Prey Selection by Urban-Breeding Merlins

Abstract

-We identified 1,332 items belonging to 36 vertebrate species from prey remains collected near 65 Merlin (Falco columbarius) nests in Saskatoon, Saskatchewan from May to July 1987-1990. Principal prey of breeding Merlins was the House Sparrow (Passer domesticus). They along with the Horned Lark (Eremophilia alpestris) were usually taken more frequently than expected from their relative abundance. Other potential prey species were usually taken in proportion to their relative abundance, or less often than expected. Proportions of adult House Sparrows in the diet decreased while juveniles increased significantly as the Merlin breeding season progressed. During incubation and nestling phases, male and female House Sparrows were taken as expected. In the fledgling phase, Merlins took adult House Sparrows less often and juveniles more often than expected. The relative availability of different species positively correlated with that in the diet during the incubation and nestling phases. Merlins selected prey based on relative availability, independent of prey mass, but not independent of relative abundance. Our data supported the prediction that increase in absolute abundance of selected prey species increased the degree of prey selectivity. Received 12 March 1992, accepted 25 November 1992. STUDIES OF prey selection in carnivorous predators have concluded that predators either take prey according to their relative abundances (e.g. Jaksic et al. 1981, Nilsson 1981, Turner 1982, Village 1982), or take prey based presumably on relative profitabilities (e.g. Korpimaki 1985, Hunter et al. 1988, Steenhof and Kochert 1988, Canova 1989, Derting and Cranford 1989, Bochenski 1990, Brigham 1990). Predators following the former strategy are called opportunistic feeders, while those following the latter are known as selective feeders (Jaksic 1989). Studies evaluating the species of prey taken usually have considered only prey abundance. Furthermore, few data are available on the sex, age, and size of prey selected by carnivorous predators. There are many reports of the diet of breeding Merlins (Falco columbarius) from North America (Oliphant and McTaggart 1977, Hodson 1978, Becker 1985, Knapton and Sanderson 1985, Laing 1985, James and Smith 1987), but the studies all suffer from small number of nests sampled or a sporadic collection schedule. Better documentation of diet of breeding Merlins comes from Britain (Newton et al. 1984, Bibby 1987). Baker and Bibby (1987) compared relative 3 Present address: Department of Zoology, University of Alberta, Edmonton T6G 2E9, Canada. abundance of birds (based on questionnaire surveys) in habitats frequented by Merlins with relative consumption of those species by Merlins. They concluded that breeding Merlins took birds in order of their relative abundance. We had three objectives. First, we described the diet of urban-breeding Merlins based on systematic collection of prey remains. Second, we tested the null hypothesis that Merlins would take prey species according to their relative abundance in nature. The alternative hypothesis, based on classical optimal-foraging theory, states that a predator should not take prey species in relation to their relative abundances, but should use cues such as relative profitability (i.e. net energy intake) for prey selection (Emlen 1966, Pulliam 1974). According to the relativeprofitability hypothesis, we predicted that Merlins would not take prey species according to their relative abundances. Also, we considered the effect of relative prey availability (percentage of individuals of each prey species outside of cover) on Merlin prey selection. Third, we examined whether Merlins show prey selectivity according to the sex and age of their primary prey (i.e. House Sparrow as shown in Table 1). Finally, we examined two qualitative predictions of the foraging-theory prey model, which have been tested in previous studies of other animals (e.g. Schluter 1981, Steenhof and Ko-