VARIABILITY AND STOCHASTICITY OF MEAL SIZE AND FEEDING FREQUENCY IN THE LITTLE SHEARWATER PUFFINUS-ASSIMILIS

The pattern of chick feeding in Little Shearwater Puffinus assimilis on Selvagem Grande was examined by weighing chicks at 4-h intervals throughout eight successive nights and daily for a further 11 days (19 days in all). Individual meals fed to chicks averaged 23.2 g (s.d. +/- 4.7) or 13.6% of adult mass. Mass increments over 24 h (NET) were linearly related to the sum of positive mass increments over 4-h intervals during the night (SUM) by the equation NET = 0.36SUM - 5.89 (r2 = 0.60). Using this relationship, I estimated that over a period of 18 nights, a mean of 95% of chicks were fed each night, and the mean interval between feeds was 1.05 nights, with a maximum of three nights. There was no significant day-to-day variation in feeding rate. 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 poor feeding conditions. On average, chicks required 16 g of food per day to maintain constant mass and converted 33% of their intake of food above this requirement into biomass. Meal size and feeding frequency were independent of chick size and body condition (body-mass corrected for body-size), and the masses of food received by individual chicks each night varied in direct proportion to previous values. These results suggest that the rate of food supply to chicks was not regulated by adjustment according to chicks' nutritional requirements. To some extent, this supports the hypothesis that lipid accumulation among Procellariiformes is related to stochastic variation in food supply rate, resulting from an absence of regulation of feeding. However, feeding was not stochastic, in that adults tended to deliver consistent amounts of food to their chicks, and the pattern of feeding among even the worst-fed chicks was inconsistent with a need for large lipid stores based upon chance variation in food delivery.