A Comparison of Foraging Behavior among Permanent, Summer, and Winter Resident Bird Groups

-Foraging behavior of permanent resident bird species in summer and winter was compared with that of summer resident and winter resident bird groups of a floodplain habitat along the lower Snake River in southeastern Washington. Ten criteria were used to compare foraging differences between seasonal groups. In summer, permanent residents differed significantly from invading summer residents in 8 of 10 foraging categories. In summer, permanent residents foraged on the forest edge, in the tree-shrub physiognomic type, and usually on the ground. Summer residents differed by feeding more frequently in trees or in the air and by gleaning insects off mulberry or willow leaves. In winter, permanent residents foraged on the ground of the forest edge, either in the tree-shrub or grass-herb physiognomic types. Winter residents fed in trees or on the ground, in the woods or forest edge, and in the tree-shrub physiognomic type. When in trees, these birds usually gleaned insects off small branches and dead leaves. Separation of foraging behavior was approximately six times greater in winter than in summer. Many avian ecologists, including Lack (1944), Hinde (1958), MacArthur (1958), Holmes and Pitelka (1968), Willson (1971) and Sealy (1973) have believed that differences in bird foraging behavior are as important to niche segregation as differences in food items selected. Even though similar food items may be eaten, different methods of foraging expose birds to prey items in different locations and thus potentially reduce competition and allow coexistence. Studies of niche segregation have shown separation of foraging behavior of a few closely related species (e.g., Hartley 1953, Gibb 1954, MacArthur 1958, Norris 1958, Recher 1966, Sturman 1968, and Morse 1970). Few ecologists have attempted to analyze the foraging differences among groups of less closely related species or among most species of an avian community (but see Cody 1968, Wiens 1969, Willson 1970, Pearson 1971, Austin and Smith 1972, and Emlen 1972). Leck (1972) found that migrant frugivores in the tropics generally were attracted to superabundant fruit resources, and thus did not seriously compete with local frugivores. Karr (1976:456) summarized his and others' work: "In general, it appears that the evolutionary strategies of migrant birds are keyed to the exploitation of superabundant and/or sporadically available resources in their tropical wintering areas." Blondel (1969:312) noted that the "food niche of wintering birds is never occupied by other birds." Emlen (1972) found that winter invader granivores in Texas were primarily small-seed foragers whereas permanent residents were primarily large-seed foragers. Willson (1970:171) stated that "neither the influx of spring foods nor the distribution of spring foods changes the degree of specialization of these birds [several species of permanent residents in Illinois]." If this is true, then invading species should evolve foraging habits that differ from those of permanent residents. Foraging preferences of permanent residents might cause invading summer and winter residents to respond to this competitive group in similar ways. Moreau (1972) reported that 65 of 90 (72%) migrant species in six African habitats appeared not to compete seriously with permanent residents. Moreau (1972) and others stated that migrants generally exploit food resources not fully used by permanent residents. Herrera (1978) found permanent residents of a Mediterranean bird community to be more specialized feeders than migrants. Finally, Holmes, Bonney, and Pacala (1979) found height, location (trunk, branch, etc.), and tree species to be the most important criteria for separating birds of a New Hampshire bird community into four foraging guilds. The purposes of this study were to determine if summer and winter migrant birds forage in a riparian habitat in southeastern Washington so that they minimize competition for food with permanent residents and to identify the most important foraging criteria which segregate resident groups. STUDY AREA AND METHODS Alpowa Creek Study Area was an 11.5-ha plot located along the lower Snake River in southeastern Washington 11.3 km west of Clarks-