Seasonal changes in the ecological niche of seabirds - a global perspective

Dates: 2018–2020

Research team: LIENSs UMR 7266 La Rochelle Université - CNRS.

Collaborators: Dr Jérôme Fort.

The seasonal variability is one of the main drivers of seasonal movements like migration. Bird migration is often thought to be driven by poor environmental conditions during one season and to permit avoidance of resource shortage or harsh weather and tracking of more favourable conditions. Yet, whether this pattern exists in the marine realm is still unknown. We tested at the global scale the hypothesis of migration being driven by unfavourable conditions for mobile marine top predators, the seabirds. First, we quantified niche occupancy during the breeding and non-breeding periods over multiple marine ecoregions, and compared seasonal dynamics across migratory strategies. Second, we quantified the availability across seasons of both breeding and non-breeding niches to test for the effect of availability on seabird migratory strategies. We informed monthly presence of species over marine ecoregions from literature and expert knowledge. Breeding and non-breeding niches were drawn from environmental conditions encountered in ecoregions in which species were present in each period. We quantified the seasonal dynamics of niches from true distributions, but also from simulated residency in breeding and non-breeding grounds to quantify the seasonality of niche availability. 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 hypothesize 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 optimizing reproductive success. This work sheds new light on the ecological drivers of migration.

This work was published in Journal of Animal Ecology. The data and codes used to complete the analyses can be found on a dedicated repository.