Migratory protogyny and condition-dependent arrival in Icelandic Red-necked Phalaropes

In many migratory birds, arrival at breeding grounds is sexually asynchronous. This pattern is often explained by the idea that early arrival benefits the sex under stronger sexual selection by improving access to mates through territoriality. However, in species where females arrive first and are non-territorial, this explanation falls short. One alternative is that early-arriving females are simply in better condition. Here we report, for the first time, evidence of migratory protogyny in the Icelandic population of Red-necked Phalaropes (Phalaropus lobatus), a shorebird with reversed sexual dimorphism. We also present the first direct test of the condition-dependent hypothesis in a protogynous species, using arrival data, morphometric measurements, and leukocyte profiles. Although females arrived significantly earlier than males, we found no evidence that early arrival was linked to superior body condition. Leukocyte profiles did not suggest marked sex differences either, although we had insufficient statistical power to test this. Our results point instead to the role of sex-specific migratory strategies and intrasexual competition in shaping arrival timing. With its unique life history, P. lobatus offers a powerful model for studying the evolution of sex-biased migration. We encourage further research integrating fine-scale tracking, reproductive monitoring, and physiological data to better understand the ecological and evolutionary drivers of migratory protogyny.