Predator-prey Interactions between Eagles and Cackling Canada and Ross' Geese during Winter in California

-Cackling Geese (Brunta canadensis minima) were preyed on heavily in northeastern California by Golden Eagles (Aquila chysaetos) and less commonly by Bald Eagles (Haliaeetus Zeucocephalus) in 19851990. Eagle predation on Cackling Geese was minimal in other wintering locations in California. In the Klamath Basin, eagles killed Cackling Geese most frequently soon (<IO days) after the geese arrived in the fall. Eagles killed fewer Cackling Geese in the Klamath Basin when Cackling Geese were less common than Ross’ Geese (Chen rossii) and Lesser Snow Geese (C. caerulescens caerulescens). We also examined spatial and temporal (daily, seasonal, and annual) variation in eagle predation on geese at a smaller scale in Big Valley, California. Most eagle-caused flushes of geese occurred during mid-day when the geese were using traditional day-roost sites. Roosting on water with most other Cackling and Ross’ Geese in Big Valley reduced the frequency of eagle attacks relative to other sites. In Big Valley, the larger Great Basin Canada Goose (Brunta canadensis mojfitti) was attacked by Golden Eagles only once during 88 observation days, while the smaller Cackling and Ross’ geese were attacked by Golden Eagles a total of 27 times. Moreover, Cackling Geese in Big Valley were attacked and killed at least twice as often as Ross’ Geese because Cackling Geese often grazed in pasture where Golden Eagle attacks were more frequent. When feeding on pasture, geese did not increase time spent vigilant or flock size compared to habitats with less eagle predation. The antipredator behavior of Cackling Geese includes maintaining high levels of vigilance, occurring in large, dense flocks, and roosting on water during nonfeeding periods. When attacked by eagles, Cackling Geese used socially-coordinated and speed-based escape tactics. Received 2 June 1993, accepted 15 Sept. 1993. An individual bird may join a flock to reduce the chance of being attacked or of being caught when attacked (see Bertram 1978). Birds in flocks may be safer than solitary individuals for at least three reasons. Individuals in a group may detect predators better or earlier than smaller groups or solitary individuals (Pulliam 1973, Siegfried and Underhill 1975, Kenward 1978, Lazarus 1979). A predator which attacks a group of prey may become confused and catch fewer prey (Neil1 and Cullen 1974, Milinski 1979, Landeau and Terborgh 1986). Finally, an individual in a group may reduce its chance of being caught simply because of a dilution-effect (Foster and Treherne 198 1). Flocking creates a tradeoff between avoiding predators and feeding efficiently (Powell 1974, Caraco 1979a, b, Caraco et al. 1980, Pulliam I Dept. of Wildlife and Fisheries Biology, Univ. of California, Davis, California 95616. ’ Present address: Dept. Biology, Univ. South Carolina, Columbia, South Carolina 29208. ’ Deceased.