The leaf warblers Phylloscopus spp. are small insectivorous birds which are adapted to foraging by leaf gleaning. Three species commonly breed in the British Isles as summer visitors, namely the Chiffchaff P. collybita, the Wood Warbler P. sibilatrix, and the Willow Warbler P. trochilus. The Chiffchaff is the smallest of them (7.9 g) and its extensive range, from the Canary Islands to eastern Siberia, covers a wide diversity of habitats, and many subspecies are recognized. In the British Isles, the Chiffchaff belongs to the nominate race collybita and inhabits mainly deciduous or mixed woodlands (Rodrigues and Crick 1997).
The Chiffchaff Phylloscopus collybita and the Willow Warbler P. trochilus are two morphologically similar species which have very different song patterns. Studies have shown that these two species also have very similar ecological requirements such as habitat selection and foraging niche, but that they occupy almost mutually exclusive areas. Two different behavioural mechanisms may cause occupation of mutually exclusive areas. (1) It is adaptive for both species as a mean to reduce interspecific competition to defend territories against each other or (2) interspecific territoriality may result when individuals of one species are dominant over the individuals of another species, and thereby exclude the sub- dominant species. No indication was found among experiments to suggest that one species is dominant over the other one however. When using playback to promote an intrusion of one species in the territory of a male of the other species, the owner of the territory usually was able to evict the intruder. These experiments suggest that the species were capable of identifying each other by means of the song (Saether 1983).
Much work has been done to try to understand the function of song in territorial passerine birds, and two main hypotheses have emerged: (1) song is used for territorial defence, and (2) song is a sexually selected trait for attracting females as mates. Males sing most actively during the fertile period of their mates partly as a paternity guard strategy and partly to maximize their own extra-pair copulation success. However, in doing so, a male also announces the fertility status of his female, thus attracting neighbouring competitors, that is, other males can use the paternity guard behaviour as an indicator of female fertility and so attempt extra-pair copulation with that female at that time. It has been suggested that males would be announcing their high quality, and these high-status males would receive fewer territorial intrusions because the neighbours would recognize them as good defenders of their mates. The dawn chorus is a well-known phenomenon for several bird species, where males start to sing, sometime before sunrise and then cease or decrease song activity for the rest of the day. Song activity at dawn is linked to the fertile period of the female. Females are supposed to be fertile early in the morning and according to the male announcement hypothesis, song activity must be higher at this time because it would signal to neighbours that the singer is a good mate defender. The Chiffchaffs’ first songs of the day were heard from 30 min before sunrise. Male chiffchaffs sang significantly more at dawn than at noon, suggesting that this hypothesis is correct for the Chiffchaff (Rodrigues 1996).
Reference List
- Rodrigues.M., (1996) Song activity in the chiffchaff: territorial defence or mate guarding? Animal Behaviour, 51, 709-716.
- Rodrigues.M., & Crick.P.Q.H., (1997) The breeding biology of the Chiffchaff Phylloscopus collybita in Britain: a comparison of an intensive study with records of the BTO Nest Record Scheme. Bird Study, 44, 374-383.
- Saether.E.B., (1983) Mechanism of Interspecific Spacing out in a Territorial System of the Chiffchaff Phylloscopus collybita and the Willow Warbler P. trochilus. Ornis Scandinavica, 14, 154-160.
