Modeling the Relationship Between Estimates of Local Occupancy and Local Abundance in Avian Species
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Occupancy, the percentage of an area occupied by one or more individuals of a species, and abundance, the number of animals in a population, are population variables often used to make management decisions. Because occupancy does not precisely estimate abundance for many wildlife species, it is not considered as vital to an informed management program. However, estimating occupancy usually requires less effort than abundance. If occupancy is strongly related to abundance then occupancy may be useful to managers as a surrogate of abundance. Describing a linear relationship between occupancy and abundance would make estimating abundance more straightforward by allowing abundance to be calibrated to occupancy. Therefore, one objective was determining whether a linear relationship existed between occupancy and abundance. I also examined whether the relationship varies due to life history traits such as habitat use (specialist or generalist) and movement (year round resident or migrant). To address these objectives I conducted two seasons of avian point counts on Camp Swift in Bastrop County, TX during the fall of 2007 and winter of 2008. I sampled 100 detection stations located 200 meters apart and conducted four point count surveys of the detection stations for both seasons. I estimated occupancy using modified mark-recapture models and abundance using binomial mixture models. The findings indicated a linear relationship between occupancy and abundance. Moreover, slopes of the regressions for migrants and residents were significantly different. My research shows that occupancy-abundance relationships estimated at the local scale have the potential to be used to estimate abundance from occupancy. This has significant management implications for the use of occupancy models in lieu of the traditional, but more laborious, abundance estimators.