The migration pattern of a monogamous shorebird challenges existing hypotheses explaining the evolution of differential migration

Differential migration by sex, where one sex migrates further than the other, occurs in many bird species. How this pattern evolves is however little understood. The first aim of this study was to investigate the extent of differential migration in the common ringed plover Charadrius hiaticula, breeding in southeast Sweden, and test three main hypotheses (the social dominance, body size and arrival time hypothesis) regarding the evolution of differential migration. Geolocators were used to derive spatiotemporal data and morphometrics were collected from the studied population. Males migrated 800 km farther compared to females, were marginally larger and there was no statistical difference in spring arrival between the sexes. In contrast to other studies none of the previously proposed hypotheses could account for the observed pattern. An additional finding was that both sexes arrive up to 1.5 months before egg laying, but males initiate territorial behavior upon arrival. Based on these observations we suggest that males have a higher energetic demand, and challenges to meet those, early in the breeding season. Therefore we hypothesize that males arrive to the breeding site with residual fuel reserves accumulated at the wintering site to cover at least parts of these demands. Based on this hypothesis we present a simple model to explain the longer migration by males. The model is contingent on a trade-off between site specific fueling rates (which we assume to increase with decreasing latitude), cost of the extra migration distance and predation risk during fueling. This framework may be applicable to other cases of differential migration, especially in temperate breeding species which exhibit long pre-egg laying periods.