Today I went to check on a collection of bird boxes a friend and I installed to see if there was any activity in or around the boxes. We installed the boxes a few weeks ago and literally had Tree Swallows following after us as we nailed the boxes up, inspecting each box not even a minute after they were installed. Last week there was evidence of nest building in one box and this week there was a completed nest and six Tree Swallow eggs in the box. I wasn't able to check the other boxes this morning because of impending rain. A House Wren seemed interested in another box and appeared to haul in a few sticks the last time I checked.
 
One egg is a different color than the other eggs. I am not sure if this off-colored egg is a Tree Swallow egg or if it is something else. It doesn't appear to be a Brown-headed Cowbird egg.