Responses to changes in prey availability by Common Murres and Thick-billed Murres at the Gannet Islands, Labrador

We quantified Common Murre (Uria aalge) and Thick-billed (Uria lomvia) Murre chick diets, chick-feeding rates, breeding success, chick growth, adult masses, and pair members' time spent together at site at the Gannet Islands, Labrador, in 1996 and 1997, after a decline in capelin (Mallotus villosus) abundance along the coast of southern and central Labrador. These results, with the exception of time spent at the site, were compared with those collected by other researchers at the Gannet Islands in 1981-1983, before the capelin decline. The two species responded similarly to the decline. After the decline, murres fed their chicks up to 75% fewer capelin and up to 65% more daubed shannies (Lumpenus maculatus). Feeding rates of both murre species varied among years, without respect to changes in the proportion of capelin. We found no evidence for declines in colony attendance, breeding success, chick growth, and adult mass. No data on time spent at the site were available before the decline in capelin abundance, but after the decline, off-duty murres of both species spent a mean of 10 min at their sites per feeding visit. This amount of time was short with respect to that recorded for Common Murres elsewhere, suggesting that murres' foraging effort at the Gannet Islands was high and buffered the effects of prey availability on other parameters measured. Taken together, our results suggest that murres responded to changing capelin abundance by changing their chicks' diet, but were otherwise little affected.