Delayed song maturation and territorial aggression in a songbird.

Asymmetries in competitive ability can determine the outcome of social interactions in animals and are often expressed through differences in sexual traits. Competitive ability (resource holding potential, RHP), trait expression and ultimately reproductive success may vary with an individual's age or experience. In some species, reproductively mature males delay acquisition of some adult traits and thereby signal their young age. Theory on animal contests predicts that individuals assess the RHP of an opponent relative to their own, such that escalation is more common between evenly matched opponents. Here, we test predictions from this hypothesis that males respond to a territorial intruder based on their RHP relative to the intruder's RHP. We simulated white-crowned sparrows (Zonotrichia leucophrys) intruding into the territory of a recruit or return. Playback of a song repertoire simulating a young male (recruit) elicited a weaker response from established territory holders (return), but a stronger response from recruits. Playback of a single song type simulating an older male elicited the opposite responses. This indicates that males distinguished between simulated young and old intruders based on song, and responded differently depending on their own experience. Our study highlights the possibility that receiver as well as sender traits should be considered when interpreting animal interactions.

Twenty-one novel microsatellite DNA loci isolated from the Puget Sound white-crowned sparrow, Zonotrichia leucophrys pugetensis.

The white-crowned sparrow, Zonotrichia leucophrys, has served as a model species for studies of song and reproductive physiology. Here, we describe primers for 21 novel microsatellite loci isolated from the Puget Sound subspecies, Zonotrichia leucophrys pugetensis, which will be useful for parentage and population genetic analyses. Based on genotypes from seven to 22 adult birds from one population, the average number of alleles per locus was 10.9 (four to 21 alleles) and observed heterozygosity varied from 0.50 to 1.00. All loci also amplified products in at least one of three other passerine species tested.