Glossy Black-Cockatoos (Calyptorhynchus lathami) feed almost exclusively on the seeds of Allocasuarina and Casuarina species. Their specialised diet and low rates of food intake may make Glossy Black-Cockatoos susceptible to even small changes in their food supply. Glossy Black-Cockatoos occupying a large cypress pine–ironbark remnant woodland in central New South Wales were studied. Food supply within the study area was known to fluctuate with climate. I studied group size and feeding rates of Glossy Black-Cockatoos and compared them at different times of the day and year and in drought and non-drought conditions. Foraging groups were larger in the morning than in the afternoon and were larger during drought, when food was less abundant, than during non-drought periods. These results are consistent with previous observations that cockatoo group size increases when food resources are limited and the theory that foraging efficiency is enhanced by associating with more experienced flock-mates or by using the presence of feeding conspecifics to assist in locating food. Age-class had a strong influence on feeding rates, with older birds processing cones at faster rates than younger birds. There was no evidence to suggest that Glossy Black-Cockatoos were able to compensate for a reduction in food supply by increasing the rate at which they processed food.