Why do cormorants and other pelecaniforms sometimes hold their wings open in the sun?

Cormorants and anhingas possess wettable plumage that traps little air, which reduces their buoyancy and allows them to swim underwater effectively. Because their feathers become wet, these birds frequently climb out of the water and adopt a characteristic spread-wing posture to dry their wings.1

While this behavior is essential for drying feathers after swimming, many species of pelecaniforms also habitually spread their wings in "sun-bathing" postures even when their feathers are already dry. The specific function of this behavior in those instances is not well understood.1

It is important to note that while some birds use wing-drooping to manage heat stress—such as the masked booby, which may droop its wings to expose shaded feather areas for cooling—this is a distinct physiological adaptation for thermoregulation rather than the drying behavior observed in cormorants.2