Food brought to nestling Rhinoceros Auklets (Cerorhinca monocerata) was sampled by applying tape or cloth muzzles to the chicks and collecting the uningested food from the burrow daily. Limited data were also gathered for Tufted and Horned puffins (Fratercula cirrhata and F. corniculata). Auklet chicks received an average of 34.1 g of food per night at Middleton Island in 1978, and 32.8 g per night at the Semidi Islands in 1979. Pacific sandlance (Ammodytes hexapterus) made up the bulk of the diet at both locations, and large, second-year or older fish predominated in the samples. In contrast, Horned and Tufted puffins took mostly small, first-year fish during the same seasons. The quantity of food delivered per night to Rhinoceros Auklets increased with the age of the chick, and wind speed may have been an important environmental factor affecting feeding rates. Proportions of first-year and older sandlance in the diet varied within and between sampling bouts, indicating spatial and temporal changes in prey availability. Parallel changes in feeding rates were also observed. The primary use of different age groups of sandlance by the three puffin species suggests the age structure of the prey population may differentially affect breeding success.
