Species at Risk: Golden Eagle Predation on Arid-Land Foxes
Clark, Howard O., Jr., Endangered Species Update
The Golden Eagle (Aquila chrysaetos) in California typically inhabits grassland foothills with scattered oaks (Quercus sp.), sycamores (Platanus racemosa), or large gray pines (Pinus sabiniana), with sustainable California ground squirrel (Otospermophilus beecheyi) and black-tailed jackrabbit (Lepus californicus) populations, either along the Coast Range or the Sierra Nevada (Grinnell and Miller 1944). The Golden Eagle is federally protected under the Bald Eagle Protection Act of 1962 (U.S. Fish and Wildlife Service 1978). Nearly 70% of direct and indirect Golden Eagle deaths are from accidental trauma, such as collisions with vehicles, power and transmission lines, and other structures, such as wind turbines (Kochert et al. 2002). Poisoning, electrocution, and illegal shooting also take their toll on Golden Eagles. Loss and destruction of habitat due to wildfires and human developments have caused loss of shrublands that support jackrabbits and have led to a decline in Golden Eagle populations (Kochert et al. 2002).
Also associated with these rolling grassland habitats are several carnivore species, including the federally endangered San Joaquin kit fox (Vulpes macrotis mutica--U. S. Fish and Wildlife Service 1998). In the literature, there are reports of predation of fox species by Golden Eagles, including Channel Island gray foxes (Urocyon littoralis) and the swift fox (Vulpes velox) of the Great Plains. If Golden Eagles are known to prey on these species, it is also likely that they prey upon the San Joaquin kit fox in California. Herein I review the literature regarding predation of arid-land foxes by Golden Eagles and its applicability to eagle-fox interactions within the Central Valley of California.
Golden Eagles are strong birds and have been known to kill ungulates, including mountain sheep (Ovis Canadensis--Bleich et al. 2004). One bird can carry up to 3.5 kg in flight (Kochert et al. 2002), and can easily take a coyote pup (Ingles 1965). Golden Eagles have also been known to take domestic calves (Phillips et al. 1996). Kit foxes, which weigh 2.3 kg on average, do not seem to be a challenge for this powerful raptor. However, the literature is sparse in documenting kit foxes and other arid-land fox species as prey items, either because it has not been a common research objective, or these interactions are infrequent, and when observed, are not being reported.
Golden Eagles are typically diurnal whereas kit foxes are nocturnal, providing an unexpected predator-prey interaction. In the northern range of the San Joaquin kit fox (Alameda, San Joaquin, and Contra Costa counties), a common prey item for the fox is the California ground squirrel which is a diurnal species (Orloff et al. 1986). The lack of well-distributed and abundant kangaroo rat (Dipodomys sp.) populations has likely led the kit fox to prey-shift to the ground squirrel, which makes the fox vulnerable to diurnal aerial predators (Clark et al. 2007).
The northern range of the kit fox also occurs in ideal Golden Eagle habitat. The rolling oak savannah habitat of the northern range has been reported to support the highest density of Golden Eagles in the world (Franklin et al. 1998, Hunt et al. 1998). The additional predatory pressure of Golden Eagles on kit foxes in the northern range may contribute to the currently declining San Joaquin kit fox populations in that portion of California (Clark et al. 2007), although this is conjectural. In the past, Grinnell et al. (1937) documented Golden Eagle predation on San Joaquin kit foxes in Fresno County. Arthur Oliver, a trapper interviewed by Grinnell, stated that trapped foxes were sometimes destroyed by Golden Eagles, which also occasionally took adult free-roaming foxes.
In the 1980s, portions of the northern kit fox range were targeted for an extensive rodent poisoning campaign, which left large expanses of grassland devoid of the California ground squirrel for many years (Orloff et al. …