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

Abstract

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.

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

“…However, once the breeding duties are completed, seabird energy requirements drop and the central-place foraging constraint disappears. This results in a dilution of intraspecific competition during the non-breeding period (Pettex et al, 2019), but also in the possibility for individuals to move to more favourable areas otherwise inaccessible during the breeding period.…”

Monitoring Abundance and Distribution of Northern Gannets Morus bassanus in Western Iberian Waters in Autumn by Aerial Surveys

“…Aerial surveys have proved effective in providing unbiased Northern Gannet abundance estimates and probability distribution maps (e.g. Pettex et al, 2017Pettex et al, , 2019Rogan et al, 2018). this type of survey is essential to assess abundance and demographic trends outside breeding grounds and their relation to the breeding population.…”

Divergent post-breeding spatial habitat use of Laysan and black-footed albatross

“…For colonial breeding seabirds that function as central place foragers when breeding, competition for resources near colonies may influence habitat use, prey selection, energetic expenditure, and ultimately population sizes (Furness and Birkhead, 1984;Lewis et al, 2001;Wakefield et al, 2013;Oppel et al, 2015;Jessopp et al, 2020;Weber et al, 2021). Habitat segregation in seabirds is well documented and occurs not just between sympatrically breeding species (Kappes et al, 2010;Young et al, 2010Young et al, , 2015Linnebjerg et al, 2013;Robertson et al, 2014;Barger et al, 2016), but also between age classes (Pettex et al, 2019), sexes (Lewis et al, 2002;Phillips R.A. et al, 2004;Camphuysen et al, 2015), and nearby populations of the same species (Rayner et al, 2011;Wiley et al, 2012;Ceia et al, 2015;Shaffer et al, 2017;Bolton et al, 2019). While resource partitioning is evident during breeding when competition for resources close to the colony is high, partitioning during the post-breeding period has received much less attention until recent years.…”