An aerial sightability model for estimating ferruginous hawk population size

Most raptor aerial survey projects have focused on numeric description of v isibility bias without identifying the contributing factors or developing p redictive models to account for imperfect detection rates. Our goal was to develop a sightability model for nesting ferruginous hawks (Buteo regalis) that could account for nests missed during aerial surveys and provide more accurate population estimates. Eighteen observers, all unfamiliar with nest locations in a known population, searched for nests within 300 m of flight transects via a Maule fixed-wing aircraft. Flight variables tested for the ir influence on nest-detection rates included aircraft speed, height, direc tion of travel, time of day, light condition, distance to nest, and observe r experience level. Nest variables included status (active vs. inactive), c ondition (i.e., excellent, good, fair, poor, bad), substrate type, topograp hy, and tree density. A multiple logistic regression model identified nest substrate type, distance to nest, and observer experience level as signific ant predictors of detection rates (P < 0.05). The overall model was signifi cant (chi(6)(2) = 124.4, P < 0.001, n = 255 nest observations), and the cor rect classification rate was 78.4%. During 2 validation surveys, observers saw 23.7% (14/59) and 36.5% (23/63) of the actual population. Sightability model predictions, with 90% confidence intervals, captured the true populat ion in both tests. Our results indicate standardized aerial surveys, when u sed in conjunction with the predictive sightability model, can provide unbi ased population estimates for nesting ferruginous hawks.