Why do Anhingas spread their wings?

The answer may surprise you -- it has nothing to do with oil glands or wetness.

The structure of cormorant and Anhinga feathers decreases buoyancy and thus facilitates underwater pursuit of fishes. Hence, their plumage is not water-repellent, but "wettable."

It has been suggested that the functions of the spread-wing postures in these birds is to dry the wings after wetting. Biologists once thought that deficient production of oils from the preen gland necessitate wing-drying behaviors.

We now know, however, that the degree of waterproofing of feathers is primarily due to their microscopic structure, not to their being oiled. In addition to helping wing feathers to dry, other suggested functions for these postures include regulating body temperature (thermoregulation), realigning of feathers, forcing parasites into motion to ease their removal, and helping the perched bird to balance.

Spread-wing postures may serve different purposes in different species.

Anhingas, for example, have unusually low metabolic rates and unusually high rates of heat loss from their bodies. Whether wet or dry, they exhibit spread-wing postures mostly under conditions of bright sunlight and cool ambient temperatures, and characteristically orient themselves with their backs to the sun. Thus, it appears that Anhingas adopt a spread-wing posture primarily for thermoregulation -- to absorb solar energy to supplement their low metabolic heat production and to offset partly their inordinately high rate of heat loss due to convection and (when wet) evaporation from their plumage.

Cormorants, in contrast, apparently use spread-wing postures only for drying their wings and not for thermoregulation. Although cormorant plumage also retains water, only the outer portion of the feathers is wettable, so an insulating layer of air next to the skin is maintained when cormorants swim underwater. The difference in feather structure may explain why cormorants can spend more time foraging in the water than Anhingas, and why cormorants can inhabit cooler climes, while the Anhinga is restricted to tropical and subtropical waters.

source: The Birder's Handbook, Simon & Schuster, 1988, pp. 25-27