This type of behaviour can have more than one purpose. It can be to dry out feathers and / or to assist in thermoregulation (control of body temperature). The expanded wings increase the surface area which can be warmed by the suns rays. Cormorants such as that of the white breasted cormorant (top left) and the darter (top right) often perch with wings outspread after swimming.
Birds or animals will orient themselves in a specific direction to the suns rays depending on how quickly and effectively they wish to warm or dry themselves. The marabou storks in the lower images are sunning themselves with wings outspread. The dark colour of the wings of cormorants, darters and marabou storks will absorb more energy than light colours which reflects sunlight.