The Breeding Diet of Different Brown Falcon ('Falco berigora') Pairs Occupying the Same Territory over Twenty Years Apart

Abstract

Food is of critical importance in determining reproductive success in birds (Lack 1954), including raptors (Newton and Marquiss 1981, Meijer et al. 1989, Wiehn and Korpimaäki 1997). Even within the same population, between-pair differences in diet can have marked influences upon fitness (Holthuijzen 1990, Slotow and Perrin 1992, Olsen et al. 1993, Swann and Etheridge 1995). This is also true of the Brown Falcon ('Falco berigora') from southeastern Australia, where the focal population maintains a very broad diet consisting of invertebrates, reptiles, birds and mammals (McDonald et al. 2003). Despite this, within-pair dietary breadth is comparatively narrow, with most pairs taking the majority of their prey items from just one of five dietary groups: lagomorphs (rabbit ['Oryctolagus cuniculus'] kittens), ground prey (e.g., rodents and invertebrates), small birds (passerines ,40 g), large birds (e.g., feral Rock Doves ['Columba livia']), and reptiles (e.g., eastern tiger snakes ['Notechis scutatus']; McDonald et al. 2003). These between-pair differences in diet had important impacts on reproduction, as pairs taking smaller prey (ground prey and small birds; geometric mean mass 59 g) were less likely to initiate breeding attempts compared to those taking larger prey (lagomorphs, large birds, and reptiles; mean mass 155 g; McDonald et al. 2004). This difference is presumably due to a difference in the amount of resources available to pairs, as smaller prey were not delivered to nests more frequently than larger prey (McDonald 2004). While McDonald and colleagues (2003) were unable to census prey abundance during their study, the population monitored was the same as that examined by Baker-Gabb (1982) in 1979–80. We therefore had the opportunity to examine dietary differences of pairs occupying the same areas over two decades apart.