Dietary shifts based upon prey availability in Peregrine Falcons and Australian Hobbies breeding near Canberra, Australia

We collected prey remains and pellets at 16 Peregrine Falcon (Falco peregrinus) nest territories (975 prey items from 152 collections) and one Australian Hobby (F. longipennis) territory (181 prey items from 39 collections) during four breeding seasons in two time periods: 1991-1992 and 2002-2003, a total of 60 peregrine nest-years and three hobby nest-years. By number, European Starlings (Sturnus vulgaris) were the main prey taken by both falcons in 1991-1992 and 2002-2003, but starlings made up a smaller percentage of the diet by number in the latter period, apparently because their numbers had declined in the wild. Although the geometric mean of prey weights and geometric mean species weights were similar in the two time periods, both falcons compensated for the decline in European Starlings in the latter period by taking a greater variety of bird species, particularly small numbers of mostly native birds, rather than taking more of one or two other major prey species. Peregrines took 37 bird species in the latter period not found among their prey remains in the earlier period, and more individuals of some large species such as Gang-gang Cockatoos (Callocephalon fimbriatum), Galahs (Cacatua roseicapilla), and Rock Pigeons (Columba livia). Prolonged drought and competition from increasing numbers of Common Mynas (Acridotheres tristis) may have caused European Starlings to decline, but neither falcon species compensated by catching Common Mynas, even though they are of optimal prey size. Though both falcons, especially peregrines, took the more common bird species found in Surveys in the Australian Capital Territory, both tended to avoid large or dangerous species, or agile species that foraged close to cover, such as Common Mynas.