At Presqu'ile provincial park, roughly 40 km to the south, the cowbird is a common migrant and summer resident, mid-March to mid-November (LaForest, 1993, pp.375-376). In Peterborough county, to the northwest, the cowbird is a nest parasite known to invade the seasonal homes of at least 20 species of bird, both larger and smaller than the interloper (Sadler, 1983, pp.158-159).
The brown-headed cowbird never builds its own nest, and originally was restricted to the North American prairies. European settlement and forest clearances expanded grasslands and facilitated the current distribution which includes almost all southern Ontario, as well as lands north of Lake Superior (Cadman et al., 1987, pp.482-483). In the revised Bird Breeding Atlas (Cadman et al., 2007, pp.602-603) the cowbird is most abundant in the most intensively cultivated areas, i.e., between lakes Huron and Erie, and north of the Bay of Quinte.
Cadman,MD, Eagles,PFJ and Helleiner,FM (1987) Atlas of the Breeding Birds of Ontario. Federation of Ontario Naturalists and Long Point Bird Observatory, published by University of Waterloo Press, 617pp.
Cadman,MD, Sutherland,DA, Beck,GG, Lepage,D and Couturier,AR (editors) (2007) Atlas of the Breeding Birds of Ontario, 2001-2005. Bird Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature, 706pp.
LaForest,SM (1993) Birds of Presqu'ile Provincial Park. Friends of Presqu'ile Park / Ontario Ministry of Natural Resources, 436pp.
Sadler,D (1983) Our Heritage of Birds: Peterborough County in the Kawarthas. Peterborough Field Naturalists / Orchid Press, Peterborough, ON, 192pp.