Sources of Inaccuracy in Black-Tailed Deer Herd Composition Counts

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Abstract
Herd composition counts to establish sex and age structure are commonly used to assess population status for deer (Odocoileus spp.) and other large ungulates. Such counts are frequently biased, which compromises their usefulness. We evaluated composition counts of black-tailed deer (O. hemionus columbianus) for biases by comparing known behavior of a sample of radio-telemetered animals with the outcome of systematic dawn and night spotlight herd composition counts. Three sources of error (differential use of habitats, activity schedules, and deer behavior relative to observers) varied (P ≤ 0.036) by diel period; only activity schedules did not vary (P ≥ 0.521) by sex and month. Variation in sources of error was complex because of interactions between monthly and diel periods, month and sex, and diel period and sex. We used stepwise regression to test for the contribution of the 3 sources of error to numbers of deer observed and bias in the sex ratio from herd composition counts. Differential use of habitats accounted for most of the bias with numbers of deer observed, and differential alarm behavior was the only variable that explained a significant amount of variation with sex ratio. Unless herd composition counts are standardized (by route, season, etc.) for deer populations in dense habitats, biases in demographic estimates may fluctuate because errors are not systematic.

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