Extra-pair paternity in eastern bluebirds: effects of manipulated density and natural patterns of breeding synchrony |
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Authors: | Sarah L M Stewart David F Westneat Gary Ritchison |
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Institution: | 1. Department of Biological Sciences, Eastern Kentucky University, 205 Moore Building, Richmond, KY, 40475, USA 2. Department of Biology, University of Kentucky, 101 T. H. Morgan Building, Lexington, KY, 40506-0225, USA
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Abstract: | The causes of variation in rates of extra-pair paternity among avian populations remain unclear, but could include environmental
factors such as breeding density and synchrony. By experimentally manipulating nest site availability, we tested the effects
of breeding density on the frequency of extra-pair paternity in eastern bluebirds (Sialia sialis). We also examined the role of breeding synchrony on extra-pair paternity using natural timing of nests. Microsatellite analysis
revealed 34 of 305 nestlings (11.2%) were the result of extra-pair fertilizations; and 21 of 79 broods (26.6%) had at least
one extra-pair nestling. Several measures of breeding density had independent effects on extra-pair paternity. First, experimental
plot type affected extra-pair paternity, with 28 of 34 (82.4%) extra-pair young from nests in high density areas, and only
six (17.6%) from nests in low density areas. Independently of plot type, the number of breeding neighbors within a 320-m radius
was a significant predictor of the likelihood of extra-pair paternity at the nest. Extra-pair paternity was associated with
temporal factors such as absolute timing of breeding and natural levels of local breeding synchrony, but only in bivariate
comparisons. We found a positive interaction between density of neighbors within a 320-m radius and local breeding synchrony;
this term reduced the main effects of synchrony and number of neighbors, but not experimental treatment. Our results demonstrate
the importance of utilizing multiple aspects of proximity in breeding density analyses and testing for interactions between
ecological factors that can influence the behavioral events leading to extra-pair fertilizations. |
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