Wing-spreading in Cormorants

The spread-wing posture of cormorants, like the individual above in a local park earlier this month, is one of their most distinctive behaviours. Multiple reasons have been offered to explain this wing-spreading behaviour, is it to dry their plumage? or is it thermoregulation (basking)? Even aiding their balance, signalling successful captures, or help swallowing fish have been put forward as possible explanations for this behaviour. The evidence in the Cormorant overwhelmingly supports the plumage drying hypothesis.
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).

Mute Swans drying and tucking foot

Mute swans often stick one foot in the air as they swim (top shot) easily steering themselves with the other foot. I had often wondered what the purpose of this habit was.
  The two Mute Swans in my local park have afforded close observation in the last couple of months. Last week I noticed that sometimes they have one of their legs tucked under their flank feathers, virtually invisible, only a bit of the 'knee' showing, while swimming with the other leg. This behaviour probably results in keeping one leg quite warm under the plumage.
Yesterday I put both behaviours together by watching how the young swan took one foot out of the water, shook it and left it out for a while. Then it behaved like it wanted its leg dry before tucking it under its feathers: it tucked his leg under the feathers, but seemed not to be happy about it and it stuck it out again and shook it a bit more before finally tucking it in for good.
 I had noticed that cygnets, even before they have feathers, have this habit of tucking a leg out of the water over their body. I just had never realised the adults would do it too, preceded by the leg-stuck-out while drying.
This behaviour has also been reported in other swan species and although thermoregulation seems to be the underlying reason, both cooling and drying/warming have been proposed as explanations for the behaviour.
 The next series of photos illustrates the sequence of behaviour.

The following photo is from 21 June of 2015, taken in a local fishing lake. A young downy cygnet a few weeks old. You can notice how its right leg is tucked by its body, outside of the water, while it preens. When it grows feathers, the same position results in a leg completely covered with feathers.

More information

Why do some swans only paddle with one leg, with the other leg tucked up under the wing?

The Whooper Swan. Here. p 93-94.