Territory Size Influences Nutritional Condition in Nonbreeding Loggerhead Shrikes (Lanius ludovicianus): A Ptilochronology Approach

Abstract

Once relatively common throughout much of North America, the Loggerhead Shrike (Lanius ludovicianus) has recently undergone a precipitous decline and is currently diminishing in numbers at about 5% per annum (Graber et al. 1973; Morrison 1981; Bystrak 1983; Hands et al. 1989). Here, we use ptilochronology (Grubb 1989) to show that a reduced number of hunting perches is correlated with reduced nutritional condition in shrikes, which could be contributing to the population loss. Ptilochronology is a recently developed method for monitoring the nutritional condition of a free-ranging bird by measuring the width of daily growth bars on a tail feather induced to grow by pulling out its predecessor (Grubb 1989). That growth-bar width is related to nutritional condition is now clear (Grubb 1989, 1991; Grubb & Cimprich 1990). A number of factors have been implicated in the Loggerhead Shrike's decline (Busbee 1977; Anderson & Duzan 1978; Craig 1978; Porter et al. 1978; Kridelbaugh 1982; Bystrak 1983; Cadman 1985; Hands et al. 1989). In the last decade of the twentieth century, however, loss of foraging habitat and hunting perches to modern agricultural practices seems the most likely explanation (Cadman 1985; Hands et al. 1989). This report focuses on hunting perches as a limiting resource in a habitat where foraging substrate and nest sites are unlikely to be limiting. Shrikes do not hunt on the wing. Therefore, we hypothesized that a territorial shrike requires some minimum amount of grassland or pasture substrate within scanning distance from hunting perches, a portion of the territory we termed the area. Recognizing that the density of hunting perches is variable, we also hypothesized that in areas with few hunting perches, a shrike must defend a very large area, and that populations have been declining because continual removal of fence rows and other hunting perches has made utilizable areas too costly to defend due to the increased dead space. Specifically, with the continual removal of hunting perches, the benefit-cost ratio of defending a territory eventually declines below some minimum threshold value as the ratio of utilizable area to dead Paper submitted March 15, 1991; revised manuscript acceptedJuly 10, 1991. * Correspondence should be addressed to this author.