Why do most pelecaniform birds have reduced or closed external nostrils?

The reduction or closure of external nostrils in most pelecaniform birds is primarily an adaptation for diving and swimming underwater. By minimizing or eliminating these openings, the birds prevent water from entering the respiratory system during submersion.1

In species where the nostrils are nonfunctional or closed, such as boobies, cormorants, and anhingas, air is instead drawn into the mouth cavity through secondary external nostrils—slitlike openings located at the angle of the mouth—before reaching the lungs via the glottis.1

Additionally, this anatomical modification is linked to the birds' physiological need to manage salt intake. Marine pelecaniforms possess lateral nasal glands that excrete concentrated sodium chloride. In species with closed nostrils, this saline solution is directed from the internal nares in the roof of the mouth to the end of the bill, rather than flowing out through the nostrils as it does in other seabirds.2