Britannica
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How do boobies and cormorants breathe when their bills are closed?
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Pelecaniform
European white pelicans (Pelecanus onocrotalus) in flight.
In boobies and cormorants, the primary external nostrils are reduced or closed, which is an adaptation for their aquatic lifestyles. When their bills are closed, these birds breathe through secondary external nostrils.1
These secondary openings are located as slitlike structures at the angle of the mouth on each side. They function where a horny flap, known as the jugal operculum, at the base of the upper mandible overlaps the lower mandible, allowing air to enter the mouth cavity and reach the lungs via the glottis.1