| Abstract: | We examine the provisioning constraints of a pursuit‐diving seabird in a cold ocean regime by comparing the behaviour of common murres Uria aalge rearing chicks at two colonies in the Northwest Atlantic during 1998‐2000. Funk Island is the largest (340,000–400,000 breeding pairs) and most offshore (60 km) colony of common murres in eastern Canada. Seventy‐five percent of the Northwest Atlantic population of common murres breeds on this island. Great Island is one island within the Witless Bay Ecological Reserve, which is the second largest breeding aggregation (100 000 breeding pairs) and is located near‐shore (2 km). The primary forage fish species in Newfoundland waters is capelin Mallotus villosus, which spawns on or near coastal beaches during summer. Therefore, the two study colonies differ in their distance to food resources and colony size. It is within this natural context that we compare: (1) prey types and frequency of delivery (amount of prey), (2) parental time budgets, and (3) the mass and condition (mass/wing length) of fledglings at both colonies. Similarly sized female capelin (100–150 mm) were delivered to chicks at both colonies. Foraging time per day per parent, a proxy of foraging effort, was similar at both colonies (Great Island: 5.1 h; Funk Island: 5.5 h), as was the percentage of time spent with mates (Great Island: 12.3%; Funk Island: 10.9%). Foraging trips, however, were longer at Funk Island (4.1 h) than at Great Island (2.9 h). This resulted in lower feeding rates of chicks (0.17 feeds per h) and poorer condition of fledglings (2.9 g/mm) at Funk Island compared to those at Great Island (0.22 feeds per h; 3.9 g/mm). We hypothesize that provisioning efforts are constrained at Funk Island by (1) distant food resources and increased competitor density, resulting in longer foraging trip durations and (2) the time spent paired with mates at the colony, which may reflect a minimum time required to maintain breeding sites due to higher breeding densities at Funk Island compared to Great Island. Demographic consequences of this poor fledgling condition at Funk Island are unknown, but fledglings may sufficiently accelerate growth at sea due to their closer proximity to an important nursery area. If fledgling survival is compromised, however, the lower potential for growth at Funk Island will impact the entire Northwest Atlantic population of common murres. |
