Notes on Hawk and Owl Pellet Formation and Identification

Abstract

This paper records the progress to date (March 15, 1940) of attempts made to establish a basis for the identification of the pellets of certain raptorial birds found in western Kansas. The fundamental purpose of this study is to show that the accurate identification and interpretation of pellets, which have been formed under natural conditions in the field, may increase their value as food habits data when carefully studied. The birds dealt with in this study are the barn owl, short-eared owl, longeared owl, great horned owl, and the marsh hawk. In conducting studies on these birds repeated field observations were made over a period of six months. With the exception of two weeks in December, there were but few times during this period when more than 24 hours elapsed between visits to the areas frequented by these birds. Although it was not expected that birds would be sighted on every visit, regularity was considered necessary in order to gather all obtainable facts on fresh pellet deposits. This periodic collecting of pellets made it possible to gain information on the nocturnal feeding habits of the owls, though the birds were seldom seen. Before beginning this study, pellets that had accumulated in the areas under observation were collected and retained for analysis. However, only those pellets which were observed to appear from day to day were considered important as field data. As field data, each ensuing pellet carried a particular value that is not found in pellets comprising mass accumulations. For example, it was possible to estimate the frequency with which owls dropped pellets in one particular place. As described elsewhere in this paper, it was found that an owl might disgorge pellets a.t one roost daily for a week or more, and then not return to that roost for several days. How much this may have been due to pressure from other species was only partially determined. However, these habits may have an important bearing on pellet analysis when used in predation studies. Thus, when a bulk of pellets is gathered from an owl roost, an attempt should be made to determine how many birds of the same or different species have visited the area; also, the length of time the pellets have been accumulating under the roost. In this way it can be estimated whether or not every pellet the owl or owls have dropped during a certain period is represented in the accumulation.