An Evaluation of Methods Used to Estimate Raptor Nesting Success

Surveys of raptor nesting populations can yield markedly different results depending on sam- pling and analytical procedures. Analysis of reproductive data for 3 raptor species in southwestern Idaho showed that productivity estimates based on pairs found early in the nesting season tend to be lower than those for pairs found late. Two factors seem to be responsible. First, researcher visits to nests early in the nesting season may cause nest failure, especially for eagles and red-tailed hawks (Buteo jamaicensis). Second, successful pairs are more conspicuous and are more easily located than unsuccessful pairs later in the season. To obtain an unbiased estimate of reproduction per pair by large raptors, researchers may use a sample of known traditional pairs to assess percent of pairs breeding, begin surveys during incubation to identify breeding pairs, and avoid disturbing nesting pairs until just before young fledge to determine nest success. Nest success per pair can be calculated in 1 of 3 ways. A method that incorporates the Mayfield technique may be most appropriate.