Spatial segregation contrasting dietary overlap: niche partitioning of two sympatric alcids during shifting resource availability

Abstract

Interspecific interactions help to shape individual- and population-level dynamics. As such, similar species breeding sympatrically are expected to have a high potential for competition and, thus, may have developed strategies to reduce niche overlap to allow coexistence. We combined GPS tracking and stable isotope analysis (δ15N, δ13C) to simultaneously compare multiple behavioral and dietary niche dimensions of common murres (Uria aalge) and razorbills (Alca torda) breeding at nearby colonies (55 km apart) on the northeastern Newfoundland coast under varying prey availability in 2017. Though foraging ranges of species had high potential overlap (5000 km2), foraging areas of the two species were spatially segregated. Dive characteristics also differed, as predicted based on body size and dive capacity. These differences contrasted a high degree of overlap in diet, habitat characteristics, and diurnal foraging patterns, suggesting that spatial segregation may be the most important factor to allow coexistence of these similar species for the same limited resources. As prey availability increased later in the breeding season, individual murres and razorbills foraged closer to their respective colonies and population-level diet composition changed, indicative of reduced energy costs and dietary plasticity to take advantage of highly available prey, capelin (Mallotus villosus). Foraging areas, however, remained segregated. Seabirds and marine systems face many potential threats, the greatest of which may be climate change. Thus, understanding variation in space, time, and habitat selection among similar species allowed for a more comprehensive view of the possible ways these species may partition their niche and how future changes to resource availability could impact interspecific relationships.