Fecal pellet analysis for diet determination of a mixed colony of bats in Western Washington

Senior research by Shawn Ferguson

A mixed colony of bats inhabiting an abandoned Weyerhauser pier in Olympia, Washington has been a focus of interest in an attempt to characterize the flora and fauna of Woodard Bay Conservation Area. It has long been known that bats are beneficial in insect control, however many of their habits and feeding preferences remain a mystery. No studies have been published on the impact of this bat colony as insect predators on the community.

Because bats are small, aerial, and nocturnal, direct observations are limited and little research has been done to describe their feeding preferences. Two methods are accepted in determining the dietary habits of bats. One method, stomach and intestinal tract examination, has been criticized because it involves sacrificing the animal. A second method, fecal pellet analysis, is an accepted, non-intrusive means of dietary analysis where the impact on the species remains minimal. Pellets dropped by bats at roosting sites are periodically collected. Chitinous parts of partly digested insects in dissolved pellets are then used to identify bat food preferences.

During the summer of 1994 I initiated a study to determine the diet of a mixed colony of insectivorous bats roosting under a pier. The colony is known to include the little brown bat, Myotis lucifugus, and the big brown bat, Eptesicus fuscus. My field work consisted of collecting approximately 75 bat fecal pellets each week during June, July and August. I am analyzing the fecal pellets and comparing the insect parts to a reference collection of local insects caught and identified in the area. I will compare my results with those reported in the literature and with Burns (1994) who collected and analyzed pellets of the same bat colony during the summer of 1993.