Black-capped Chickadee, Jackson Bottom Wetlands, Hillsboro, Oregon on 13 July 2010 by Greg Gillson.
There is nothing unusual in the above photo. On the free Wednesday morning bird walk I lead, we encountered this Black-capped Chickadee feeding nestlings at a nest box at Hillsboro, Oregon's Jackson Bottom Wetlands Preserve. The nestlings were grown enough to meet the parents at the nest entrance to receive food.
What is unusual, is that White-breasted Nuthatches were feeding nestlings at this same nest box 68 days earlier (see photo below)!
White-breasted Nuthatch, Jackson Bottom Wetlands, Hillsboro, Oregon on 6 May 2010 by Greg Gillson. Click on photo for larger views.
Here is what John K. Terres has to say about Black-capped Chickadee nesting in his book The Audubon Society Encyclopedia of North American Birds. He says that incubation is by both parents, 11-13 days and that the young leave the nest when 14-18 days old. The incubation and fledgling period is nearly the same for White-breasted Nuthatch (12 and 14 days, respectively).
It appears that the nest box might have been unoccupied approximately 42 days between the nesting of the nuthatch and chickadee.