Chick diet, provisioning rates, foraging trip duration and chick growth of an increasing population of Guillemots Uria aalge were studied on Skomer Island, Wales, in 1985-87. Results are compared with similar data collected in 1973-75 when numbers were fairly stable at about half of the 1985-87 population levels.
Chicks were fed a higher proportion of sandeels Ammodytes tobianus (2I%) than in 1973-75 (<1%). There was a seasonal increase in the proportion ofsprats Sprattus spruttus and the size of prey fed to young, but this was probably a consequence of selection of prey according to chick age rather than an effect of date per se. There was also a diurnal increase in the proportion of sprats fed to chicks and a peak of chick feeding in the early morning. Feeding rates varied with chick age, 6-8-day-old chicks being fed at the highest rate of 4.6 feeds/ chick/day, with an estimated daily calorific intake of about 370 kJ. The mean calorific value of prey did not differ from 1973 to 1975 but provisioning rates were significantly higher than in 1973-75 and foraging trips were significantly shorter, suggesting an increase in food availability. However, there were no differences either within this study or between studies in the hatching weight, growth rate or fledging weight of chicks.
The observed changes in feeding ecology could be explained by changes in the relative abundance of the two main prey species and increased food availability. The recent increase of the Guillemot population on Skomer was not a consequence of the effects of the observed changes in feeding ecology on fledging weight or fledging success.
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