Sympatric nesting seabird species are often found to differ in one or more aspects of their foraging ecology. This is usually interpreted as resource partitioning, potentially due to current or past competition, but other explanations have been proposed. Three closely related species of alcids breeding together in subarctic southwest Greenland differed in several aspects of their foraging ecology during chick rearing. Thick-billed Murres (Uria lomvia) and Common Murres (U. aalge) did not differ in their diving behavior but both species differed markedly with Razorbills (Alca torda). Thick-billed Murres foraged mainly close to the colony, whereas Common Murres and Razorbills also made foraging trips to the mainland coast. Common Murres made significantly more bouts (series of dives) per trip than Thick-billed Murres, but significantly fewer dives per bout than Razorbills. Median dive depth of Thick-billed and Common murres was twice that of Razorbills. Thick-billed Murres nested on open ledges and spent most of their non-foraging time on the ledge attending the chick. Common Murres and Razorbills nested under boulders and in crevices and often left their chicks alone (particularly at night) and rested on the water. One interpretation of this pattern is that the risk of predation from Glaucous Gulls (Larus hyperboreus) was much higher on open ledges, and that Thick-billed Murres therefore had to guard their chicks at all times.
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Vol. 38 • No. 2