How do cormorants and anhingas reduce buoyancy for underwater swimming?

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Three eastern white pelicans with black-tipped wings take off from a muddy shoreline, wings outstretched, with blurred green vegetation in the background. — Pelecaniform European white pelicans (*Pelecanus onocrotalus*) in flight.

Cormorants and anhingas reduce buoyancy to facilitate underwater swimming primarily through the physical properties of their plumage and skeletal structure.

Wettable Plumage: Unlike many other pelecaniforms that possess water-repellent feathers, cormorants and anhingas have large contour feathers that are wettable. These feathers trap very little air when the birds are submerged, which keeps their buoyancy low and allows them to dive directly from the surface without needing the momentum of an aerial dive.1
Skeletal Density: While many aerial birds have extensively pneumatized (air-filled) bones, this feature is least developed in anhingas. By having less pneumatization, anhingas maintain a higher specific gravity, which helps them remain underwater even while motionless.1

These adaptations allow these birds to be highly effective underwater hunters compared to more buoyant species that must plunge from a height to reach significant depths.1