ASSESSING RELATIVE ABUNDANCE AND REPRODUCTIVE SUCCESS OF SHRUBSTEPPE RAPTORS

From 1991-1994, we quantified relative abundance and reproductive succ ess of the Ferruginous Hawk (Buteo regalis), Northern Harrier (Circus cyaneus), Burrowing Owl (Speotyto cunicularia), and Short-eared Owl (A sio flammeus) on the shrubsteppe plateaus (benchlands) in and near the Snake River Birds of Prey National Conservation Area in southwestern Idaho. To assess relative abundance, we searched randomly selected plo ts using four sampling methods: point counts, line transects, and quad rats of two sizes. On a per-sampling-effort basis, transects were slig htly more effective than point counts and quadrats for locating raptor nests (3.4 pairs detected/100 h of effort vs. 2.2-3.1 pairs). Random sampling using quadrats failed to detect a Short-eared Owl population increase from 1993 to 1994. To evaluate nesting success, we tried to d etermine reproductive outcome for all nesting attempts located during random, historical, and incidental nest searches. We compared nesting success estimates based on all nesting attempts, on attempts found dur ing incubation, and the Mayfield model. Most pairs used to evaluate su ccess were pairs found incidentally. Visits to historical nesting area s yielded the highest number of pairs per sampling effort (14.6/100 h) , but reoccupancy rates for most species decreased through time. Estim ates based on all attempts had the highest sample sizes but probably o verestimated success for all species except the Ferruginous Hawk. Esti mates of success based on nesting attempts found during incubation had the lowest sample sizes. All three methods yielded biased nesting suc cess estimates for the Northern Harrier and Short-eared Owl. The estim ate based on pairs found during incubation probably provided the least biased estimate for the Burrowing Owl. Assessments of nesting success were hindered by difficulties in confirming egg laying and nesting su ccess for all species except the Ferruginous hawk.