Food Resource Use and Diet Overlap of Common and Thick-Billed Murres at the Gannet Islands, Labrador

-We studied food resource use by two similar seabird species, Common (Uria aalge) and Thick-billed murres (U. lomvia), breeding sympatrically at the Gannet Islands, Labrador, to examine the overlap in their chicks' diet and thus to indirectly evaluate whether the two species were in competition for food. We used Monte Carlo randomization to establish whether murre chick diet overlap in 1996 and 1997 were greater than would be expected by chance. Diet overlap was higher than 75% in both years and was not lower than that predicted by the null model. To determine whether the two murre species' chick food resource use converged in ways other than diet composition, we compared timing of breeding, sizes of fish delivered to chicks, maximum dive depths and diurnal feeding patterns. In both years, the murres' chick-rearing periods overlapped almost exactly. The size of the principal item in their chicks' diets did not differ significantly. During one of two all-day feeding watches in 1997, the murres' chick-feeding peaks were concurrent, but during the other they were not. In 1997, foraging Common and Thickbilled murres dove to similar maximum depths. Taken together, these results suggest that chick food resource partitioning might have been negligible between Common and Thick-billed murres breeding at the Gannet Islands in 1996 and 1997. Received 5 November 1998, accepted 21 January 1998.