Western Bluebirds are among the 88 species of cavity nesting birds that often nest in birders’ artificial nest boxes.

By now your bluebirds, chickadees, wrens, Tree Swallows and other cavity nesting birds have finished raising their nestlings and all that’s left in the nest box is the old nesting material. It’s time to clean things up, right? Not so fast!

One reason for cleaning out old nests from nest boxes is to reduce parasites, which might deter or affect future nesting. There isn’t much research on this topic, but one study explored the question with House Wrens. House Wrens prefer nest boxes that contain old nests over those from which the nest was removed. Male House Wrens remove old nests before renesting, apparently to reduce initial mite populations. Ornithologists found mite populations were about the same, whether old nests were removed by wrens or people. So, for House Wrens at least, it doesn’t seem to matter if wrens or people clean the nest boxes, the impact on mites is the same.

Bluebirds, in contrast to wrens, don’t clean out old nests—they simply build on top of them. But if you don’t clean out the old nest, as the bluebirds add new nesting material, their eggs will be closer to the entrance hole, perhaps making them more vulnerable to predators.

In a North Carolina study, 71 percent of Eastern Bluebirds choose to move to a nest box that had been cleaned, rather than to build on top of an old nest. Curiously, a Kentucky study reached the opposite conclusion. There, bluebirds preferred nest boxes with old nests. Biologists in the Kentucky study found that parasitic wasps take refuge in the old nests and kill blowfly larvae, a bluebird parasite, so removing old nests could disrupt this helpful process.

Some people prefer to leave old nests in nest boxes so mice or other small mammals can use them for shelter. Others, though, caution that because mice are a host for hantavirus in some areas, you shouldn’t encourage them. Fire ants may be drawn to old nests too, posing a hazard to future bird broods, so cleaning could prevent ant infestations.

Given these somewhat confusing results, what’s a responsible nest box landlord to do? Consult with your local wild bird retail store owner to see if they can offer guidance for your local area. Here in South Carolina, I remove old nests from nest boxes used by Carolina Chickadees, Tufted Titmice, Brown-headed Nuthatches and Eastern Bluebirds after each brood, but I leave the nest from the last brood in each nest box over the winter, cleaning things out in late winter in preparation for the new season.

Article by Peter Stangel

Read more at https://nestwatch.org/connect/blog/to-clean-or-not-to-clean-your-nest-box/