ESTIMATING CORE RANGES - A COMPARISON OF TECHNIQUES USING THE COMMON BUZZARD (BUTEO-BUTEO)

The need to describe the relative intensity with which an animal uses different parts of its home range has been recognized for at least hal f a century. Such descriptions are particularly important for wide-ran ging raptors with home ranges covering a variety of habitats, in studi es of many tars, the description of internal range structure is addres sed by describing a core range of most intensive use. However, there i s still no broadly accepted definition of a core or method of objectiv ely estimating core ranges. Here, we propose that a core range can be usefully defined by the exclusion of excursive activity with the assum ption that behavior differs between core and excursive activities. Two methods of excluding excursive activity are presented for winter rang es of the Common Buzzard (Buteo buteo) in lowland U.K. The first invol ves subjective exclusion of outlying locations, using the outermost di scontinuing in the utilization distribution (UD). Incremental Cluster Polygons are used to produce the UD because this method provides the c losest spatial relationship to the animal locations and the most clear ly defined discontinuities. The potential for error or bias in this su bjective method may often be unacceptable, particularly for home range s which do not have well-defined core areas. The second method is a ne w application of incremental cluster analyis that objectively excludes excursive locations. The objective and subjective approaches are comp ared, and implications of core range definition in habitat and sociali ty analysis of raptors are explored in the context of published analys es on raptors and other taxa.