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Abstract

Female dark-eyed juncos (Junco hyemalis) are socially monogamous, but they engage in extra-pair copulations (EPCs). We examined spatial activity and behavior of female juncos during their fertile period to determine whether they engaged in tactics likely to facilitate EPCs and whether any such tactics varied with the attractiveness of their social mates. We manipulated the attractiveness of social mates by implanting experimental males with tubes containing testosterone (T-males) and control males with empty tubes (C-males). Previous findings in free-living juncos showed that females mated to C-males were more likely to produce extra-pair young than females mated to T-males. We radio-tracked 13 females (eight C-mated, five T-mated) for an average of 15 h each over 3 d during their fertile periods. We predicted that C-mated females, to compensate for the induced relative unattractiveness of their social mates, would foray from their territories to seek EPCs and as a result would have larger home ranges than T-mated females. Females of both treatment groups made extra-territorial forays, some of considerable distances, but we observed no EPCs during forays. Further, neighboring T- and C-males frequently made incursions into the home ranges of T- and C-mated females but we saw no EPCs during these incursions. Our ability to detect statistical differences was limited by sample size, but given that constraint, we found no detectable difference in female home-range size in relation to the treatment of their mates, nor did other female behavior differ according to male treatment. Male behavior was significantly affected by testosterone treatment. C-males guarded their mates more closely than did T-males. We conclude that female juncos make extra-territorial movements during their fertile period without regard to male attractiveness (testosterone treatment), but we found no evidence that these function as a special tactic to gain EPCs.