Seasonal Aspects of the Predatory Behavior of Loggerhead Shrikes

Abstract

-Aspects of the foraging behavior of Loggerhead Shrikes were analyzed in the nonbreeding and breeding seasons in southern California. The birds obtained more food during the breeding season by attacking more often, not by taking larger prey. Attack rate during the breeding season was twice that in the nonbreeding period, with no apparent difference in capture efficiency. No distinct relationship between attack distance and capture efficiency was noted. Time between attacks decreased during breeding in response to greater food demands. Pursuit, encounter, and handling times did not significantly change between seasons. A positive significant correlation between prey handling time and the square of prey length was found. Handling time did not severely reduce the amount of time that could be devoted to hunting during any season. However, total movements averaged considerably higher during breeding than attack rate alone, adding to the energy expenditures of a hunting shrike and possibly placing constraints on the amount of hunting time available. An important part of the daily activity budget of an animal is that portion devoted to foraging. More food is needed during the breeding season, when the nutritional requirements of the nestlings must also be satisfied. Studies of the temporal patterns of foraging behavior thus lend insight into changing energy needs of a species. This study reports on the foraging behavior of the Loggerhead Shrike (Lanius ludovicianus). Impaling (caching) behavior has been discussed by Watson (1910) and Applegate (1977), and methods for carrying prey were noted by Esterly (1917) and Caldwell (1967). Slack (1975) hypothesized that shrikes select small prey that would be easier to catch, instead of larger prey that would provide more food once captured. Food items taken by shrikes were listed by Judd (1898), Stephens (1906), and Wayne (1921). An important paper by Craig (1978) described the various components of shrike attack behavior, mostly during the nonbreeding season. Attack behavior of shrikes might be expected to differ between nonbreeding and breeding seasons owing to the need to obtain additional food during breeding. I report here on differences in the components of a shrike's attack between breeding and nonbreeding seasons. STUDY AREA AND METHODS My 6-ha study area was in the southeast end of the Whittier Narrows Nature Center, South El Monte, Los Angeles County, California (elevation 67 m). The center was bordered by a channeled river on the south and by major roads and freeways on the remaining sides. Vegetation on the site was relatively homogeneous, consisting mainly of a low (< 1 m) growth of mustard (Brassica campestris) interspersed with grasses and herbs. Widely scattered elderberry (Sambucus mexicana) and walnut (Juglans californica) grew to 7 m. Transmission lines supported by three towers crossed the site. A chain-link fence and a dirt road ran parallel with the lines and b rdered the south edge of the site adjacent to the river; the river was about 50 m from the towers. From 24 December 1977 through 23 August 1978 I visited the study area for about 60 h on 31 days. During the pre-breeding season I collected data on seven adult shr kes occupying the area. Data were collected on one pair during the breeding and post-breeding seasons; this pair raised two broods of three young each during