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Assessing relative abundance and reproductive success of shrubsteppe raptors

Journal of Field Ornithology

This product was produced from the BLM/IDARNG Research Project (Snake River FS)
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Abstract

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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Assessing relative abundance and reproductive success of shrubsteppe raptors
Series title:
Journal of Field Ornithology
Volume
69
Issue:
2
Year Published:
1998
Language:
English
Contributing office(s):
Forest and Rangeland Ecosystem Science Center
Description:
p. 244-256
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Field Ornithology
First page:
244
Last page:
256
Number of Pages:
13