VARIATION AND REGULATION OF MEAL SIZE AND FEEDING FREQUENCY IN CORYS SHEARWATER CALONECTRIS-DIOMEDEA

1. We examined the pattern of chick feeding in Cory's shearwater Calonectris diomedea on Selvagem Grande by weighing 50 chicks at 4-h intervals throughout nine successive nights, then daily for a further 11 days (20 days in all). Meal sizes were estimated from the sum of positive mass increments recorded over 4-h intervals (SUM). Mass increments over 24-h (NET) were linearly related to SUMs by the equation NET = 0.83 SUM -40.1 (r2 = 0.82). 2. Individual meals fed to chicks averaged 74 g (SD = 26.4 g, range = 20-100 g). A mean of 79% of chicks were fed each night and the mean interval between feeds was 1-3 nights, with a maximum of four nights. There was little day-to-day variation in feeding rate, and no relationship with ambient light conditions over a half lunar cycle. The variance in chick masses each morning arose over approximately 5-5 days, indicating that any large reductions in mass resulting from a lack of feeding were short-lived. Further evidence suggested that variation in the intervals between meals for individual chicks resulted, in part, from changes in adult behaviour. These results were not compatible with the prevalent idea that the purpose of large fat deposits in procellariiformes is to tide chicks over periodic fasts resulting from unpredictable food supplies or poor feeding conditions. 3. On average, chicks required 48 g of food day-1 to maintain constant mass, and converted 60% of their intake of food above this requirement into biomass. Meal size and feeding frequency were independent of chick size, and the rate of food delivery (mean SUM; 56.1 g chick-1 night-1) was similar to the rate of 61.9 g chick 1 night-1 recorded in the Azores Islands by Klomp & Furness (1992). This was despite a large difference in chick size and growth rate, and in the temporal pattern of food delivery at the two colonies. We present evidence to suggest that changes in mass growth rate of chicks with age resulted from changes in metabolic energy requirements and food conversion efficiency rather than from changes in food supply rate. 4. Meal size was independent of the interval since last feeding. Meal size and feeding frequency were not related to chick body condition (body mass corrected for body size) and the masses of fod received by individual chicks each night were independent of previous values. These results suggest that the rate of food supply to chicks was not regulated by adjustment according to chicks' nutritional requirements, and support a previous hypothesis that chick feeding in procellariiformes is not regulated, but is controlled by an intrinsic rhythm in adults.