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 equally available. We also demonstrate how these indices can be
used to compare relative use of available habitats, assign them ranks,
and assess statistical differences between pairs of indices. The set
of these indices for all habitats can be compared between groups of
animals that represent different seasons, sex or age classes, or
experimental treatments. 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 four categories of sea ice concentration to their
use by adult female polar bears, whose movements were monitored by
satellite radio tracking in the Bering and Chukchi Seas during 1990.
Use of ice categories by bears was nonrandom, and the pattern of use
differed between spring and late summer seasons.