Where do Tree Swallows build their nests?

Tree Swallows are cavity-nesters. They nest inside holes in trees. However, Tree Swallows have a problem.

They are unable to dig their own cavities. Under natural conditions they must find an old woodpecker hole or a cavity in a dead tree if they are going to raise any young. Perhaps because there is usually a shortage of natural nest sites, Tree Swallows quickly accept nest boxes, even ones far from water.

And although they are not colonial like some other swallow species, pairs of Tree Swallows will nest as close as 100' from one another if there are suitable nest sites and a good food supply. This lets us create Tree Swallow projects with multi-box grids. More.

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