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Why are tree swift eggs glued to their nests with saliva?

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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 build tiny, cup-shaped nests on small, high-up tree limbs that are frequently moved by the wind. Because these nests are barely large enough to hold a single egg, the parent glues the egg to the bottom of the nest using its sticky saliva.

This practice prevents the egg from being displaced or falling out of the nest due to wind agitation. Furthermore, while most birds manually turn their eggs during incubation, it is believed that the constant waving of the nest branch in the wind may act as a substitute for this behavior, moving the egg's contents within the shell to ensure proper development.1