The chestnut-sided warbler is a rarely-seen local visitor which we have noted just 4 times, in the three-week window 01-19 June, over a 22-year period.
The northern extremity of the bird's breeding range extends across southern Canada from the prairies to the maritimes (Godfrey, 1979, pp.335-336).
It breeds across Ontario in a broad swathe west from the Ottawa Valley across Muskoka and Algoma to a narrow band running along the Minnesota border to Manitoba (Cadman et al., 2007, pp.476-477). It quite probably breeds locally, but we have no firm evidence --- this warbler avoids old-growth forest in favour of dense, shrubby second-growth habitat, and evidence suggestive of breeding includes the appearance of adults carrying food, or distraction displays by females upon the arrival of predators.
This warbler is quite common as a migrant at
Presqu'ile provincial park, roughly
40 km to the south.
Extreme early and late dates in the park are 01 May to 30 September
(LaForest, 1993, pp.312-313).
Similar dates are noted in Peterborough county, to the northwest
(Sadler, 1983, p.132).
References
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.
Godfrey,WE (1979) The Birds of Canada. National Museums of Canada, Ottawa, 428pp.
LaForest,SM (1993) Birds of Presqu'ile Provincial Park. Friends of Presqu'ile Park, Brighton, Ontario / 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.