Since 1998, IWS has been involved in the research program to aid in the recovery of the critically endangered San Clemente Loggerhead Shrike (Lanius ludovicianus mearnsi). At the onset of IWS’s involvement with the shrike program, the San Clemente Loggerhead Shrike was probably the rarest vertebrate in North America, with a wild population reaching a low of 14 adults.

From 1998 to 2000, the San Clemente Loggerhead Shrike research program implemented a project to understand seasonal abundance and vegetation type associations of shrike prey (insects, lizards, and mice; Lynn et al. 2001). We found that small arthropods, an important source of nestling diet, were most abundant in the canyon bottoms where shrikes typically locate their nests. Mice and small arthropods responded quickly to winter rainfalls and were more abundant following the 1997–1998 El Nino event, which may explain the high reproductive success of shrikes in the spring of 1998. Because most shrike prey was more abundant in vegetation types with thick ground cover, our shrike prey abundance research suggest that prey availability may be more important than prey abundance.

Between 1998 and 2003, our research program investigated several scales of shrike habitat, from foraging perch and nest sites (Lynn et al. 1999) to home range level habitat descriptions. We found that shrikes perched in areas with short vegetation, low percent litter cover, a thin litter layer, low visual obstruction, and relatively high but patchy vegetative ground cover. Shrike nests were situated in canyon bottoms, in shrubs (Ceanothus, Rhus, and Artemisia) as well as trees (Prunus, Heteromeles, and Quercus). Shrike nests were situated to take advantage of mild microclimatic conditions (away from prevailing winds, toward morning sun). Occupied shrike breeding territories contained more trees and shrubs than historically occupied shrike breeding territories (Lynn et al. 2001), and wintering territories contained fewer trees and shrubs than breeding territories (Lynn et al. in prep.).

In 1998, we characterized shrike foraging behavior and diet during the breeding season (Lynn et al. 1999). Shrikes foraged most frequently from tall perches (>2 m), foraged successfully 70% of the time, and captured and consumed mostly small arthropods, similar to shrikes in other populations. Shrikes on San Clemente, however, performed more aerial forages than shrikes in other populations.

In an effort to determine if perches were limited on San Clemente Island, we installed supplemental perches at some shrike breeding territories in 1999 and studied their effect on shrike foraging behavior. We found that shrikes gave up on a foraging perch more quickly at sites without supplemental perches, and also used areas that they had not used before supplemental perches were installed (Lynn et al 2000). Following this experiment, in 2001 the Soil Ecology and Restoration Group (SERG) installed perches in sites scheduled for release of captive bred shrikes and also removed ground covering vegetation in patches throughout these same areas. We studied the resulting foraging behavior of shrikes in areas with and without vegetation removal but found little difference in foraging efficiency, possibly due to rainfall during the study which caused a burst of annual vegetation growth (Lynn et al. 2002). During the same time period, we surveyed for raptors at sites with and without supplemental perches, but found that densities of American Kestrels (Falco sparvarius) were high throughout San Clemente Island, and did not differ between areas with and without supplemental perches.

Up until 2002, virtually all research on the San Clemente Loggerhead Shrike was conducted during the breeding season. Shrike populations worldwide are decreasing (Cade and Woods 1997). Changes in distribution and abundance of the Loggerhead Shrike. Conservation Biology 11:21-31), and general declines have been linked to low winter survivorship. In October 2002, we began research into San Clemente Loggerhead Shrike winter ecology, in an effort to elucidate possible limiting factors of this population. Though our study was limited by low shrike detectability, preliminary conclusions indicate that our shrikes tend to shift their habitat use in the winter away from the canyons used during the breeding season, and toward the flat marine terraces with little cover from predators and inclement weather (Lynn et al. in prep.). We hope to enhance our understanding of shrike wintering ecology with follow-up studies on shifts in habitat use and foraging and predator avoidance behavior during the next few years.

Literature Cited

Cade, T. J., and C. P. Woods. 1997. Changes in distribution and abundance of the loggerhead shrike. Conservation Biology vol. 11, no. 1.

Lynn, S., J. A. Martin, K. M. Wakelee, D. M. Cooper, G. A. Schmidt, and D. K. Garcelon. 1999. Research efforts to aid in the recovery of the San Clemente loggerhead shrike – 1998, final report. U. S. Navy, Natural Resources Management Branch, Southwest Div., Nav. Fac. Eng. Command, San Diego, California. 112pp.

Lynn, S., J. A. Martin, D. M. Cooper, K. M. Wakelee, G. A. Schmidt, and D. K. Garcelon. 2000. Research efforts to aid in the recovery of the San Clemente loggerhead shrike – 1999, final report. U. S. Navy, Natural Resources Management Branch, Southwest Div., Nav. Fac. Eng. Command, San Diego, California. 95pp.

Lynn, S. and D. K. Garcelon. 2001. Research efforts to aid in the recovery of the San Clemente Loggerhead Shrike – 2000. Unpublished report prepared by the Institute for Wildlife Studies, Arcata, California for the U. S. Navy, Natural Resources Management Branch, Southwest Div., Nav. Fac. Eng. Command, San Diego, California. 62pp.

Lynn, S., L. S. Cesh, E. Kershner, and D. K. Garcelon. 2002. Research efforts to aid in the recovery of the San Clemente Loggerhead Shrike – 2001. Unpublished report prepared by the Institute for Wildlife Studies, Arcata, California for the U. S. Navy, Natural Resources Management Branch, Southwest Div., Nav. Fac. Eng. Command, San Diego, California. 47pp.

Lynn, S., A. M. Condon, E. L. Kershner, and D. K. Garcelon. 2003. Winter ecology of the San Clemente Loggerhead Shrike – Final Report. Unpublished report prepared by the Institute for Wildlife Studies, Arcata, California for the U. S. Navy, Natural Resources Management Branch, Southwest Div., Nav. Fac. Eng. Command, San Diego, California. 29 pp.