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How do tree swifts keep their eggs from falling out of small nests?

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Illustration of a chestnut-collared swift (Cypseloides rutilus) in flight, showing its dark brown body and wings with a distinctive reddish-brown collar around its neck. — Apodiform Chestnut-collared swift (*Cypseloides rutilus*)

Tree swifts construct tiny, cup-shaped nests on small, high tree limbs that are frequently moved by the wind. To prevent their single egg from falling out of these precarious structures, the parent glues the egg to the bottom of the nest using its own sticky saliva.1

This unique method of securing the egg also serves a secondary purpose: because the egg is fixed in place, it cannot be turned during incubation as is common with most other birds. It is believed that the natural swaying of the nest branch in the wind may act as a substitute for this turning process by moving the egg's contents within the shell.1