Diet of two insectivorous bats, Myotis lucifugus and Myotis keenii, in relation to arthropod abundance in a temperate Pacific Northwest rainforest environment

We assessed the diet of two morphologically similar bats ( Myotis lucifugus (LeConte, 1831) and Myotis keenii (Merriam, 1895)), which both used hydrothermally heated nursery roosts at Gandll K’in Gwaayaay (Hotspring Island), Haida Gwaii (Queen Charlotte Islands), British Columbia, in 1998 and 1999. Our purpose was to determine if they fed opportunistically or actively selected prey, and whether they partitioned prey resources. We determined diet by analyzing feces collected from captured bats and compared it with the relative abundance of insects captured in light traps. Myotis lucifugus fed mainly on lepidopterans, medium-sized to large dipterans, neuropterans, and hymenopterans, while M. keenii fed on lepidopterans, arachnids, medium-sized to large dipterans, and neuropterans. We found that both species were selecting prey, although selection may have been more a function of prey size than particular taxa. Arachnids occurred in feces of both species, implying that both were capable of gleaning prey from surfaces, although only M. keenii regularly fed on spiders. We concluded from the preponderance of flying insects in the diet of M. lucifugus that it was primarily aerial hawking prey, while we took the frequent occurrence of both flying insects and spiders in the diet of M. keenii to indicate that it was both aerial hawking and gleaning prey. Weather conditions between years affected relative abundance of insects and bat diet, with species diversity being lower in light-trap samples and diet of M. lucifugus in 1999, which was cooler and wetter than in 1998. Species diversity in the diet of M. keenii was higher in 1999. Similarities in diet indicated that some interspecific competition was occurring, although this competition was likely minimized by their different foraging strategies.