I've used Ken Kaufman's "Field Guide to Birds of North America" extensively, and I really like it as a complement to Sibley. However, it includes all birds of NA, not only Ontario.
I have and like the Kaufman's as well. I also have the ROM book and the Sibley's. I find the ROM book too limited. The Sibley's is great, but rather large and unwieldy for a field guide. The Kaufman's fits in my cargo pants or coat pocket.
