Assessing relative abundance and reproductive success of shrubsteppe raptors

From 1991-1994, we quantified relative abundance and reproductive success of
the Ferruginous Hawk (Buteo regalis), Northern Harrier (Circus cyaneus), Burrowing Owl
(Speotytoc unicularia), and Short-eared Owl (Asio 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 plots using
four sampling methods: point counts, line transects, and quadrats of two sizes. On a persampling-effort basis, transects were slightly 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 determine 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 during incubation,
and the Mayfield model. Most pairs used to evaluate success were pairs found incidentally.
Visits to historical nesting areas yielded the highest number of pairs per sampling effort
(14.6/100 h), but reoccupancy rates for most species decreased through time. Estimates
based on all attempts had the highest sample sizes but probably overestimated success for
all species except the Ferruginous Hawk. Estimates of success based on nesting attempts
found during incubation had the lowest sample sizes. All three methods yielded biased nesting
snccess estimates for the Northern Harrier and Short-eared Owl. The estimate 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 success for all species except the Ferruginous hawk.