Bias in Estimation of Bird Density and Relative Abundance When the Closure Assumption of Multiple Survey Approaches Is Violated: A Simulation Study
Ornithologists are increasingly cognizant of the effect of detection error on abundance estimation and are using multiple visit surveys with occupancy and N-mixture models to account for detection error. These approaches assume the population of interest is closed. The effects of violating closure on density and relative differences between habitats are not fully appreciated. Closure might be violated during point count surveys for birds because of within-territory movement. A spatial simulation was used to generate data that would occur in a multiple visit survey if birds move within their territories between repeat visits. We varied bird density, territory size, and number of visits and studied how density estimates from various analytical techniques changed with bird movement. Large biases (up to 900% overestimation) in density estimates were observed using maximum occurrence, maximum count, occupancy, and Nmixture models. The relative abundance ratio between habitats was generally underestimated using the maximum or multiple visit approaches. Average presence and count were not biased as these metrics do not require closure. Importance of detection error in avian studies cannot be denied. However, given closure is likely violated in most point count applications due to bird movement, density estimates or even relative comparisons of bird abundance among habitats obtained by multiple visits must be checked for ecological plausibility. There is a clear need to develop metrics of bird abundance that do not rely on population closure but account for detection error.