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To investigate the covariation of adult body condition and nestling growth, we weighed adult Brünnich's Guillemots Uria lomvia rearing chicks at Coats Island, Nunavut, Canada, each year between 1988 and 2002. We estimated chick mass at 14 days for a sample of chicks reared in the same years. Adult mass and chick mass at 14 days were highly correlated, suggesting that, as feeding conditions deteriorate, adults compromise by reducing their own body reserves, while at the same time delivering less food to their offspring. We compared the prediction of the least-squares regression for the Coats Island data with observations made at Digges Island, a much larger colony about 300 km away, where birds are similar in linear body measurements to those at Coats Island and have a similar body mass while incubating. Adult mass at Digges Island averaged 11% less during chick-rearing than during incubation, compared with only a 5% difference at Coats Island. Mean chick mass at 14 days at Digges Island was lower in all years than was observed for chicks at Coats Island in any year. The observed 14-day chick masses at Digges Island in two years were close to values predicted by adult mass and somewhat lower in two other years (those when chick growth was slowest). At Digges Island, the distribution of mass for brooding adults was right skewed and suggested a lower threshold at 800–850 g, below which Brünnich's Guillemots terminate breeding. We conclude that the correlation between adult and chick mass represents a dynamic equilibrium in which adults simultaneously adjust their own energy reserves and their delivery rate to the chick. This compromise must be based on behavioural choices made by individual birds and is unlikely to be a passive consequence of fluctuating conditions.