Migration is often thought to be driven by poor environmental conditions
during one season and to permit avoidance of harsh weather or resource
shortage and tracking of more favourable conditions. Here, we tested
this hypothesis in seabirds at the global scale by quantifying niche
occupancy during the breeding and non-breeding periods over multiple
marine ecoregions and exploring whether the niche dynamics reflects
changes in environmental conditions at the breeding and non-breeding
grounds. We demonstrate that migratory species exhibit more divergent
seasonal niches than resident and dispersive ones. In most cases,
migratory status was not related to unavailability of favourable
conditions at the breeding or non-breeding grounds, suggesting that
niche availability is not the main driver of migration. We hypothesize
that this unexpected pattern might arise from strong constraints imposed
on seabirds by scarcity of suitable sites breeding which constrain the
range of environments available for optimizing reproductive success.