Geographical variation in birdsong is taxonomically widespread and behaviourally salient, with females often preferring local over non-local song. However, the benefits associated with this preference remain poorly understood. One potential explanation is that song may reflect a male's place of origin and thus allow females to obtain genes well adapted to the local environment. We studied naturally occurring variation in the degree to which the elements of a male's song repertoire matched those of the local population (‘syllable sharing’) in migratory song sparrows (Melospiza melodia melodia). Syllable sharing was correlated with genetic similarity to the local population, suggesting that song reflects population of origin. Males sharing more syllables also had larger testosterone-dependent traits, fewer blood-borne parasites and reduced indicators of stress. Our findings are consistent with locally good genes models. Alternatively, immigrants' condition may suffer due to unfamiliarity with the breeding site or inability to match song elements during territorial interactions. Females preferring ‘local-sounding’ males may thus obtain genetic and/or direct benefits for their offspring.
We studied song, genetic similarity and condition in song sparrows breeding near Newboro, Ontario. The study site supports 30–40 breeding pairs, and is not physically isolated from other surrounding populations. Song sparrows in this area are migratory, but philopatry is high. Almost half of the breeding adults (presumably all those surviving) return to the site the following year. Each year, 5–15% of new recruits have been banded on the study site as nestlings, providing a minimum estimate of natal philopatry. This suggests the potential for considerable population genetic structuring (but see Zink & Dittmann 1993).
We captured 29 adult male song sparrows in mist nets or Potter traps, from April to June 2006. Each subject received a unique combination of coloured leg bands to permit field identification. All subjects defended territories and bred (or attempted to breed) at the study site.
Song recording and analysis
We recorded the song repertoires of 19 subjects in spring 2006. For the remaining 10 subjects, repertoires were recorded in previous years. Adult song sparrows do not alter their repertoires (Nordby et al. 2002), so we were confident that these repertoires remained unchanged. We considered a repertoire sampled in full after recording 300 consecutive or 450 non-consecutive songs (Pfaff et al. 2007).
Whole song matching is very rare at our site (in 2006, only one song type was shared), as in other M. m. melodia populations (Harris & Lemon 1972; Hughes et al. 1998). However, males often share song elements with their neighbours. Partial song matching is used in territorial interactions (Anderson et al. 2005), suggesting that alternative forms of song matching exist (Burt et al. 2002). We estimated song similarity based on the proportion of shared syllables in a male's repertoire (‘syllable sharing’). We generated spectrograms in Syrinx (J. Burt; www.syrinxpc.com), classified song types by visual inspection and generated a catalogue of syllables across all song types. Similar to Podos et al. (1992), we defined a syllable as one or more traces on the spectrogram which always occurred together. Shared syllables were identified by two independent observers' consensus. We identified a total of 135 syllables (19–48 per male), and screened each male's repertoire for the presence of each syllable. We generated a pairwise matrix of syllable sharing, using Jaccard's similarity coefficient adjusted for differences in repertoire size (Tracy & Baker 1999), calculated as
where c is the number of syllables common to both birds' repertoires; a is the syllables in A's repertoire but not in B's; b is the syllables in B's repertoire but not in A's; and d is the difference between A's and B's syllable repertoires. For each male, we calculated the average Sj with all other subjects, as an index of syllable sharing with the local population.