I was cleaning out my nest boxes and I found a dead adult Tree Swallow in one of them. How can this happen?

Many factors can affect avian mortality. Tree Swallows migrate quite a long way, some from Central America. If they arrive after such a long journey to wet, cold conditions, and a lack of insects (which is what they feed on), they may die. If you find dead young Tree Swallows in the box, make sure the inside wall of the entrance hole is roughly grooved so that the young birds have something to grip onto when fledging from the box.