No evidence that seasonal changes in large-scale environmental conditions drive migration in seabirds

Seasonal variability is one of the main drivers of seasonal movements like migration. The literature has suggested that bird migration is often driven by poor environmental conditions during one season and permits avoidance of resource shortage or harsh weather by tracking the more favourable conditions. We tested at the global scale, and focusing on seabirds, whether this pattern exists in the marine realm. Specifically, we tested the hypothesis that seabird migration permits achieving stability in niche occupancy, and that it is triggered by seasonal variations in niche availability. We collated data on monthly presence of species over marine ecoregions from literature and expert knowledge. First, we quantified niche occupancy during breeding and non-breeding periods from environmental conditions encountered in ecoregions in which species were present at each periods and compared seasonal dynamics across migratory strategies. Second, we quantified the seasonal niche dynamics from simulated residency in breeding and non-breeding grounds to quantify the seasonality in niche availability and to test its effect on seabird migratory strategies. We demonstrated that all seabirds are niche trackers, yet resident and dispersive seabirds displayed higher levels of niche tracking throughout the year, regardless of the environmental seasonality, while migrants exhibited more divergent seasonal niches. In most cases, migratory status was not related to the unavailability of favourable conditions at the breeding or non-breeding grounds, suggesting that the availability of the favourable niche is not the main driver of migration. We hypothesise that this unexpected pattern might arise from strong constraints imposed on seabirds by the scarcity of suitable breeding sites which constrain the range of environments available for optimising reproductive success. This work sheds new light on the ecological drivers of migration.