Food resource use and diet overlap of common and Thick-billed Murres at the Gannet Islands, Labrador

We studied food resource use br two similar seabird species, Common (Uria a alge) and Thick-billed murres (U. lomvia),breeding sympatrically at the Gan net Islands, Labrador, to examine the overlap in their chicks' diet and thu s to indirectly evaluate whether the two species were in competition for fo od. We used Monte Carlo randomization to establish whether murre chick diet overlap in 1996 and 1997 were greater than would be expected by chance. Di et overlap was higher than 75% in both years and was not lower than that pr edicted by the null model. To determine whether the two murre species' chic k food resource use converged in ways other than diet composition, we compa red timing of breeding, sizes of fish delivered to chicks, maximum dive dep ths 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 feedi ng watches in 1997, the murres chick-feeding peaks were concurrent, but dur ing the other they were not. In 1997, foraging Common and Thick-billed murr es dove to similar maximum depths. Taken together, these results suggest th at chick food resource partitioning might have been negligible between Comm on and Thick-billed murres breeding at the Gannet Islands in 1996 and 1997.