Behavioural and physiological responses of breeding common murres (Uria aalge): exploring inter-annual variability within individuals

Most avian studies investigating variation in parental behaviour have focused on differences between species, populations, or the sexes. Despite the importance of understanding variation at multiple levels, few have investigated parental care at the individual level. The same pairs of individually marked Common Murres (Uria aalge) breeding on Great Island, Witless Bay, showed variable behavioural and physiological responses across three consecutive breeding seasons (1998-2000). Despite high foraging effort, chick-feeding rates were low in the early chick rearing period of 2000. Low chick feeding rates in 2000 were related to the late arrival of inshore spawning capelin (Mallotus villosus). In contrast, high chick feeding rates in 1999 may have resulted from chicks hatching later and well after capelin had arrived in the vicinity of the breeding colonies. Hatching was delayed in 1999 because more chicks hatched from re-laid eggs. Female pre-lay attendance in that year was low, which resulted in less synchronous breeding and higher predation during early incubation. Frequency of feeding rates was positively correlated with chick body mass but no relationship was found with wing length. However, chicks had longer wings in 2000 compared to 1998 and 1999. Chick diet did not differ between these two years, with chicks being fed primarily capelin supplemented with sand lance (Ammodytes spp.). -- Collectively, females showed higher foraging effort and exhibited overall more direct parental care. At the pair level, however, both sexes tended to engage equally in behaviours related to chick rearing. It is possible that role partitioning between the sexes, in which one sex does most of the brooding while the other does most of the provisioning, may not be an adaptive strategy for Common Murres due to the high energetic costs associated with flying. -- Overall, males and females showed similar physiological responses to breeding. Both sexes had comparable haematocrit and corticosterone levels during chick rearing. In all years, body condition of males and females increased during incubation, dropped immediately after chick hatching and continued to decrease during chick rearing. Females maintained lowered body condition after their chick had departed the colony. Body condition of murres does not appear to decrease during chick rearing solely in response to reproductive stress. Rather, mass loss may reduce energetic costs associated with flying. In 2000, murres had lower body condition during the first week of chick rearing and showed elevated corticosterone levels compared to 1998 and 1999. These results suggest that murres were stressed during early chick rearing in 2000, supporting the view that breeding conditions were less favourable in that year. -- Because the same breeding pairs were monitored across years, the observed inter-annual differences were likely due to varying environmental conditions rather than individual variation. These results are consistent with other studies suggesting that seabirds relying on capelin in the Northwest Atlantic are experiencing highly variable breeding conditions as a result of delayed spawning and changes in the distribution of capelin. Including long-term behavioural and physiological information of marked individuals provides additional insight on the health of a population and may help further assess the quality of the breeding environment.