Northern Elephant Seals and California Sea Lions: New Hosts for Cattle Egrets

Abstract

The colonization and range expansion of the Cattle Egret (Bubulcus ibis) along the Pacific coast of the United States during the last two decades is well documented (Cogswell, H. L., Water birds of California, Univ. of California Press, Berkeley, 1977). The species has established several breeding colonies in California, including one at the Salton Sea where several thousand birds now breed annually (L. Jones, pers. comm.). Since the late 1970's, egrets have also been regular winter visitors on the southern California Channel Islands (L. Jones, pers. comm.). We report here on interactions observed between foraging Cattle Egrets and northern elephant seals (Mirounga angustirostris) and California sea lions (Zalophus californianus) on San Nicolas Island, California. We observed five immature Cattle Egrets on San Nicolas Island on 10 days between 5 December 1980 and 25 January 1981, while conducting research on the pinnipeds. Egrets spent most of each day foraging near breeding northern elephant seals on two beaches at the extreme southwestern end of the island. The egrets moved among small groups of breeding elephant seals, striking at kelp flies (Fucellia costalis, possibly other species, especially Coelopa vanduzeei), which densely covered some of the larger seals. They gleaned flies from adult and subadult males more often than they did from juveniles or females, perhaps because the bulls were more tolerant of them and also were more densely covered with flies. Flies were widely distributed over the seals' bodies, but were heavily concentrated around anal and penile openings, eyes and mouth, and especially wounds. An egret would stalk flies for a short period, wagging its head from side to side, and then remove them with a quick jab. The egrets also fed on flies on the sand and kelp between the seals in the same manner, and on various insects in the coastal vegetation, which is dominated by iceplant (Gasoul sp.). They generally foraged singly when gleaning flies from seals but in groups when in the iceplant. Although sea lions also occurred on these beaches, we did not see birds glean flies from them during this period. On 12 days between mid-January and early May we also observed immature Cattle Egrets feeding from and among elephant seals and sea lions on several other beaches. On 4 April, for example, we saw an egret perched on a dead sea lion removing flies from the carcass. On 8 and 9 May, another egret spent several hours gleaning flies from the carcasses of sea lion pups. Adult and immature Western Gulls (Larus occidentalis), which had been feeding on the dead pups earlier, were displaced by egrets, and would approach no closer than 1 to 2 m while an egret was near the carcasses; they did, however, resume feeding on the carrion once the egret departed. Occasionally, gulls attempted to approach carcasses on which egrets were foraging. When this happened, the egret would become alert, straighten its neck, and look around without moving from its perch. This posture was generally sufficient to cause gulls to retreat with their heads tucked in California sea lions were present during this period on several beaches where egrets and northern elephant seals occurred, but we saw an egret removing flies from a live sea lion only once (9 May). Sea lions may be less tolerant to gleaning attempts by egrets. The Cattle Egret's opportunistic foraging behavior and use of food sources avoided by other herons, as illustrated by our observations, is probably an important factor in its success in colonizing the New World and expanding its range in the Old World. We thank Judy Newman and Pamela Yochem for sharing their observations with us and J. R. Jehl, Jr., J. Diamond, D. Power, W. T. Everett, H. L. Jones, G. McCaskie, L. C. Binford, and J. C. Ogden for reviewing the manuscript.