Behavioral Ecology and Sociobiology

, Volume 64, Issue 3, pp 463–473 | Cite as

Extra-pair paternity in eastern bluebirds: effects of manipulated density and natural patterns of breeding synchrony

  • Sarah L. M. Stewart
  • David F. Westneat
  • Gary Ritchison
Original Paper


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.


Breeding ecology Mating system Nest box density Extra-pair copulation Sialia sialis 



We thank the University of Kentucky for use of laboratory facilities for paternity analysis, Jackie Bennett, Emily Clemons, and Barb Kieffer for help in the field, and Diana Torres and Ian Stewart for help in the lab; all of these individuals contributed significantly to the success of this project. We also thank Jackie Augustine, Radovan Vaclav, and two anonymous reviewers for providing helpful comments on previous versions of the manuscript.

Ethical standards

The authors declare that all experiments comply with current United States laws.

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

265_2009_862_MOESM1_ESM.pdf (43 kb)
Table S1 Breeding density and extra-pair paternity in individual high and low density plots on the Blue Grass Army Depot for eastern bluebirds. All means are represented ± standard error, EPY extra-pair young. (PDF 42 kb)


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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Sarah L. M. Stewart
    • 1
    • 2
  • David F. Westneat
    • 2
  • Gary Ritchison
    • 1
  1. 1.Department of Biological SciencesEastern Kentucky UniversityRichmondUSA
  2. 2.Department of BiologyUniversity of KentuckyLexingtonUSA

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