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In the “Since You Asked” section in each issue of BirdWatching, editor-in-chief Julie Craves answers readers’ questions about birds and their behavior. Here’s a question from our September/October 2018 issue.
A European paper wasp rasps wood fibers to build a nest. Photo by pjt56 (Wikimedia Commons)
I just fixed up a few bluebird houses and found an abandoned paper wasp nest and a dead bluebird in one. Did the wasps sting the bird to death? The bird’s nest was finished, so I assume the birds were in the house before the wasps arrived. If this is the case, why didn’t the bird eat the wasps before they became too numerous? — Dan Innamorato, via Internet
Wasps will generally not attempt to build a nest in a box already occupied by bluebirds, although this depends on the species of wasp and likely the availability of suitable nesting sites for the insects. A wide variety of North American birds have been known to feed on bees and wasps, but most reports I have found involve smaller, more docile stinging insect species than the wasp. European paper, quite aggressive, which commonly uses nest boxes. A bluebird might be more inclined to abandon a box overrun with wasps rather than trying to wiggle its way out of the situation.
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Likewise, bluebirds would not try to nest in a box that already had wasps in it. Without a nest to defend, the wasps probably wouldn’t try to sting the bluebird. Although I couldn’t find any data on the number of wasp stings needed to kill a bird, the stings would have to penetrate the feathers, which might require several attempts. It seems unlikely that wasps could kill a healthy adult bluebird that might simply flee the scene.
It is difficult to say what the sequence of events might have been in this situation. I agree that bluebirds were the first occupants. The dead bluebird may have been the owner of the nest and died before the eggs were even laid. Or the bird could have died in the box after use. Parasites, starvation or hypothermia are all causes of mortality in adult bluebirds. I think it’s very likely that at some point after this the wasps found the box and built a nest.
For tips on how to prevent wasps from building nests in birdhouses, see this 2016 article.