They're everywhere

Having noticed one or two reports on the GOS sightings page of mixed singing phylloscopus warblers, and not having the time/inclination to write anything original at the moment, I thought I'd post a slightly shortened version of my note that appeared in the Gwent Bird Report 2008 regarding the aforementioned confused songsters,...

Willow Warbler singing a mixed Willow Warbler/Chiffchaff song

On 26th April 2008, whilst at the Newport Wetland Reserve, I found a Willow Warbler on the Saltmarsh Grasslands singing mixed songs including both Willow Warbler and Chiffchaff syllables. Some songs appeared to be standard Willow Warbler, however, on occasions the bird would introduce a song with a varying number of 'tsilp tsalp' Chiffchaff syllables.

The only mixed singing I had heard before was a probable Willow Warbler near Blackbushe Airport, Hampshire, on 5th June 1997 which interspersed normal Willow Warbler song with soft 'trett trett' syllables performed in a typical Chiffchaff rhythm (as often heard from Chiffchaffs before or between their normal phrases). It was surprising then that, on 5th May 2008, whilst ringing at the Uskmouth Lagoons, I again noticed a Willow Warbler singing a combination of Willow Warbler and Chiffchaff song phrases. Several other observers saw and/or heard the bird (including Richard Clarke and Chris Jones), all agreed with the identification as Willow Warbler. This bird was present in an area of scrub alongside Reedbed 10 and was relocated at the same location on 10th May, at which point I recorded a short series of five songs including that shown in the sonogram below. The bird's song consisted of between one and three phrases, which became quieter as the song progressed. The first phrase began with two or three Willow Warbler-type introductory syllables, followed by between three and eight Chiffchaff-type syllables ('tsilp tsalp' notes, delivered at a slightly faster rate and higher frequency than in normal Chiffchaff) and then by five to ten syllables forming a Willow Warbler-type descending series. The second phrase, when present, was linked to the first by an intermediate, rising syllable and consisted of another Willow Warbler-type descending series of between four and seven syllables. Then followed either between two and nine Chiffchaff-type syllables or, when a third phrase was present, another intermediate, rising syllable and then a final three syllable Willow Warbler-type descending series.

The sonagram shows a representative song made up of a first phrase of three Willow Warbler-type introductory syllables (A), four Chiffchaff-type syllables (B) and a Willow Warbler-type descending series of nine syllables (C); this is then followed by a quieter second phrase (linked with a single syllable [D] rising in pitch and joining the final 'bottom' syllable of one descending series with the starting 'top' syllable of the next) consisting of another Willow Warbler-type descending series of eight syllables (E) followed by two Chiffchaff-type syllables (F).

On 1st June Craig Constance and David Hutton relocated the bird in the same area, noticed it was ringed, and managed to read the inscription (not a bad feat given each figure in the alphanumeric code is approximately 2 mm tall). The bird had initially been ringed at the site 18th June 2005 and had been retrapped on 30th April, 24th June and 8th July 2006, 29th April 2007 and on 5th May 2008. On each occasion biometrics and the basic wing formula had been noted confirming the identification of the bird as Willow Warbler. The presence of a cloacal protuberance, on all but one occasion, also confirmed the sex as, unsurprisingly, male. The combined sightings and ringing information suggests the bird held a territory throughout the breeding season in at least 2006 and 2008.

Mixed singing is widely, if uncommonly, reported in Willow Warbler and Chiffchaff (e.g. Fray 2007; Frost 1986; Norman 1985). The songs usually consist of Chiffchaff-type syllables followed by Willow Warbler song of varying length (Cramp 1992). Whilst natural hybridisation is known to occur between the two species (e.g. Asteling Strandberg 1998; McCarthy 2006) and hybrids have been reported singing mixed song (da Prato & da Prato 1986); mixed singers have usually been reported to be Willow Warblers (e.g. Lloyd 1926; Butlin 1940; Hopkins 2006). However, the behaviour has also been reported to occur in Chiffchaff (e.g. Wilson 1986) and the first genetically determined mixed singer was a Chiffchaff (Bensch et al. 2001). Hybridisation does not appear to explain the existence of most mixed singing birds, it is more likely that this behaviour is the result of anomalies in the song learning process.

References
Asteling, R. & Strandberg, A. (1998). Polygyny, hybridisation, double brooding, a case of site fidelity and time of laying of the Chiffchaff Phylloscopus collybita in southern Sweden. Ornis Svecica, 8: 125-128.
Bensch, S., Nilsson, L.G.R., Nothagen, P., Olsson, P. & Akesson, M. (2001). A Chiffchaff Phylloscopus c. collybita with mixed Chiffchaff and Willow Warbler Ph. trochilus song - genetic evidence. Ornis Svecica, 11: 108-111.
Butlin, S.M. (1940). Willow-Warbler with Chiffchaff-like notes in song. British Birds, 34: 65-66.
Cramp, S. (ed.) (1992). The Birds of the Western Palearctic Vol. VI. Oxford University Press, Oxford.
da Prato, S.R.D. & da Prato, E.S. (1986). Appearance and song of possible Chiffchaff x Willow Warbler hybrid. British Birds, 79: 341-342.
Fray, R. (2007). Mixed singing in Phylloscopus warblers. British Birds, 100: 307.
Frost, R.A. (1986). Phylloscopus warbler with songs of Chiffchaff and Willow Warbler. British Birds, 79: 340-341.
Hophins, G.R. (2006). Unusual song of Willow Warbler. British Birds, 99: 576-582.
Lloyd, B. (1926). Abnormal song of Willow Warbler. British Birds, 20: 153-154.
McCarthy E.M. (2006). Handbook of Avian Hybrids of the World. Oxford University Press, New York.
Norman, D. (1985). Possible hybrid Chiffchaff/Willow Warbler. Cheshire Bird Report, 1984: 89.
Wilson, C. (1986). Chiffchaff with songs of Chiffchaff and Willow Warbler. British Birds, 79: 342.

PS. The mixed singer reported from Goldcliff Pools on 28th March 2011 may well be a returning bird; there was a Willow Warbler near the car-parking area on 9th May 2010 whose song consisted of a Willow Warbler descending phrase followed by 'tsilp-tsilp-tsalp-tsilp...'
PPS. I have a sneaking suspician it isn't that rare at all.