In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here is a question from our September/October 2018 issue.
I just repaired some 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 nest was complete, so I assume the birds were in the house before the wasps arrived. If that’s the case, why didn’t the bird eat the wasps before they became too numerous? — Dan Innamorato, via the Internet
Wasps typically will not attempt to build a nest in a box already occupied by bluebirds, although it depends on the species of wasp and probably the availability of appropriate nesting sites for the insects. A wide variety of North American birds have been known to eat bees and wasps, but most reports I have located involve smaller and more docile species of stinging insects than the fairly aggressive European paper wasp, which commonly uses nest boxes. A bluebird might be more apt to abandon a box invaded by wasps rather than try to eat its way out of the situation.
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Similarly, bluebirds would not try to nest in a box that already contained wasps. Without a nest to defend, the wasps would probably not attempt to sting the bluebird. Although I’ve not found any data regarding how many wasp stings it would take to kill a bird, the stings would have to penetrate the feathers, perhaps requiring multiple attempts. It seems improbable that wasps could kill a healthy adult bluebird that could simply flee the scene.
It’s hard to say what the sequence of events might have been in this situation. I agree the bluebirds were the first occupants. The dead bluebird may have been the nest owner and died before eggs were even laid. Or the bird could have died in the box after it was used. Parasites, starvation, or hypothermia are all causes of adult bluebird deaths. I think it’s most likely that at some point after that, the wasps found the box and built a nest.
For tips on how to prevent wasps from building nests in nest boxes, see this post from 2016.