Preen gland secretion causes hoopoes’ eggs to change colour,
possibly giving signals about the robustness of the mother bird, researchers
have found.
Hoopoe females use cosmetics on their eggs - and the eggs
gradually change colour when they are incubated, from bluish-grey to a more
saturated greenish-brown. This happens because secretion from the preen gland –
a substance birds use to preen and protect their feathers – is transferred from
the female hoopoe’s gland to her eggs directly with the bill and by means of
belly feathers.
(Springer
Science + Business Media)
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