Hi Julie,
I'm also in the same boat (minus all those cats!, there's only 1 that seems to roam our yard). I'm also in the East-end of the city with an average size backyard. Last summer I built a small nest box with about a 1 inch hole in the hopes of luring in a Chickadee or Wren. So far only one Chickadee has poked his head in and looked around. We have a large number of House Sparrows (50-60) that frequent the yard on a daily basis to use the feeders, bird bath, and pond. Also, there are 3 nest boxes that the sparrows use each spring/summer. Don't know if I've ever seen a more aggressive bird than the House Sparrow, they really don't like other birds, especially ones smaller than themselves. Not sure if they'll let any other bird species nest in the same area or not. Time will tell. Should I fill the box with nesting material or not, does this help?
If anyone has had any success with simular setups, we'd love to hear from you.
Thanks,
Walter