PREDATOR-PREY INTERACTIONS BETWEEN EAGLES AND CACKLING CANADA AND ROSS GEESE DURING WINTER IN CALIFORNIA

Cackling Geese (Branta canadensis minima) were preyed on heavily in no rtheastern California by Golden Eagles (Aquila chrysaetos) and less co mmonly by Bald Eagles (Haliaeetus leucocephalus) in 1985-1990. Eagle p redation on Cackling Geese was minimal in other wintering locations in California. In the Klamath Basin, eagles killed Cackling Geese most f requently soon (< 10 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 t o other sites. In Big Valley, the larger Great Basin Canada Goose (Bra nta canadensis moffitti) was attacked by Golden Eagles only once durin g 88 observation days, while the smaller Cackling and Ross' geese were attacked by Golden Eagles a total of 27 times. Moreover, Cackling Gee se in Big Valley were attacked and killed at least twice as often as R oss' 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 wi th less eagle predation. The antipredator behavior of Cackling Geese i ncludes maintaining high levels of vigilance, occurring in large, dens e flocks, and roosting on water during nonfeeding periods. When attack ed by eagles, Cackling Geese used socially-coordinated and speed-based escape tactics.