Nolte, Eric G. – Raptors present but unobserved: Detectability at a western migration watch-site and its effect on trend analysis. 2012.
Annual counts of migrating raptors (Accipitriformes, Falconiformes) are used as indices of population size. Variation in the proportion of the raptor population counted may decrease precision of trend estimates, thereby reducing power of inference. The proportion counted is the product of sample coverage and probability of detection. It is possible to improve the power of trend analysis by the adoption of techniques, such as double-observer or distance sampling, which estimate the probability of detection. I used a dependent double-observer method to estimate detectability at the annual fall raptor migration count at Lucky Peak, Idaho, in 2009 and 2010. I used Huggins closed-capture removal models and information-theoretic multi-model inference to describe important factors affecting detectability. The most parsimonious model included effects of observer identity, distance, wingspan, genus, and day of the season. Competitive models also included wind-speed, cloud cover, and hour of the day. These results demonstrate the importance of controlling observer effort and training at watch-sites, and the potential utility of adjusting daily counts to account for differences in flight distance. I used model-averaging to account for selection-uncertainty in estimating coefficients, and used the resulting equation to simulate 30 years of counts of Sharp-shinned Hawks (Accipiter striatus) and Northern Harriers (Circus cyaneus) with heterogeneous detectability, a known population trend, and a degree of unexplained random variation in the number of available birds. Imperfect detection did not substantially bias trend estimation, but did increase variance in counts, decreasing power. Correcting for detectability did little to improve power to detect long-term declines when there was a realistically high variation in the number of available raptors (CV = 0.26). Detectability-correction by means of double-observer or distance sampling may, in the case of raptor migration counts, not be warranted for the purpose of long-term population monitoring. Efforts may be better focused on improving our understanding of mechanisms that cause changes in the number of migrants available to count.