Interspecific cross-fostering of great tits ( major) by blue tits (Cyanistes caeruleus) affects inter- and intraspecific communication

Bo Terning Hansen1), Lars Erik Johannessen & Tore Slagsvold (University of Oslo, Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), P.O. Box 1066, 0316 Oslo, Norway)

(Accepted: 1 November 2009)

Summary song is used to attract mates and deter rivals, and also functions as a species recognition cue. It is a flexible trait affected by learning, hence the choice of song tutors may affect an individuals’ singing. By interspecifically cross-fostering great tits (Parus major) to blue tits (Cyanistes caeruleus) in the wild, we have manipulated the species recognition of great tits, which in turn has influenced their song. In the present study, we presented breeding great and blue tit control males with playback of the aberrant song of cross-fostered males and playback of the normal song of control great tit males. Blue tit males responded more to cross-fostered than control great tit song while great tit males showed the inverse response pattern. This shows that interspecific cross-fostering may affect both inter- and intraspecific communication. However, the response of males of both species towards the song of cross- fostered great tit males was not of the same magnitude as the response towards ordinary blue tit song; thus, the real species identity is to some extent maintained in the aberrant song of cross-fostered males.

Keywords: cross-fostering, imprinting, song learning, vocal mimicry, species recognition.

Introduction Bird song is strikingly species-specific and serves an important function in avian species recognition (Emlen, 1972; Becker, 1982). Most respond only to the song of conspecifics, although some species also respond to

1) Corresponding author’s e-mail address: [email protected] © Koninklijke Brill NV, Leiden, 2010 Behaviour 147, 413-424 DOI:10.1163/000579509X12578482434926 Also available online - www.brill.nl/beh 414 Hansen, Johannessen & Slagsvold heterospecific song (Catchpole & Slater, 1995). Species pairs that exist in both sympatric and allopatric populations may differ in their response to het- erospecific song (Gill & Murray, 1972; Emlen et al., 1975; Catchpole, 1978) showing that exposure to congeneric species may affect song perception, possibly through the experience of interspecific competition. Some birds also incorporate heterospecific elements into their song. Mixed singing of conspe- cific and heterospecific song has been documented in a variety of species, and its function is debated (reviewed in Dobkin, 1979; Baylis, 1982; Helb et al., 1985). First, imitation of heterospecific song (vocal mimicry) could function in intraspecific communication, for instance if the singer is favoured by con- specific females because of an increased repertoire size. Second, it may func- tion in communication between species that exhibit interspecific territorial- ity. Mimicking the song of the heterospecific competitor may, thus, increase the efficiency of the interspecific communication. Third, vocal mimicry may simply be nonfunctional, for instance resulting from an erroneous choice of song tutor. Great tits and blue tits are related species (Gill et al., 2005) that occupy the same habitat, have overlapping territories during the breeding season and compete for nest holes and food (Hinde, 1952; Perrins, 1979; Minot, 1981). We have over several years cross-fostered great tits to blue tits (and vice versa), and this treatment has influenced the species recognition of cross-fostered individuals of both species (Slagsvold et al., 2002; Hansen & Slagsvold, 2003, 2004). Further, it has affected their song learning. Great tits, in particular, are capable of producing song of their foster species and most cross-fostered great tit males end up as mixed singers, uttering both great tit and blue tit song (Johannessen et al., 2006). Naturally occurring vo- cal mimicry of blue tits by great tits has been reported in other populations (Gorissen et al., 2006), hence interspecific cross-fostering need not be a pre- requisite for vocal mimicry among these species. We have previously shown that both great tit and blue tit control males respond far more towards play- back of conspecific control song than heterospecific control song (Hansen & Slagsvold, 2003). Hence, even though there is interspecific competition for resources between great tits and blue tits, the information contained in the ordinary song of both species largely seems to be for intraspecific communi- cation (but see Doutrelant et al., 2000). In the present study we want to investigate how the aberrant song of cross- fostered great tit males is perceived by conspecific and heterospecific males.