Distribution and Behaviour of Birds on Small Islands in Northern Minnessota

Abstract

Seven small islands and two nearby mainland plots were visited weekly throughout the late spring and summer. Of thirty-five bird species observed on one or more of the islands, only nine nested or occurred regularly on any particular site. Yellow-rumped warblers and song sparrows inhabited all of the islands, red-eyed vireos and chipping sparrows used the three largest sites, while pine warblers, blackburnian warblers and red-breasted nuthatches were regularly seen on the largest two. Isolation seemed to influence only the number of transient visitors, since distant islands supported the same regular species as similar-sized areas close to the mainland. There was some evidence that yellow-rumped warblers and song sparrows expanded their use of microhabitats in the absence of potential competitors. This was not consistent, however, because both species used a wide variety of foraging sites wherever they occurred. I suggest that the ability to utilize a large portion of available microhabitats is at least partly responsible for their success on small islands. Such ecological considerations help to explain island patterns where many species have opportunities to colonize, as they apparently do on the islands discussed here.