Is geographical variation in the size of Australian shorebirds consistent with hypotheses on differential migration?

In differential migrants the members of different age-classes or sex travel to geographically separate non-breeding areas. Here, we test five competing hypotheses explaining differential migration using more than 40 000 records of 22 species of shorebirds (Charadriiformes) occurring at two non-breeding areas at different distance from the breeding grounds and that also differ in climate. We showed that across species, the larger sex was more abundant in south-eastern than in north-western Australia. Size, as indicated by wing-length, was greater in the south-east than in the north-west for both males and females, whereas bill-length showed the opposite pattern. Based on these trends we conclude that the interaction between ambient temperature, body-size and bill-length determines the geographical distribution of shorebirds wintering in Australia. Our findings are not consistent with the resource partitioning, dominance and arrival time hypotheses. This is the first study that disassociates overlapping predictions of competing hypotheses on differential migration, thus contributing to our understanding of the evolution of differential migration in birds.