Ecological and Reproductive Relationships of Black-Capped and Carolina Chickadees

Abstract

THE pattern of coexistence of populations differentiated in geographical isolation depends on their reproductive and ecological relationships. For distinct populations to exist sympatrically, two conditions must be met: the populations must be reproductively isolated and they must avoid continued competition. When reproductive isolation is lacking, the two forms will not persist sympatrically because swamping-out of differences between them will result in their being reunited as a single species. Continued competition will result in the more successful species spreading throughout the common range and replacing the second, unless a region or situation is met where the second is the more successful (Lack, 1944). This study is concerned with Black-capped and Carolina chickadees, Parus atricapillus and P. carolinensis. These two species of birds are extremely similar or identical in every aspect of size, weight, plumage, life history, and voice investigated (Brewer, 1959; also see Dwight, 1900; Odum, 1941, 1941a, 1942; Bent, 1946; Lunk, 1952; Tanner, 1952; Brewer, 1961). The probable origin and dispersal of these two species and their present-day geographical pattern of distribution are examined in light of their ecological and reproductive relationships. The period of study extended from October 1954 to June 1959, with scattered observations past that date. Most of the field work was done in Illinois, but supplemental observations were made in Indiana, Michigan, Missouri, North Carolina, Tennessee, and Texas. Principal areas of observation in Illinois were located in the following counties: Bond, Champaign, Clinton, Coles, Cumberland, Douglas, Effingham, Fayette, Jackson, Piatt, Randolph, St. Clair, Shelby, Washington, and Williamson.