A study by Robin Sellers on wintering cormorants in the river Severn provided strong evidence for wing-drying as the function of this behaviour. Cormorants engage in the open-wing posture almost exclusively after having been diving, when their plumage is wet. After a bout of diving, the cormorant will do some bathing movements in the water, flap its wings and shake its plumage before flying to a perch, where the wing-spreading takes place. Not only the wings, the tail is also spread at the same time. The longer they've been immersed, the longer they will stay open-winged.
Wing-spreading was almost exclusively observed in individuals that had been in water the previous 30 min, and most of the cormorants that had been in the water did engage in wing-spreading shortly afterwards.
Wing-spreading is often followed or preceded by preening the plumage. The duration of the behaviour is also inversely correlated to temperature and wind-speed, supporting the drying hypothesis. Rain tends to inhibit the behaviour.
Wing-spreading in Cormorants is not associated to a successful fishing event, rejecting the 'digestion-aiding' hypothesis.
Sunny conditions did not trigger wing-spreading, but at low wind speeds cormorants tended to orientate away from the sun.
Super-wettability
So, why don't other diving birds, such as grebes, auks or goosanders engage in this behaviour? The mechanistic, proximate reason is that cormorant feathers are not very water-proof, unlike other diving birds, in fact, they are very become waterlogged after a few minutes in the water. This is not due to a deficiency of their oil glands, but to a different microstructure of their feathers, which is thought to help reduce their buoyancy, and therefore energy requirements, during diving.
Different birds different functions?
Evidence also supports wing drying as the primary reason behind wing-spreading in other studied cormorant species (Shag, Double-crested Cormorant, Galapagos Cormorant, Bank Cormorant and Little Cormorant). However, in the tropical Anhingas, a group related to the cormorant family, wing-spreading takes a much larger proportion of their time than in cormorants, and the behaviour tends to happen for longer in sunny, cool weather, with their back oriented to the sun, and doesn't require wet plumage. Anhingas have a very low metabolic rate, which might favour efficient basking behaviour, and in captive, controlled conditions, oxygen consumption decreased as they engaged in wing-spreading. Therefore, the primary function of the behaviour in anhingas appears to be thermoregulatory ('sunning') rather than drying (although this may also have a secondary role). It is a cautionary tale that apparently identical behaviour in related species can have different functions.
More information
Sellers, R. M. Wing-spreading behaviour of the cormorant Phalacrocorax carbo. Ardea-Wageningen, 83, 27–27 (1995).
Hennemann, W.W. Energetics and Spread-Winged Behavior in Anhingas and Double-Crested Cormorants: The Risks of Generalization. Am. Zool. 28, 845–851 (1988).
