Date of this Version
PROCEEDINGS OF THE NORTH AMERICAN CRANE WORKSHOP 13:25-32
Abundance estimates allow wildlife managers to make informed management decisions, but differential detectability of individuals can lead to biased estimates of abundance. Our objective was to quantify detectability for non-territorial and territorial sandhill cranes (Grus canadensis tabida) during summer. We hypothesized that territorial sandhill cranes would be detected more often than non-territorial cranes. In 2009, 3 wetland areas were surveyed 2 days per week during the nesting season near Briggsville, Wisconsin. We created capture histories for color-marked territorial (n = 52) and color-marked nonterritorial cranes (n = 23) and used the Huggins closed capture model in program MARK to estimate detection probability and abundance for each group. A priori models were developed that explained daily crane detection over the sampling period using distance from road, territorial status, observation event, and time of season as variables. The best approximating model included the variables territorial status and observation event (AICc weight = 0.92). Probability of detection was higher for territorial (0.11, 95% CI = 0.08-0.14) than for non-territorial ( 0.03, 95% CI = 0.01-0.07) sandhill cranes. In subsequent observation events, detection probability almost doubled to 0.18 (95% CI = 0.17-0.20) for territorial cranes, and almost tripled to 0.11 (95% CI = 0.09-0.14) for non-territorial cranes. Potential reasons for differential detection during subsequent observations include differing degrees of movement by birds and/or an observer effect in which the ability to observe birds or the perception by technicians of birds increased over time.