THE EFFECTS OF FOOD ABUNDANCE ON BREEDING PERFORMANCE AND ADULT TIME BUDGETS OF GUILLEMOTS URIA-AALGE

The breeding performance, food fed to chicks and adult time budgets of Guillemots Uria aalge were examined in a year of high and a year of low food availability. There was no difference between the 2 years in reproductive Success, although the rate of chick feeding, chick weight and fledging success were greater in the year of high food availability. On average, chick prey items were larger in the poor food year, but this was insuffiCient to compensate for the lower feeding frequency. Chick feeding frequency did not, differ between days in the good year but did increase later in the season in the poor food year. Compared with the high food availability year, adult Guillemots in the year of low food availability spent much less time resting at the breeding colony, and their foraging trips were twice as long. Foraging birds tended to make several successive trips before resuming brooding duties from their mates when food supplies were good, but in the low food availability year single trips were the norm. These results demonstrate that predators experiencing reduced food supply may mitigate the effects on their reproductive output by shifting their time allocation such that more time is available for foraging.