The diet and movements of a breeding population of the Red-tailed Black Cockatoo Calyptorhynchus banksi escondidus was studied between spring 1974 and the end of 1981 at Nereeno Hill in the northern wheatbelt of Western Australian. The birds fed almost exclusively on the seed of Double Gee Emex australis, an introduced, prolific, agricultural weed. When incubating and raising young nestlings, parents foraged within 5 km of their breeding area. However, once their nestlings were older, they often foraged more than 20 km from their nest hollows, commuting over an hour in the early morning and again in the evening. In doing so, they passed over available food, to join large, non-breeding, foraging flocks. Once the birds finished breeding they foraged widely in large flocks of over 1000 birds, up to 200 km from their breeding area. The dependence on the seed of an agricultural weed for food has allowed the birds to increase their distribution and abundance, but is a potential threat to the cockatoos, as any control measure that severely reduces the density of Double Gee may adversely impact on the distribution of the cockatoos in the southern part of their range.