The Foraging Ecology of Parus atricapillus and P. rufescens in the Breeding Season, with Comparisons with Other Species of Parus

Abstract

Situations in which morphologically similar, closely related species coexist in the same geographical area have stimulated interest and investigation by ecologists for several decades. These cases of peaceful are relatively common in the animal world, and offer us an opportunity to learn the mechanisms responsible for them. Studies of these cases have demonstrated the numerous behavioral and ecological adaptations that have been of selective advantage for species in competition with a close relative. Such a situation exists in western Washington where the Chestnut-backed (Parus rufescens) and Black-capped (P. atricapillus) Chickadees occur as common, permanent residents in the lowlands. Because these two species are similar in so many respects-holenesting, insectivorous gleaners of mid-latitude forests and groves-they are potential competitors. A study of these two species was undertaken in an effort to discover the extent and character of their coexistence and to determine the ecological and behavioral characters that adapt each species to the part of the environment that it occupies in the breeding season. The breeding habitats of the two species overlap very little. A description and analysis of their habitat occupancy is presented elsewhere (Sturman 1968). Aspects of the species' foraging ecology are presented in this paper. Species of the genus Parus have drawn the scrutiny of ecologists because they are conspicuous, in many places common, and are important insect predators in forest communities (Kluyver and Tinbergen 1953). European and American workers have described the reproductive behavior (Odum 1941, 1942; Hinde 1952; Gibb 1956; Brewer 1961; Stefanski 1967), habitats (Snow 1954a; Dixon 1961; Sturman 1968), population dynamics (Kluyver and Tinbergen 1953; Lack 1958, 1966; Perrins 1965), and foraging pattern (Hartley 1953; Dixon 1954, 1955; Gibb 1954, 1960; Colquhoun and Morley 1943; Brewer 1963; Root 1964; Smith 1967) of many of the most abundant species.