One of the first things that a birder in Europe learns when getting to grips with the seperation of Willow Warbler (Phylloscopus trochilus) and Common Chiffchaff (Phylloscopus collybita) in the field is the fact that both species have very distinct songs from each other, enabling easy identification. The song of the Willow Warbler is a musical descending series of notes, whereas that of Common Chiffchaff is an almost stuttering series that is responsible for the species’ onomatopoeic common name in several languages. Indeed, these two species’ songs usually differ so much that even a relative novice can easily and accurately identify what can otherwise be rather subtle birds to the uninitiated.
Pretty much every keen birder in Europe already knows this, of course, but things aren’t always as straightforward as one might expect. Sometimes, individual Phylloscopus warblers are encountered that either switch between one song type and another (‘song-switching’) or incorporate elements of one species with those of the other (‘mixed singing’). Sometimes, birds can do both, and, should the singer remain unseen, arriving at a reasonably confident identification can be tricky.
It is believed that most ‘song switchers’, in particular, are Willow Warblers that have picked up elements of Common Chiffchaff song. In my personal experience, such birds tend to give away their likely true identity by the Willow Warbler strophes sounding typical while the Common Chiffchaff-like strophes sound a bit wrong, perhaps the rhythm is a little too fast, or the song strophes are a bit shorter than is normal for Common Chiffchaff. A recording of such a bird, which was remaining loyal to an area of willows and alders in Cork city can be found here :
While the confident exclusion of a hybrid without in-hand examination and DNA analysis isn’t possible, it does seem likely that such birds are almost always pure Willow Warblers, as stated. Any such ‘song-switcher’ that I have also managed to see has matched the phenotype of Willow Warbler, though I concede that a hybrid Willow Warbler x Common Chiffchaff could, theoretically at least, closely resemble a Willow Warbler.
Things become a little more complicated with ‘mixed singers’, as it is known that the majority of true ‘mixed singers’ within the hybrid zone between Common and Iberian Chiffchaffs are hybrids, and it is often surmised that ‘mixed singers’ in the abietinus/tristis hybrid zone are also most likely hybrids. Therefore, it may well be that at least some of the true ‘mixed singers’ with elements of Willow Warbler and Common Chiffchaff in their songs may be ‘cryptic’ hybrids, resembling one parent or the other enough in the field to be mistaken for that species. But some birds that incorporate occasional mixed song strophes tend to more frequently give typical song of one species as well, and it may well be that ‘pure’ examples of one or both species can indulge in occasional mixed singing. An example of a mixed singer can be heard here :
In life, this bird mainly gave typical Willow Warbler song strophes, but also gave what sounded like Willow Warbler song ‘corrupted’ by some Common Chiffchaff-like notes, as can be heard here.
It seems that such ‘song switching’ and ‘mixed singing’ isn’t uncommon in Willow Warblers. I have heard of ‘mixed singers’ being trapped and found to be Common Chiffchaffs, but this seems to be genuinely rarer in that species. Why this should be so is unclear, as is the frequency of hybridisation between these two species, but we, as birders and sound recordists, can help to provide some of the answers to these questions, in conjunction with other methods such as DNA analysis and biometrics, by recording any interesting song types and making an effort to see and perhaps photograph, where possible, the birds