Effect of sampling effort on bias and precision of trends in migration counts

ABSTRACT Hourly or daily counts of animals during migration are used to assess change in population status. However, due to financial and logistical constraints, it is sometimes not possible to sample the entire migration, resulting in an unknown impact on accuracy and precision of estimated trends. Using simulated migration counts of 3 raptor species, commonly detected Sharp-shinned Hawks (Accipiter striatus), rarely detected Merlins (Falco columbarius), and super-flocking Broad-winged Hawks (Buteo platypterus), we used a hierarchical modeling framework to test whether sampling effort influenced accuracy and precision of trends. We subsampled simulated datasets in various ways: weekends only; a random sample of 40%, 60%, or 80% of the migration; or the first 25%, 50%, or 75% of the migration. Bias in trend estimates and probability of error (estimating a false but significant trend) varied little with sampling effort for the common and rare species, which suggests that for these species, sampling only a ...