Chiffchaffs and What Are 'Old World Warblers' Anyway?

The wooded places of Europe, Asia and northern Africa are home to a warbler with a distinctive song, a song that explains its English common name…

I still simply haven’t had time to finish anything new, so here – once again – is an article from the archives. It originally appeared at TetZoo ver 3 in August 2014; the full version is viewable here at SciAm (I’m linking to a wayback version because the original is paywalled to me).

Caption: Common chiffchaff, co-operatively posing in profile. Image: Darren Naish.

This is a Common chiffchaff Phylloscopus collybita, a familiar, ubiquitous leaf warbler here in western Europe. I photographed this particular bird in Carmarthenshire, Wales, and you wouldn't believe the time and effort it took to get the images you see here (or, if you photograph birds yourself, maybe you would). Leaf warblers (the Phylloscopus species) are slim-billed insectivores, greenish-greyish above and whitish below. There have been occasional efforts to recognise new genera or ‘subgenera’ within Phylloscopus (namely Cryptigata, Seicercus and Acanthopneuste), the result being radical non-monophyly and more confusion than ever (Olsson et al. 2005).

Caption: PASSERINES, the most diverse, more successful dinosaurian group of them all. Here are some (not all) of the many passerines I’ve drawn for my giant in-prep textbook project (which you can support at patreon). Image: Darren Naish.

Where do leaf warblers and kin belong in passerine phylogeny? They’re uncontroversially part of Sylvioidea, the major passeridan passerine clade that includes all Old World warblers as well as bulbuls, cisticolas, white-eyes, Old World babblers and numerous similar birds (Passerida = the clade that contains all sparrow-like, finch-like and warbler-like passerines). However, the idea that all Old World warblers should be classified together is not supported by molecular data. Convention would have it that leaf warblers belong to Sylviidae – the 'Old World warbler family' – but the fact that recent phylogenies find Sylvia warblers to be closer to Old World babblers, white-eyes, cisticolids and so on means that this is very wrong, since leaf warblers and kin are clearly not part of this clade (Beresford et al. 2005, Alström et al. 2006, Johansson et al. 2008). The best way to classify leaf warblers may, actually, be to give them their own ‘family’ – Phylloscopidae – that’s close to (but distinct from) Cettidae and related sylvioid clades (Alström et al. 2006, Johansson et al. 2008).

Caption: these Old World warbler taxa are always placed together in field guides. But – shock horror – actually represent three phylogenetically distinct radiations, well separated by tropical taxa. From left: Hippolais (this is H. icterina), Acrocephalus (this is A. palustris) and Locustella (this is L. fluviatilis). Images: Vogelartinfo; Dirk-Jan Kraan; Lasse Olsson; all CC BY SA 3.0 and 2.5.

As surprising and radical as it might seem, other Old World warblers conventionally discussed alongside leaf warblers – namely, the Locustella, Acrocephalus and Hippolais warblers – aren’t part of the same group as Phylloscopidae, Cettidae and so on, but are instead part of that bulbul-babbler-cisticola clade (Alström et al. 2006, Johansson et al. 2008). Remember this when you open your field guide and see all those warbler-type birds arranged together on the same few pages: they actually belong to at least three distinct evolutionary radiations, are well separated in the sylvioid tree (except for Acrocephalus and Hippolias, which really are close relatives), and are surrounded in the phylogeny by tropical African and Asian taxa.

Caption: just to help remind you of the current approximate shape of passerine phylogeny, here’s this. It’s a little dated (I last modified it in 2019) but I haven’t had time to update it. Anyway, the point is that this article is devoted to animals included within Sylvioidea. Image: Darren Naish.

Chiffchaffs are famously similar to Willow warblers Ph. trochilus in appearance (but not in voice: the name 'chiffchaff' is semi-onomatopoeic). Some studies indicate that both are essentially sister-taxa, only the Plain leaf warbler Ph. neglectus of central Asia being ‘intermediate’ between the two (it seems to be closer to chiffchaffs than to the Willow warbler) (Badyaev & Leaf 1997, Helbig et al. 1996, Olsson et al. 2005). Of course, this picture becomes more complex if various of the traditional chiffchaff ‘subspecies’ are recognised as worthy of species rank, like the Iberian chiffchaff Ph. brehmii, Canary Islands chiffchaff Ph. canariensis, Eastern or Mountain chiffchaff Ph. sindianus and Siberian chiffchaff Ph. tristis. Yes, as is so often the case, ‘the Chiffchaff’ is a polytypic entity that encompasses substantial diversity, with some of its ‘subspecies’ being 'more distinct' than many universally recognised ‘species’ in other bird groups, and easily differentiable based on morphology, acoustics, hybridisation patterns and DNA (Helbig et al. 1996).

Caption: Common chiffchaff (at left) compared with Willow warbler: note darker legs in chiffchaff. the other differences are less easy to appreciate due to the very different lighting conditions of these images... birds often don’t look exactly as they do in books. Images: Andreas Trepte (CC BY-SA 2.5); © Rob Bendall/Highfields.

Regardless, Common chiffchaff and Willow warblers are known to hybridise in places (the hybrids have a ‘hybrid song’, featuring song phrases of both parent species) (da Prato 1993). Despite the stereotype about them being similar except for voice, they’re actually not that difficult to distinguish if you know what to look for: Willow warblers are larger with a shallower, less domed head, slightly longer primary projection, and they’re 'brighter', with light brownish/pinkish legs (as opposed to darker grey legs) and have a more washed-out look overall. The habitat preferences of the two are also slightly different. Chiffchaffs are generally restricted to areas where there are tall deciduous trees whereas Willow warblers are more associated with woodland edges, scrubland and clearings. However, the two must be pretty similar in ecological terms since some studies indicate that, when one of the two species occupies a habitat, the other one is discouraged from doing so (Saether 1983). Willow warblers are somewhat larger so tend to have the advantage if they occupy a habitat first (Saether 1983).

Caption: I’ll just leave this here. I haven’t yet seen this book — hopefully I will soon!

This article was written as a quick summary, the original plan being that I was going to use it to showcase some of my Welsh chiffchaff photos. It goes without saying that a huge amount of research has been done on leaf warbler hybridisation, distribution, ecology, foraging behaviour, systematics and phylogeny, and you can all help to make this article (and its comments) fantastically complete by saying smart and brilliant things in the comments below. So, over to you...

For previous TetZoo articles on passerine birds, see…

If you enjoyed this article and want to see me do more, more often, please consider supporting me at patreon. The more funding I receive, the more time I’m able to devote to producing material for TetZoo and the more productive I can be on those long-overdue book projects. Thanks!
Refs - -

Alström, P., Ericson, P. G. P., Olsson, U. & Sundberg, P. 2006. Phylogeny and classification of the avian superfamily Sylvioidea. Molecular Phylogenetics and Evolution 38, 381-397.

Badyaev, A. V. & Leaf, E. S. 1997. Habitat associations of song characteristics in Phylloscopus and Hippolais warblers. The Auk 114, 40-46.

Beresford, P., Barker, F. K., Ryan, P. G. & Crowe, T. M. 2005. African endemics span the tree of songbirds (Passeri): molecular systematics of several evolutionary ‘enigmas’. Proceedings of the Royal Society of London B 272, 849-858.

da Prato, S. R. D. 1993. Chiffchaff. In Gibbons, D. W., Reid, J. B. & Chapman, R. A. (eds) The New Atlas of Breeding Birds. T & A D Poyser, London, pp. 348-349.

Fuchs, J., Fjelds, J., Bowie, R. C. K., Voelker, G. & Pasquet, E. 2006. The African warbler genus Hyliota as a lost lineage in the oscine songbird tree: molecular support for an African origin of the Passerida. Molecular Phylogenetics and Evolution 39, 186-197.

Helbig, A. J., Martens, J., Henning, F., Schottler, B., Siebold, I & Wink, M. 1996. Phylogeny and species limits in the Palaeoarctic Chiffchaff Phylloscopus collybita complex: mitochondrial genetic differentiation and bioacoustic evidence. Ibis 138, 650-666.

Johansson, U. S., Fjelds, J. & Bowie, R. C. K. 2008. Phylogenetic relationships within Passerida (Aves: Passeriformes): a review and a new molecular phylogeny based on three nuclear intron markers. Molecular Phylogenetics and Evolution 48, 858-876.

Olsson, U., Alström, P., Ericson, P. G. P. & Sundberg, P. 2005. Non-monophyletic taxa and cryptic species – Evidence from a molecular phylogeny of leaf-warblers (Phylloscopus, Aves). Molecular Phylogenetics and Evolution 36, 261-276.

Saether, B. 1963. Mechanisms of interspecific spacing out in a territorial system of the Chiffchaff, Phylloscopus collybita, and the Willow warbler, Phylloscopus trochilus. Ornis Scan 14, 154-160.