We have two bird boxes around our house. The ideal time to clean them out is about August or September, right after nesting season. At work I'm very focused on deadlines and schedules. At home I downshift into a slower, less frenzied pace. Because of that, the bird boxes usually don't get cleaned until January or February, just before nesting season.
Here's the location of bird box #1, about 12 feet up on the trunk of a large Douglas fir just on the other side of our back fence (photo in May 2010).
Bird box #1 has been used by chestnut-backed chickadees for almost 20 years, except for one year when I didn't clean it out and it was colonized by bumblebees.
Bird box #2 was moved out by our back door last year and black-capped chickadees nested here for the first time. Before I moved it by the back door it was hanging in an old camellia bush and was used only once by house wrens.
This year's clean out shows the chickadee's nests constructed of moss and lined with soft fuzz and dog hair, no doubt from our dog.
Doing yard work, trimming back western sword ferns that create a fern grotto in a corner of the backyard, I found a couple nests. These are probably spotted towhee nests, who keep their nests low. The outer foundation layer is shredded cedar bark then lined with woven grass.