Pacific Southwest Research Station

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General Technical Report

Title: Estimating the occupancy of spotted owl habitat areas by sampling and adjusting for bias

Author: Azuma, David L.; Baldwin, James A.; Noon, Barry R.

Date: 1990

Source: Gen. Tech. Rep. PSW-124. Berkeley, Calif.: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 9 p

Station ID: GTR-PSW-124

Description: A basic sampling scheme is proposed to estimate the proportion of sampled units (Spotted Owl Habitat Areas (SOHAs) or randomly sampled 1000-acre polygon areas (RSAs)) occupied by spotted owl pairs. A bias adjustment for the possibility of missing a pair given its presence on a SOHA or RSA is suggested. The sampling scheme is based on a fixed number of visits to a sample unit (a SOHA or RSA) in which the occupancy is to be determined. Once occupancy is determined, or the maximum number of visits is reached, the sampling is completed for that unit. The resulting data are summarized as a set of independent Bernoulli trials; a zero (no occupancy) or one (occupancy) is recorded for each unit. The occupancy proportion is the sum of these Bernoulli trials divided by the sample size. The bias adjustment estimates this occupancy proportion for the estimated number of units on which a pair of owls was present but not detected. The bias adjustment requires the recording of the number of the visit during which occupancy was first detected. The distributional assumptions are checked with five different sets of data.

Key Words: spotted owl, SOHA, sampling scheme, bias adjustment, occupancy

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Citation

Azuma, David L.; Baldwin, James A.; Noon, Barry R.  1990.  Estimating the occupancy of spotted owl habitat areas by sampling and adjusting for bias  Gen. Tech. Rep. PSW-124. Berkeley, Calif.: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 9 p.