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 behaviour 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
hypothesise 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 fuelling rates (which we assume to
increase with decreasing latitude), cost of the extra migration distance
and predation risk during fuelling. This framework may be applicable to
other cases of differential migration, especially in temperate breeding
species which exhibit long pre-egg laying periods.