The Effect of Breeding Synchrony On Extra-Pair Mating Systems in Songbirds

Abstract

Extra-pair mating systems are now considered to be typical of songbirds because DNA fingerprinting has revealed high rates of extra-pair fertilizations (EPFs) in so many species previously thought to be monogamous. However, there is no general framework to explain the tremendous variability among songbirds in EPF rate. Here, in a comparison of EPF rates and temporal patterns of nesting among songbirds, we show that EPFs are most common in species where females breed synchronously. We argue that for both males and females, a temporal concentration of female fertility (breeding synchrony) increases the net benefits of seeking EPFs. One implication of this finding is that tropical songbirds, most of which breed asynchronously, are predicted to have low levels of extra-pair matings. Testes size is positively correlated with frequency of EPFs, and as predicted, tropical songbirds had small testes size relative to temperate zone species. The few DNA fingerprinting studies that exist for socially monogamous tropical songbirds support the correlation between breeding synchrony and EPFs. We believe that breeding synchrony is the most important factor promoting the evolution of extra-pair mating systems.