Behavioral, demographic, and environmental correlates of extrapair fertilizations in eastern bluebirds, Sialia sialis

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Abstract
Extrapair fertilizations occur in eastern bluebirds, Sialia sialis, accounting for as much as 35% of nestlings. To test the assumption that females are passive participants in extrapair fertilizations and to supplement existing information about the costs and benefits of extrapair fertilization for males, we examined behavioral variation during females' fertile periods and incubation, demographic variation including the ages of care givers, duration of consortship, the sexes of offspring, and the number of cavities within territories for correlation with the likelihood of caring for nondirectly descendant nestlings (NDDN). We use a method of estimating the frequency of NDDN that eliminates “the allele problem,” a possible explanation for differences between individuals detected and those not detected caring for NDDN. Our method controls for the possibility that observed differences in caring for NDDN between, say, individuals of different ages are due to variations within age groups in the frequencies of rare alleles that allow detection of nonkin. The frequency of NDDN was not significantly greater in one-cavity territories than in two. It was greater for males in their first breeding season than for older males. Pairs nesting together for longer times had lower frequencies of NDDN than pairs breeding together for the first time. The frequency of caring for NDDN was greater for males paired with females who were off territory more during their fertile periods than for males paired with females that seldom left their territories during egg laying. Males that stayed closer to fertile females had significantly higher frequencies of NDDN in their nests, rather than lower frequencies. Males that stayed with fertile females longer also had significantly higher frequencies of NDDN in their nests. NDDN was greater for males who were more likely to be off territory during incubation than for males that were more often on territory. We conclude that female eastern bluebirds are not passive participants in extrapair fertilizations. [Behav Ecol 1991; 2:339–350]

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