White-breasted Nuthatches are one of the 85 species of North American birds that are classified as cavity nesters. Although the young of birds nesting in cavities are protected from the elements, they are still vulnerable to predators.
White-breasted Nuthatches have an unusual strategy for discouraging uninvited guests. They “bill-sweep” with an object, usually a crushed insect, in their bill, sweeping back and forth on the tree both outside and inside the nest, often for many minutes at a time. It’s thought that chemical defense secretions from the crushed insect may discourage squirrels from entering the cavity. (Red-breasted Nuthatches apply sticky conifer resin to both the inside and outside of their nesting cavity, which presumedly serves the same purpose.)
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