Overlap in Diets and Foraging of Common Murres (Uria Aalge) and Rhinoceros Auklets (Cerorhinca Monocerata) After the Breeding Season

Abstract

AbstractCommon Murres (Uria aalge; hereafter “murres“) and Rhinoceros Auklets (Cerorhinca monocerata; hereafter “auklets“) breed and forage sympatrically over much of their range. They have similar diets during the breeding season, which suggests that they partition prey during the breeding season by foraging (1) at different locations, (2) at different times of day, (3) at different water depths, (4) on different proportions of the same prey species, or (5) some combination of the four. We examined possible mechanisms of niche partitioning during late summer and fall in Puget Sound, Washington, in 1993-1996. Murres and auklets fed mainly on Pacific herring (Clupea pallasii, occurring in 74.2% and 48.1%, respectively, of gastrointestinal tracts with contents), Pacific sand lance (Ammodytes hexapterus; 45.8% and 62.3%), and salmonid (Oncorhynchus spp.) species (21.9% and 9.7%). Auklets also consumed considerable amounts of threespine stickleback (Gasterosteus aculeatus; 26.6%). Murres and auklets did not differ significantly (1) in their diet (between age classes or sexes of either species, or among years); (2) in mean lengths of Pacific herring (101 and 109 mm, respectively) and Pacific sand lance (82 and 86 mm) they consumed; or (3) in the mean depth (7–8 m) at which they were entangled in gill nets. Dietary diversity was low, with most gastrointestinal tracts containing only one or two prey species in both murres and auklets. Murres were caught and therefore presumably feed more frequently in the afternoon and evening; whereas auklets were entangled more often in early morning. We found differences between murres and auklets in the diel chronology of prey taken, which may partly explain how murres and auklets coexist during the breeding season and months thereafter, prior to auklet emigration from Puget Sound.Chevauchement du Régime Alimentaire et de la Quête Alimentaire chez Uria aalge et Cerorhinca monocerata après la Saison de Reproduction