Aggressive Encounters between Tundra Swans and Greater White-Fronted Geese during Brood Rearing

Abstract

Interspecific aggression in waterfowl (Anatidae) is relatively common (McKinney 1965; Kear 1972; Savard 1982, 1984), but interactions leading to mortality of one of the combatants are rarely-observed in the wild. A recent debate (Livezey and Humphrey 1985a, 1985b; Nuechterlein and Storer 1985a, 1985b; Murray 1985) has centered on the proximate and ultimate causes of interspecific territoriality and killing in the steamerducks (Tachyeres spp.), a group of large-bodied anatids. We report here aggressive encounters between Greater White-fronted Geese (Anser albifrons) and Tundra Swans (Cygnus columbianus) during rearing on the Yukon-Kuskokwim Delta, Alaska, which on two occasions resulted in the death of a Whitefronted Goose gosling. Greater White-fronted Goose families aggregate after hatch (Ely 1979) and remain in brood until fledging (C. Ely, unpubl.). Little is known of the behavior of Tundra Swans during rearing, but families remain solitary after hatch, and probably hold territories until the young fledge (Kear 1972). We observed families of Greater White-fronted Geese between 9 July and 10 August 1985 when goslings and Tundra Swan cygnets were between two and seven weeks old. Observations were made from blinds atop 4-m high towers which were placed adjacent to two areas used by the geese during rearing. Without disturbing broods, blinds were entered through canvas tubes connecting the entrances of the blinds to access sloughs. We observed 12 encounters between adult swans and two different groups of Greater White-fronted Goose families. In every instance an adult swan, acting alone, attacked one of the two groups. For six of the incidents we were able to determine that the attacking swan was associated with a mate and cygnets (brood size two to eight); the attacking bird was probably the cob as females rarely fight (Scott 1977). Interactions were always initiated by swans, which often attacked the geese at distances greater than 300 m from their own family. Swans initiated attacks by flying or running (after they molted their remiges) into the middle of a group and repeatedly chasing the nearest bird. During four encounters, the swan grabbed a goose by the nape of the neck and sat or stood on its back, often trampling and biting the back of the neck and pulling out feathers. The pinned goose generally lay motionless on the ground until the swan stepped off, at which time the goose, if capable, fled. Geese responded to charging swans by attempting to run away or by flying (prior to wing molt). If overtaken, geese often threatened (Fischer 1965) the swan, and on two occasions counter-attacked. In the latter instance, adult geese within the group repeatedly bit the swan, and beat the swan with their wings until they were repelled. On one occasion the goose being trampled by a swan was released when another adult goose from the group attacked the swan. Attacks by two different swans (with two and three cygnets) on the same group of geese resulted in the ' Received 6 March 1986. Final acceptance 6 October 1986. 2 Present address: Alaska Fish and Wildlife Research Center, U.S. Fish and Wildlife Service, 1011 E. Tudor Road, Anchorage, AK 99503. 3 Present address: Department of Wildlife Resources, University of Idaho, Moscow, ID 83843.