Factors affecting bird counts and their influence on density estimates Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/f4752m92x

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  • Variable area surveys are used in large geographic regions to estimate the density of birds distributed over a region. If some birds go undetected, a measure of the effective area surveyed, the amount of area occupied by the birds detected, is needed. The effective area surveyed is determined by observational, biological, and environmental factors relating to detectability. It has been suggested that density estimates are inaccurate, and that it is risky to compare bird populations intraspecifically over time and space, since factors influencing bird counts will vary. There have been several controversial studies where variable area survey density estimates were evaluated using density estimates calculated from spot mapping as the standard for comparison. Spot mapping itself is an unproven estimator that the previously mentioned factors also influence. Without a known population density, determining how the different density estimators perform is difficult to access. Variable area surveys of inanimate objects whose densities were known have been conducted under controlled circumstances with results generally supporting the variable area survey method, but time and inability to control for all factors limit the application of this type of study. A simulation program that distributes over a region vegetation and a known density of birds, and then simulates the process of gathering bird detection data is one tool accessible to evaluate variable area density estimates. Within such a simulation study various observational, biological, and environment factors could be introduced. This thesis introduces such a simulation program, VABS, that was written with the objectives of identifying factors that influence bird counts and determining the limitations of the variable area survey. Within this thesis are discussions concerning the several factors that have been identified as influencing bird counts and the effects that these factors had on the Fourier series, exponential power series, and Cum-D density estimates when these factors were simulated in VABS. Critical assumptions of the variable area survey are identified, and the ability of the variable area survey to estimate density for different detectability curve is examined. Also included are discussions on the topics of pooling data gathered under different detectabilities and monitoring population trends.
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