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Friday, 27 April 2012

More on IBERIAN CHIFFCHAFFS

I have received a very long essay from my friend and expert Spanish birder Dani Velascuo Lopez. He knows Iberian Chiffchaffs very well and has large numbers of them breeding near his home. He states that the most critical constituent is the CALL, which sadly I have not heard from the Castor Hanglands bird. He states that if this diagnostic call is not uttered by the bird, it can be instantly dismissed as not being a pure ibericus. In his experience, Iberian Chiffchaff never produces anything other than classic calls, although he acknowledges a hybrid could produce either (nobody knows the outcome of that one).


I shall summarise a few other salient points he highlights...........

Regarding the contact zone with colybitta, it is a falacy that Iberian and Common Chiffchaffs only overlap in a narrow area in the western Pyrenrees as claimed in previous peer-reviewed papers on Iberian Chiffchaff taxonomy and occurrence. COMMON CHIFFCHAFF is a widespread, though usually uncommon (being common only in localised areas), breeding species in the lowlands of Asturias and Cantabria regions of North Central Spain, thus overlapping with ibericus on an almost 300 kilometre stretch of the Cantabric coastline. Within that overlap zone, ibericus tends to be much commoner, but most importantly, both species can be found breeding at the same locations.

Dani sees many hundreds of singing ibericus in an average season in his home region so I feel that he is best placed to make informed comment on the species. The vast majority of ibericus in this region sing more or less typical ibericus song, which as I have explained previously, is quite straightforward to hear. There is quite a lot of variation though within the population, although songs will always follow the same general structuire. Almost all of the songs uttered differ slightly from each other resulting in individual singers producing constantly slightly differing versions of the song, with some easily recognisable variation between them. Sometimes, they can lack the typical ibericus trill at the end, although sooner or later, the typical trill will eventually be added. Thus, it is important to hear a single bird sing over an extended period. The somewhat Common Chiffchaff-like abstract at the beginning of ibericus song is highly variable but again, even if uttered alone, and without its trill, is still easily identifiable from collybita, being amongst other things, distinctively shorter in length and more irregular in rhythm. Those individual birds producing versions without the final trill still NEVER produce longer versions of perfect Common Chiffchaff songs and the majority will after a while sing typical ibericus songs. Summarising once more, these variant singers will still not sound like typical collybita.

In this region of overlap in northern Spain, birds singing both perfect collybita and ibericus songs are extremely rare and impossible without trapping to know wxactly what they are. Some individuals could be pure ibericus that have learnt to perfectly mimic collybita song. Others might be hybrids, as it appears that hybrids might be prone to learn both songs. Any form of mixed singing by both species of chiffchaff is rare in northern Spain so why so many are appearing further north in NW Europe is unexplainable, but perhaps the theory that hybrids are more likely to migrate could be the reason.

Regarding the physical appearance of the Castor bird, Dani states that the bird is not the most typical but could fit within the variation of that species. A small proportion of ibericus do look extremely similar in the field to some brighter collybita and are almost impossible to identify without hearing them. The strong yellow tones on the fore of the supercilium is a strong pointer towards ibericus, though the legs are noticeably dark. The yellow wash to the undertail coverts are not as bright as you would expect of an Iberian and it lacks in depth, the green component that Iberian has in the upperparts. It is very dirty white below too, which is not a good sign. Primary projection too seems slight, though the bill does look quite long, often an Iberian feature.