Occupancy modeling of bird point counts: Implications of mobile animals

ABSTRACT

Occupancy modeling has been applied to a wide variety of taxa and sampling methods, including bird point counts. A critical assumption of basic occupancy models is that sites are occupied throughout the duration of the study, which is unlikely to be true for typical bird point-count studies. As such, we evaluated the implications of mobile animals on parameter estimates. We simulated the movement and detection of individual birds using an individual-based simulation model. We fit the basic occupancy model to data that represented a range of animal mobility, and determined the bias relative to known parameters used in the simulation. Occupancy depends on the size of the site selected, with smaller sites leading to lower occupancy for a given area and number of individuals present. At low animal density, occupancy scales approximately linearly with the area of sites, but at very high density, occupancy asymptotes at 1.0 across all site sizes. Even small amounts of movement lead to bias in estimates of occupancy and detectability, and the typical size of bird home ranges can lead to highly biased parameters. Moreover, variation in home range size over time or across habitats can lead to varying degrees of bias. Because of the potential for large bias in occupancy estimates, and their sensitivity to behaviors of birds (e.g., home range size), we recommend against applying current occupancy models to bird point-count data. © 2015 The Wildlife Society.