Just finished cleaning the nest boxes I manage at a golf course near our home. There are 40 boxes on the trail. Use splits about 50/50 between Tree Swallows and Eastern Bluebirds. House Wrens and Black-capped Chickadees use a handful of the boxes.
Bluebirds boxes are simple to clean: just pull the neat, unsullied grass nest from the box. Chickadee nests are made of moss, tightly fitted to the dimensions of the box base. They come out intact, and clean. Wrens weave sticks into hollow forms that fill boxes. Sometimes I can slide nest out of the box intact. I get to marvel at this piece of intricate engineering. Wren nests also are clean.
Clean means no feces in the nest. Bluebirds, chickadees, wrens, most songbirds keep a clean house by removal of chick feces in a most ingenious way. The chick packages its waste in a strong membrane that is produced upon the little bird being fed. This is called the fecal sac. The parent bird brings food and leaves with the sac.
Which brings us to Tree Swallows.
Tree Swallows remain in the nest for as much as a week longer than, say, bluebirds. Bluebirds fledge in 13 or 14 days, swallows in about 20. Swallows produce fecal sacs for all but the final five days or so. As they near fledging, the sac mechanism slows up and quits. The swallows then just relieve themselves where they sit.
A used Tree Swallow nest is a nasty thing. The grass and feathers used as construction material, when mixed with feces, become bricks. They stick to the box. I pry them out.
Violet-green Swallows, the western version of our Tree Swallow, have evolved to what I consider a more advanced state. When their fecal-sac mechanism gives out they point their tails to the corners of the nest (or box) when the need arises. The nest stays relatively clean.
So what is it about the evolution of these two closely related species that produces different approaches to the same problem?
Below, a fresh Tree Swallow nest. I have no photos of the post-nesting mess.