The Manner and Efficiency of Raptor Depredations on Bats

Abstract

Large bat colonies such as those in the Carlsbad Caverns, New Mexico, attract many species of predators. The most significant are birds of prey, for they commonly feed on bats and are capable of taking them in flight (Bailey, 1928; Stager, 1941, 1948; Constantine, 1948; Eads, Wiseman and Menzies, 1957; Medway, 1958; Baker, 1961; Downing and Baldwin, 1961). Reptiles and carnivores, that might prey heavily on bats were they not terrestrial, are limited by the virtual inaccessibility of bats in their daytime roosts and by their flight habits. Only in a few congested bat caves are carnivores known to consume large numbers of bats. There are isolated cases of bat depredations by fish (Baker, 1961), amphibians (Kinsey, 1961; Elwell, 1962), birds other than raptores (Clay, 1959; Herreid, 1960; Wilks and Laughlin, 1961) and by rodents (Martin, 1961). Allen (1940) summarizes numerous references to fish, reptiles, birds, and mammals preying on bats and also lists spiders and ants as bat predators. Of all these, avian raptores are likely the most effective predators because when locally abundant, bats can constitute major proportions of raptor diets. Owl pellets from the Carlsbad Caverns are composed almost entirely of bat bones and fur. The number of bats captured, however, is relatively insignificant, and birds of prey are but minor threats to colonies as a whole. The lateness of the hour at which bats emerge almost eliminates them from the diets of diurnal birds, and nocturnal birds are incapable of pursuing and capturing individual bats. Therefore, raptores are a threat only for the relatively short time that bats exit en masse from a cavern entrance in the early evening. During that short time bats are extremely vulnerable. They emerge in dense streams for hours and by hundreds of thousands. In flight, bats are slow, compared to birds, and can be overtaken quickly. Yet, it is surprising that some birds are inefficient at catching bats and the number of captures versus the number of attacks varies greatly among the different species. For example, Red-tailed Hawks (Buteo jamnaicensis) and Great Horned Owls (Bubo virginianus) have difficulty at times in catching bats, and they may miss repeatedly. Accipiters and falcons, however, seldom miss and the reasons lie in the manner of attack and the nature of the bird. At Carlsbad Caverns National Park, park naturalists have had the opportunity for many years to witness bird depredations on the large colonies of Mexican freetail bats (Tadarida brasiliensis mexicana) that exit each evening from the caverns. Through the years, six species of hawks, one falcon, and one owl have been observed. They are: the Cooper Hawk (Accipiter cooperii), Sharp-shinned Hawk (Accipiter striatus), Swainson Hawk (Buteo swainsoni), Red-tailed Hawk, Ferruginous Hawk (Buteo regalis), Marsh Hawk (Circus cyaneus), Sparrow Hawk (Falco sparverius), and Great Horned Owl.