The tawny owl is one of a small number of owl species in which calling from inside the egg has been recorded, says Mike Toms. Before a chick hatches from its egg it has to make the switch from breathing within the egg's amniotic fluid to breathing in air.
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As hatching approaches the chick penetrates the air chamber at the blunt end of the egg, a result of the evaporation of water from the egg throughout its incubation. It is at this stage, two or three days before hatching that the chick begins to call.
It is thought that the calling alerts the parent to the imminent arrival of its offspring, and may also encourage the adult to help the youngster break free from the egg once it has pierced the eggshell with its egg tooth.
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Top image: a tawny owl's egg by Klaus Rassinger und Gerhard Cammerer, Museum Wiesbaden, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
