Document: JEF-3-53-7

Raptor population trends discerned from 23 years of migration monitoring in Nevada, Utah, and New Mexico.

SMITH, J.P.*

HawkWatch International, Inc., Salt Lake City, Utah 84115 ¹

Abstract:
HawkWatch International has monitored migrations of diurnal raptors at a network of sites primarily in western North America (currently 15 sites) since 1977. The primary goal is to track long-term population trends and thereby provide early warnings about negative trends and a means of documenting conservation successes. Here I discuss trends at our 4 longest-term sites. Trained observers conducted season-long counts of primarily 16 species in the Wellsville Mts., UT, 1977-79, 87-99 (autumn); Goshute Mts., NV, 1983-99 (autumn); and Manzano (autumn) and Sandia (spring) Mts., NM, 1985-99. Annual counts average: Goshute 14,931; Wellsville 3,729; Manzano 5,282; Sandia 4,145. I used linear regression to identify trends in annual passage rates (birds/100 hrs) derived from daily counts recorded using standardized methods and sampling periods, and adjusted for variation in observer numbers and effort (1987-99 only for Wellsvilles). At the high-volume Goshute site, 12 of 16 species increased (P 0.10) and 4 species showed no trends. Species that increased at 3 sites included: Turkey Vulture (Cathartes aura; 5-12%/yr), Osprey (Pandion haliaetus; 5-8%/yr), Broad-winged Hawk (Buteo platypterus; 3-9%/yr at 3 sites), Swainson's Hawk (B. swainsoni; 3-25%/yr), Merlin (Falco columbarius; 8-15%/yr at 3 sites), and Peregrine Falcon (F. peregrinus; 4-14%/yr). Based on regressions, declines emerged for Ferruginous Hawk (B. regalis) in the Manzanos (5%/yr) and Sharp-shinned Hawk (Accipiter striatus; 4%/yr) and American Kestrel (F. sparverius; 7%/yr) in the Wellsvilles. However, t-tests comparing mean annual passage rates for 1977-79 vs. 1987-99 in the Wellsvilles revealed declines for Northern Goshawk (A. gentilis; 56% overall decline) and Golden Eagle (Aquila chrysaetos; 51%). These tests also revealed significant increases for 6 species that have shown no trend since 1987. I discuss factors that confound interpretations and offer plausible explanations for noteworthy trends. For example, Turkey Vultures are likely expanding their range northward in response to global warming and increasing in density in response to greater availability of open habitat and carrion associated with human activities.

Keywords: raptor migration, diurnal raptors, population trends, western North America, Nevada, Utah, New Mexico

This abstract is being presented at: 8:45 AM in session:
Oral Session #3: Avian Ecology.