Hi Robin,

Nesting birds are most susceptible to abandoment during the nestbuilding, egg laying, and early incubation phases. Females lay eggs in the morning, so I'd check the boxes in the early afternoon. That's what I do and I've never had any problems.

By the way, check out Cornell's Birdhouse Network at:
http://www.birds.cornell.edu/birdhouse
Note the Chestnut-backed Chickadee is one of their "most wanted" species.

Mike Donahue
Seattle