Female eavesdropping on male song contests in songbirds.

Male song reflects the quality of the singer in many animals and plays a role in female choice of social and copulation partners. Eavesdropping on male-male vocal interactions is a means by which females can compare different males’ singing behavior directly and make immediate comparisons between potential partners on the basis of their relative vocal performance (1, 2). Using an interactive playback experiment followed by microsatellite paternity analysis, we investigated whether female black-capped chickadees (Poecile atricapilla) base their reproductive decisions on information gained through eavesdropping. Black-capped chickadees are socially monogamous songbirds that follow a mixed reproductive strategy in which one-third of broods include young that are not related to their social father (3). From 1999 to 2001, we assessed dominance ranks in a free-living population of chickadees at Queen’s University Biological Station, Canada, to predict which males were likely to be sought for extrapair copulations (high-ranking males) and which males were likely to lose paternity within their nests (lowranking males) (3, 4). At the start of the breeding season, when male-male song contests are common and females actively solicit copulations, we used interactive song playback to engage territorial male chickadees in countersinging interactions with a simulated intruder (5). We performed 6.0-min playback trials to dyads of neighboring high-ranking and lowranking males from the same winter flock. In control treatments, we mimicked natural territorial encounters; we simulated an intruder that sang submissively (Fig. 1A) with the high-ranking playback subject and sang aggressively (Fig. 1B) with the low-ranking neighbor. In experimental treatments, we attempted to alter eavesdropping females’ perceptions of their social mates; we simulated an intruder that sang aggressively with the high-ranking playback subject and sang submissively with the low-ranking neighbor. To test whether interactive playback altered the normal pattern of paternity in the nests of subject males, we conducted paternity analysis on blood samples collected from offspring (6). High-ranking males that lost song contests with a simulated intruder lost paternity in their nests (Fig. 1C); high-ranking males that received playback simulating an aggressive intruder showed a significantly greater level of paternity loss than high-ranking males that received playback simulating a submissive intruder (control I; Fisher’s exact test, P 5 0.05) and a significantly greater level than a control group of high-ranking males that received no playback (control II; P 5 0.05). As predicted (3, 4), we observed little