Our findings suggest that singing locally common song elements does indeed indicate a locally common genotype. This represents one of only a few demonstrated links between neutral-locus genotypes and geographical variation in song (Searcy & Nowicki 2005
), and the first such finding (to our knowledge) in a system where song varies gradually among populations (Searcy & Nowicki 1999
). If song reflects population of origin, this represents an important prerequisite for it being an isolating mechanism. Assignment tests, taking into account allele frequencies in surrounding populations, are clearly warranted.
Males producing locally common song elements were in better condition, with larger testosterone-dependent traits, fewer parasites and reduced indicators of physiological stress. We suggest several possible explanations for this finding. First, song and condition may be linked indirectly through dispersal, if each reflects population of origin. Philopatric males might outperform immigrants for several reasons, including locally good genes effects (Nottebohm 1969
; Reinhold 2004
), familiarity with the local area and/or inherent differences in quality. Second, the link between song and condition may be mediated through territorial interactions. Sharing song elements with rivals may affect the efficacy of territorial defence, and thereby condition. This possibility is supported somewhat by the nearly significant partial correlation between syllable sharing and condition, when controlling for genetic similarity. In M. m. morphna
, song matching is clearly important to territoriality (Beecher et al. 2000
). In M. m. melodia
, similarly, locally and distantly recorded songs elicit different levels of aggression from territorial males (Harris & Lemon 1974
; Searcy et al. 2002
). However, the geographical scale at which this discrimination occurs may exceed typical dispersal distances (Zink & Dittmann 1993
). Moreover, naturally occurring variation in whole and partial song sharing is not associated with territorial advantages in Pennsylvania M. m. melodia
(Hughes et al. 2007
). Third, local-sounding song may reflect song copying ability (Searcy et al. 2002
), which may in turn advertise resistance to developmental stress (Nowicki et al. 1998
). Thus, males that accurately produce local song elements may confer material and/or genetic benefits to their offspring.
Despite the potential advantages to preferring local song, to the extent that inbreeding depression occurs, females may face an important trade-off. Selecting local-sounding mates may provide direct and/or genetic benefits for their offspring, but at the same time increase inbreeding risk. The associations between song, genotype and condition suggest a clear advantage to local song preferences, but also raise questions as to the mechanisms underlying these relationships.