Ecological segregation among Thick-billed Murres (Uria lomvia) and Common Murres (Uria aalge) in the Northwest Atlantic persists through the nonbreeding season

To study the influence of inter-and intra-specific interactions on patterns of ecological segregation in nonbreeding habitat, we used geolocators to track year-round movements of congeneric and partially sympatric Thick-billed Murres (Uria lomvia (L., 1758)) and Common Murres (Uria aalge (Pontoppidan, 1763)) from seven Canadian colonies during 2007-2011. Locations from 142 individuals were (i) examined for species-and colony-specific spatiotemporal patterns, (ii) mapped with environmental data, and (iii) used to delineate core wintering areas. Compared with Common Murres, Thick-billed Murres dispersed across a wider range of latitudes and environments, had larger winter ranges, and showed greater variation in seasonal timing of movements. These interspecific differences were consistent at two scales: among colonies spanning a wide latitudinal range and at a sympatric colony. Intraspecifically, nonbreeding ecological segregation was more pronounced among colonies of Thick-billed Murres than of Common Murres: colonies of Thick-billed Murres tended to follow distinct movement patterns and segregate by latitude, whereas colonies of Common Murres segregated very little; moreover, the extent of segregation was more variable among Thick-billed Murres than Common Murres. For Thick-billed Murres, rather than complete divergence of winter ecological niche from Common Murres, we found a widening of an overlapping niche. This strategy of increased movement flexibility may enable Thick-billed Murres to mitigate competition both intra-and inter-specifically; we propose this movement strategy may have played a role in species divergence.