Great areas to bird is August
1) Luther marsh. why other than the Scissor-tailed Flycatcher presently there. A) 100+ Great Egrets, Bald Eagles, Sandhill Cranes, possible Dickscissels, lots of waterfowl and marsh birds... and my favorite reason the low/absent crowds and a nice drive. From South Mississauga maybe 1.5 hrs
2) Darlington Provincial Park/ Oshawa Second Marsh
My long term stomping grounds
Why is it one of the best areas to consistently bird in the GTA year round. In August the park get large numbers of warblers and very few birders. The secret of success there is to listen for the chickadees and find them. Warblers, vireos, flycatchers roam around in foraging flocks with the Chickadees at this time.
The beach At Darlington is a great spot for beach shorebirds like Red Knots, Sanderlings Ruddy Turnstones. Also the reef offshore of the park has produced Red-throated Loons and Common Eider in the summer
Oshawa Second Marsh/Mclaughlin bay area produces songbirds, lots of waterfowl species, lots of herons etc. Also early raptor migration is readily seen here. Currently this morning the long staying White Pelican was easily seen.
I consistently get over 100 species of birds here in August walking from the Oshawa Second Marsh to the campgrounds In Darlington Provincial Park
3) Beeton Sod farms
Massive area of sod farms which is favored by dry tundra shorebirds species like plovers, baird's Sandpipers and Buff-breasted Sandpipers
4) Cranberry Marsh in whitby. Produces a great diversity of marsh birds and is the site of the Hawk Watch which starts in late August.
There are more but these are off the top my head within 1.5 hrs of Mississagua