that Hume's and Mandelli's respond to each other's vocalisations (Irwin et al 2001).
The acoustic experiments are enough reason to regard them as ssp. and as far as I know not a single world list has split them. It's likely that they are in the process of becoming separate species, but are just not there yet.
Figure 2. Genetic variation between Yellow-browed, Hume's Leaf and Mandelli's Leaf Warbler, including the bird ringed in Meijendel.
So what about the vagrants in Europe?
Due to their respective ranges, it's of course the nominate ssp. that most likely shows up in Europe. Both the plumages and calls of Dutch birds I checked also imply that. But I think it has never really been sorted out. So Peter de Knijff analysed some feathers I collected of the bird I trapped on 7 November 2017 in Meijendel, Wassenaar, the Netherlands (the bird on top of this post). The analysis proofs that, as expected, it was indeed a humei. This is the first genetic proof of this subspecies in Europe (Martin Collinson in litt). See figure 2 (made by Peter de Knijff).
No surprises here, but at least we have now established with certainty that humei is the default taxon in Europe.
More details will be published in a Hume's Leaf Warbler overview in Dutch Birding magazine later this year.
Martin Collinson confirmed no other humei has been genetically tested in Europe before. Peter de Knijff is once more thanked for his genetic analyses. Peter made figure 2.
Irwin, D E , P Alström, U Olsson & Z M Benowitz-Fredericks 2001. Cryptic species in the genus Phylloscopus (Old World leaf warblers). Ibis 143: 233-247
Clement, P 2017. Hume's Leaf-warbler (Phylloscopus humei). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona.