THE INFLUENCE OF FOOD-SUPPLY ON THE BREEDING ECOLOGY OF KITTIWAKES RISSA-TRIDACTYLA IN SHETLAND

We measured the breeding performance, body condition, time budgets and foraging ranges of Kittiwakes Rissa tridactyla at Sumburgh Head, Shet land, in two years of contrasting food availability. Kittiwakes in She tland generally feed their young almost entirely on sandeels, and fish eries data indicated that stocks of sandeels in Shetland waters were a t least ten times higher in 1991 than in 1990. Fledging success of Kit tiwakes was nil in 1990 and 68% of eggs laid in 1991, although clutch- size and hatching success were no different between years. Post-hatchi ng foraging trips in 1991 were of comparable duration to those recorde d at other colonies in conditions of good food supply (2-3 h), while t rips recorded during incubation or post-hatching in 1990 were approxim ately three times longer on average than at corresponding stages of th e breeding season in 1991. Radio-tracking data indicated that adults g enerally stayed within 5 km of the colony in 1991 but flew more than 4 0 km from the colony on each trip in 1990. Eggs were apparently not le ft unattended in either year, despite the fact that this required adul ts to incubate for periods in excess of 44 h in 1990. The extent to wh ich adults were able to increase trip durations, foraging ranges and i ncubation shift lengths between years, while maintaining hatching succ ess, indicates the degree to which Kittiwakes are normally buffered ag ainst adverse feeding conditions during incubation. Reduced nest atten dance and lower body-condition of adults post-hatching in 1990, in con junction with complete post-hatching breeding failure, indicate that a dults were beyond the limits of their buffering capacity during chick- rearing in 1990.