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USGS1990

Recent trends in counts of migrant hawks from northeastern North America

Using simple regression, pooled-sites route-regression, and nonparametric rank-trend analyses, we evaluated trends in counts of hawks migrating past 6 eastern hawk lookouts from 1972 to 1987. The indexing variable was the total count for a season. Bald eagle ( Haliaeetus leucocephalus ), peregrine falcon ( Falco peregrinus ), merlin ( F. columbarius ), osprey ( Pandion haliaetus ), and Cooper's hawk ( Accipiter cooperii ) counts increased using route-regression and nonparametric methods (P < 0.05). Northern harrier ( Circus cyaneus ) counts increased based on the nonparametric method (P < 0.01) but not the route-regression method (P > 0.10). We found no consistent trends (P > 0.10) in counts of sharp-shinned hawks ( A. striatus ), northern goshawks ( A. gentilis ) red-shouldered hawks ( Buteo lineatus) , red-tailed hawks ( B. jamaicensis ), rough-legged hawsk ( B. lagopus ), and American kestrels ( F. sparverius ). Broad-winged hawk ( B. platypterus ) counts declined (P < 0.05) based on the route-regression method. Empirical comparisons of our results with those for well-studied species such as the peregrine falcon, bald eagle, and osprey indicated agreement with nesting surveys. We suggest that counts of migrant hawks are a useful and economical method for detecting long-term trends in species across regions, particularly for species that otherwise cannot be easily surveyed.

Michigan, Minnesota, New Jersey, New York, Pennsyl↗

USGS1990

Winter use of douglas-fir forests by Blue Grouse in Colorado

We studied winter use of Douglas-fir ( Pseudotsuga menziesii ) forests by blue grouse ( Dendragapus obscurus ) from 1981 to 1983 at 2 study areas in northcentral Colorado. Comparisons of used and available stands indicated grouse were concentrated spatially, but there were no consistent differences related to basal area of tree species, conifer stem densities, and topography that were common to both areas. Blue grouse used dense (2,000 stems/ha) second growth (40-75 yr old), open to dense (200-1,900 stems/ha) mature (100-200 yr old), and open (<100 stems/ha) old-growth (200-600 yr old) stands. Stands used were composed of Douglas-fir alone or in association with subalpine fir ( Abies lasiocarpa ), Engelmann spruce ( Picea engelmannii ), lodgepole pine ( Pinus contorta ), limber pine ( P. flexilis ), Rocky Mountain juniper ( Juniperus scopulorum ), and quaking aspen ( Populus tremuloides ). Grouse used stands on mesic northern and eastern aspects, on xeric southern and western aspects, at elevations of 2,530-2,960 m, and on slopes of 1-45°. Preferential use (P < 0.05) of Douglas-fir trees occurred within stands that had an abundance of limber pine (use = availability) and subalpine fir (use < availability). Large Douglas-fir (20-90 cm dbh) were preferred (P < 0.05) within stands that had an abundance of smaller (≤15 cm dbh) trees. Both sexes used similar trees.

Colorado↗

USGS1990

Error in telemetry studies: Effects of animal movement on triangulation

We used Monte Carlo simulations to investigate the effects of animal movement on error of estimated animal locations derived from radio-telemetry triangulation of sequentially obtained bearings. Simulated movements of 0-534 m resulted in up to 10-fold increases in average location error but <10% decreases in location precision when observer-to-animal distances were <1,000 m. Location error and precision were minimally affected by censorship of poor locations with Chi-square goodness-of-fit tests. Location error caused by animal movement can only be eliminated by taking simultaneous bearings.