Pellets of the Short-Eared Owl and Marsh Hawk Compared

Abstract

Twenty-four pellets each from wintering short-eared owls ( Asio flammeus ) and marsh hawks ( Circus cyaneus hudsius ) were staffstically compared. Considerably less osseous material was found to comprise the hawk pellets (17 percent of the total by weight) than the owl pellets (44 percent). exereta and dropped feathers at the pellet site as clues, I ascertained the source of 24 pellets gathered in the field. Weller et al. (1955) indicated that the droppings of short-eared owls were black and string-like and that iose of marsh hawks were green and pellet-like. In addition, I noted that uric acid exereted by the owls was generally buffy as contrasted with the chalky white uric acid of the hawk exerement. As 24 of those pellets collected in the field were known with certainty to be hawk pellets, an equal number of owl pellets were selected at random from the large number collected at their quarry roost. After equal periods of air-drying, the pellets svere weighed and the diameters measured. The pellets were then soaked in preparation dishes filled with water. After soaking 24 hours, they were teased apart with forceps and teasing needles. I found that by rotating the dish a centrifugal action was set up concentrating ffie bone on the bottom of the dish. The fur floated to the surface of the water and was skimmed off. This procedure was repeated until all fur had been renzoved. The bone was then dumped on filter paper to dry in preparation for veighing. Similarities in preference of habitat and food type frequently bring ffie short-eared owl and the marsh hawk into close proximity ( Weller et al. 1955 ). Such was the case in an area of central New York where I was observing short-eared owls in the winter of 196748. From one to SLY hawks roosted in an opexl field of uncut alfalfa. This field was about 6S0 yards north of an abandoned limestone quarry used by the owls as a roost. Because of this situation, I had access to pellets from both of these Moon ( 1940:465 ) stated ffiat Small marsh hawk pellets were easily confusable with short-eared owl pellets, since limited amounts of bone were found to be characteristic of the pellets of both species. As a subjective appraisal of pellets from known sources indicated the oontrary, I decided to compare statistically quantitative characteristics of pellets from both I acknowledge the American Museum of Natural History for a Frarlk M. Chapman Memorial Fund grant. Drs. D. Q. Thompson, D. A. Lancaster, and M. E. Richmond provided several suggestions on the manu-