Resource Dependence and Territory Size in Loggerhead Shrikes (Lanius ludovicianus)

Abstract

Population declines of Loggerhead Shrikes (Lanius ludovicianus) have not been strongly correlated with the loss of preferred habitat (Bohall-Wood 1987, Brooks and Temple 1990). These findings suggest that factors other than breeding habitat are at least partially responsible and although, from time to time, researchers have searched for these other factors, no remedies for the situation have been found. U.S. Fish and Wildlife Service Breeding Bird Surveys (1966-1989) demonstrate that the Loggerhead Shrike is declining at a rate of about 5% per year continentwide. Of the 43 states censused during this period, shrike populations declined in 37 (86%; Robbins et al. 1986, Droege and Sauer 1990, Peterjohn and Sauer 1993). Although peninsular Florida, Oklahoma and New Mexico have been thought to be strongholds, Tyler (1992) and Yosef et al. (1993) have demonstrated that in both Oklahoma and peninsular Florida the species is declining at 5 to 9% per year. A number of factors have been implicated in the Loggerhead Shrike's decline (Busbee 1977, Anderson and Duzan 1978, Bystrak 1983, Lymn and Temple 1991). However, in the last decade of the 20th century, the continuing loss of foraging habitat and hunting perches to modern agricultural practices seems the most likely explanation (Novak 1989, Pittaway 1991). Shrikes are primarily sit-and-wait predators that hunt from conspicuous perch sites (Bohall-Wood 1987), and hunt on the wing only during adverse conditions (e.g. seasonal growth of annuals; Yosef and Grubb 1993), or when substantial numbers of prey are flying and can be caught easily. Loggerhead Shrike territories must include suitable hunting perches and a nest site. Although trees and bushes may be important for other activities (e.g. hiding from predators, roosting), their importance as hunting perches has been demonstrated in Northern Shrikes (L. excubitor) by Askham (1990) and Yosef (1993), and in Loggerhead Shrikes by Yosef and Grubb (1992). In the latter study, areas with sparse trees and fence posts were devoid of shrike territories. Furthermore, with the use of ptilochronology (Grubb 1989), Yosef and Grubb (1992) illustrated that territory size influenced the nutritional condition of the defending bird. Shrikes defending smaller territories were in better condition.