Reproductive asynchrony and population divergence between two tropical bird populations

High-latitude vertebrates generally breed seasonally and synchronously as the primary environmental cue used to time seasonal processes is photoperiod. Investigations of tropical vertebrates have also documented seasonal reproduction, but it is unclear how synchronous reproduction is, both within and between populations. In this study, we investigated whether seasonal reproduction in a tropical species is synchronous between two populations in close proximity and, if not, whether asynchrony is correlated with genetic and cultural differentiation. We describe two equatorial populations of rufous-collared sparrows (Zonotrichia capensis), at the same latitude and separated by 25 km, that each breed seasonally but out of phase with each other. This asynchronous reproductive phenology is associated with local weather patterns and is independent of photoperiod. At a finer scale, reproductive timing is more highly synchronized within monogamous pairs than within the population as a whole. Associated with the difference in reproductive phenologies, we document that males in each population sing different song dialects. Using microsatellite DNA analysis, we found limited gene flow and significant genetic differentiation between the two populations. From these results we hypothesize that cultural and genetic differentiation between populations, which can be greater in tropical populations than temperate ones, can be associated with locally adapted reproductive phenologies.