ASSESSING HABITAT SELECTION WHEN AVAILABILITY CHANGES

We present a method of comparing data on habitat use and availability that allows availability to differ among observations. This method is applicable when habitats change over time and when animals are unable to move throughout a predetermined study area between observations. We used maximum-likelihood techniques to derive an index that estimates the probability that each habitat type would be used if all were equal ly available. We also demonstrate how these indices can be used to com pare relative use of available habitats, assign them ranks, and assess statistical differences between pairs of indices. The set of these in dices for all habitats can be compared between groups of animals that represent different seasons, sex or age classes, or experimental treat ments. This method allows quantitative comparisons among types and is not affected by arbitrary decisions about which habitats to include in the study. We provide an example by comparing the availability of fou r categories of sea ice concentration to their use by adult female pol ar bears (Ursus maritimus), whose movements were monitored by satellit e radio tracking in the Bering and Chukchi Seas during 1990. Use of ic e categories by bears was nonrandom, and the pattern of use differed b etween spring and late summer seasons.