EFFECTS OF PREY ABUNDANCE ON THE FORAGING BEHAVIOR, DIVING EFFICIENCYAND TIME ALLOCATION OF BREEDING GUILLEMOTS URIA-AALGE

The foraging behaviour of Guillemots Uria aalge at sea was compared be tween 2 years of radically different food abundance. Radio telemetry w as used to determine foraging locations and diving patterns. In the po or compared with the good food year, foraging trips were much longer, the birds foraged more than six times further from their breeding site s, they spent over five times as much time diving when at sea and thei r estimated energy expenditure was twice as great. Time spent foraging in the poor food year was at the expense of time spent sitting at the colony. The duration of a foraging trip was a poor indicator of dista nce travelled but a good indicator of the amount of time spent diving. Mean dive durations, surface pause durations and interbout periods di d not differ between years, but individuals made more than four times as many dives per diving bout in the poor food year. Surface pause len gths did not vary with water depth in either year. In the poor food ye ar, birds made shorter surface pauses for a dive of a given duration t han in the good food year, possibly accepting a lactic acid debt in or der to maximize searching time. The duration of the interbout period w as positively related to the number of dives in the previous bout, and dives tended to get shorter in long diving sequences, suggesting poss ible exhaustion effects. These data demonstrate that breeding Guillemo ts have the capacity to adjust their foraging behaviour and time budge ts in response to changes in food abundance, but this flexibility was not sufficient to compensate fully for the very low food abundance exp erienced by birds in this study.