Spatial segregation between immatures and adults in a pelagic seabird suggests age-related competition

Individual competitiveness conditions access to resources when they are limited. Immature individuals that are less skilled than adults have to adapt their foraging strategies to survive. Among strategies to reduce competition, spatial segregation has been widely demonstrated. However, the use of spatial segregation by immatures to limit intra-specific competition with adults has rarely been tested. In this study, we investigated and compared habitat preferences and distributions of free-ranging immature and breeding adult northern gannets Morus bassanus in order to determine whether they compete for similar habitats during the year, and if this results in a spatial segregation between birds of different age groups. Based on > 66 000 km of aerial surveys conducted in the North-East Atlantic Ocean during winter and summer 2012, habitats selected by immatures and adult birds were modelled independently, linking gannet density to a set of oceanographic and physiographic predictors. Their large-scale seasonal distribution was then predicted. We found that gannets displayed a strong season-dependent competition between immatures and adults, as a consequence of immatures and adults using similar habitats in both summer and winter. During summer, when adults are constrained by reproduction, both groups were spatially highly segregated despite similar habitat preferences (thermal fronts), with youngest individuals selecting habitats out of range of central-place foragers, highlighting intra-specific competition. Contrastingly during winter, when reproductive constraints disappear, immature and adult distributions largely overlapped. Our study provides new insights into the role played by age, foraging experience and reproductive constraints on the distribution of marine predators. More specifically, these results highlight in seabirds how the youngest fraction mitigates, through spatial segregation, the competition with experienced adults, and suggest a progressive strategy along the maturation process.