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Raptor Occurrences, Incidence of Nesting, and an Assessment of Prey Availability on Retired Agricultural Lands in the San Joaquin Valley, California

Poster presented at:

The Raptor Research Foundation 2004 Annual Conference, Bakersfield, CA

Krista R. Garcia, Curtis E. Uptain, and Patrick A. Kelly, California State University, Stanislaus, Endangered Species Recovery Program, 1900 N. Gateway Blvd. Suite 101, Fresno, CA 93727, USA.


From 1999 to 2003 we monitored wildlife use on approximately 800 ha of retired farmland near Tranquillity, Fresno County, California. Four experimental restoration treatments were applied to 20, 4-ha study plots configured in a randomized block design. We conducted winter raptor surveys along a 17.7 km road transect adjacent to these restored lands and we performed annual nest surveys and quarterly small mammal trapping on study plots. Survey results allowed an assessment of the value of restoration in providing habitat and prey for raptors. The number of wintering raptor species increased each year (except in 2002) reaching seven in 2001 and 2003. Among the raptors using the site were the American Peregrine Falcon (Falco peregrinus anatum), a California endangered species, the Prairie Falcon (Falco mexicanus), Northern Harrier (Circus cyaneus), Ferruginous Hawk (Buteo regalis), Burrowing Owl (Athene cunicularia), and the Short-eared Owl (Asio flammeus) which are California special concern species. Raptor abundance increased annually, except in 2003. American Kestrels (Falco sparverius), Red-tailed Hawks (Buteo jamaicensis), and Northern Harriers were the most frequently observed raptors. Approximately 25% of the Red-tailed Hawks observed in 2002 and 2003 exhibited dark (melanistic) morphology, and were often ground roosting. Three Short-eared Owl nests were found in 2002 and high numbers were observed roosting in winter of 2003. One Northern Harrier nest and one Burrowing Owl burrow occurred on study plots in 2003. The abundance of small mammals fluctuated seasonally and yearly. In 2,400 trap nights conducted each season, we captured the fewest number of small mammals (27 individuals) in fall 1999 and the greatest number (996 individuals) in summer 2001. Most (98%) of the small mammals captured were deer mice (Peromyscus maniculatus), but six other species also were captured. This relatively high diversity and abundance of small mammals is suspected to provide an adequate prey base for raptors in the project vicinity.

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