THE EFFECT OF BREEDING SYNCHRONY ON EXTRA-PAIR MATING SYSTEMS IN SONGBIRDS

Extra-pair mating systems are now considered to be typical of songbird s because DNA fingerprinting has revealed high rates of extra-pair fer tilizations (EPFs) in so many species previously thought to be monogam ous. 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 E PFs are most common in species where females breed synchronously. We a rgue that for both males and females, a temporal concentration of fema le fertility (breeding synchrony) increases the net benefits of seekin g EPFs. One implication of this finding is that tropical songbirds, mo st of which breed asynchronously, are predicted to have low levels of extra-pair matings. Testes size is positively correlated with frequenc y 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 cor relation between breeding synchrony and EPFs. We believe that breeding synchrony is the most important factor promoting the evolution of ext ra-pair mating systems.