What can cause an adult bird to die inside a birdhouse or nest box?

For the most part, nest boxes are safe places for birds, but they can still be vulnerable to predation and can't protect birds if food supplies are low. Photo by Mike Thirkell via Birdshare. — For the most part, nest boxes are safe places for birds, but they can still be vulnerable to predation and can’t protect birds if food supplies are low. *Photo by Mike Thirkell via Birdshare.*

There are a few reasons why adult birds may die in a nest box.

If it had obvious injuries, especially on its head, it may have been killed by a House Sparrow or European Starling trying to take over the box. In most cases, though, these competitors toss out birds after they kill them.
Was the inside front of the box, below the hole, rough or grooved? Sometimes birds get stuck inside boxes because the inside walls are so smooth that they can’t climb out. Tacking sandpaper or small strips of wood, making sort of a ladder, will prevent this in the future.
Sometimes an infestation of blowflies or other parasites can become so intense that it kills not only nestlings but also adults. If there was no sign of dead young with the adult, that’s probably not the answer in this case.
Some wood preservatives may release harmful gases, especially in hot weather. Make sure any paints or varnishes that you use on your nest boxes are rated safe for indoor or playground use.
It is also possible that migrants may arrive before adequate food is availble for them to survive. For example, Tree Swallows migrate a long way—some of the birds that nest in northern Canada and Alaska winter down in Central America. If they arrive when the temperature is too cold for flying insects, their primary food, they may die of starvation or hypothermia. This is probably what happened to your swallow, especially if swallows or bluebirds have used your nest box successfully in past years.

Nestwatch, a citizen-science project of the Cornell Lab of Ornithology, has a wealth of information for people who have nest boxes. For features of a good nest box, check out the page, “Features of a Good Birdhouse.” You might also like to join Nestwatch and let us know what is happening in your nest boxes.