The usual explanation for this peculiar habit is that the birds are drying their wings after diving to search for food. But there are other theories: better balance on land; a signal of feeding success for fellow cormorants to exploit; an aid to swallowing and digestion; regulating body heat, and casting a shadow over the water so that they can spot the next moving snack.
For some time, the wing-drying theory has been the front-runner. But if it is correct, why is such wing-drying confined to cormorants and closely related birds such as darters? How come we do not see rows of diving ducks standing like crucifixes around the edges of lakes?
Dr Robin Sellers, a professional engineer and amateur ornithologist, claims to have found the answers after spending many winter hours observing their behaviour in the Severn Estuary. He has just published his findings in the journal Ardea.
During his research, only once did he see a cormorant hold its wings out when it had not dived in the water. And of those birds he observed diving, 93 per cent of them held their wings spread afterwards.
But wing-stretching had nothing to do with their success in catching fish. It was performed whether or not they had anything to digest, ruling out the theory that wing-spreading helps swallowing or digestion, at least in wet birds.
The time spent in this posture varied considerably, from a minute to a marathon 18 minutes. Dr Sellers explains this variation with some neat observations. Most of it he attributes to wind speed. When the wind was up to, or more than, a strong breeze on the Beaufort scale (4 or more), cormorants spent, on average, 2.5 minutes standing like crucifixes. With hardly a flutter in the air (0 to 1 on the scale) they stood with their wings out for nearly five minutes and generally extended them further.
The birds also faced the wind - more than half of them orienting within one compass point of its direction when conditions were virtually still, but more than 80 per cent when a good breeze got up.
They also tended to hold their wings out for longer in lower temperatures and when they had been under water for longer. Light rain reduced their fervour and heavy rain put a stop to any spreading at all.
His painstaking observations seem to point to one conclusion: that cormorants are drying their wings and making the best of the weather in order to do so.
If wing-spreading signalled feeding success, the birds would not do it when diving proved unsuccessful. And the theory of casting a shadow on the water only helps birds that adopt a stand-and-wait strategy - like herons - rather than those that dive as much as 10 metres down in murky water.
Dr Sellers calculates that between 30 and 90 grams of water have to be expelled from a cormorant's plumage after an average dive. It would need up to 222 kilojoules of body heat to evaporate it, an amount that could be sacrificed providing enough fish were caught to make up for the loss. "But in winter, cormorants can only just get enough food to survive, so using body heat to dry their plumage isn't likely to hold water," he said. Air-drying is more energy-efficient.
Disproving the balance theory is not easy, except that previous studies quoted by Dr Sellers found no evidence for it. To confuse matters, though, his observation that cormorants spread their wings less in windy weather could be assumed to improve their balance.
As a means of regulating body heat, wing-spreading appears to be a non- starter. Cormorants spend too little time - and then only intermittently - doing it. In the cool of the Severn Estuary in winter, what self-preserving bird would want to lose more heat anyway?
So is wing-drying the sole explanation for the cormorant's unusual posture? Probably not. According to another study, albeit a much smaller one, just published by Dr David Gremillet of Neumunster Zoo in Germany, wing- spreading may, after all, be a digestive aid.
Dr Gremillet found that cormorants fed cold fish held their wings out, often for 20 minutes. But fewer than a quarter of the birds fed fish pre- warmed to cormorant body temperature bothered to make a wing movement, and then only for a few minutes.
The implication - albeit using zoo-confined birds that did not dive and get wet - is that cormorants are exposing their bellies to absorb the sun's heat, a warm glow that aids the digestion.
Dr Chris Mead of the British Trust for Ornithology, while acknowledging that wing-drying and digestion aids may jointly explain the cormorant's striking stance, says: "Personally, I betheating up their food is the most important reason. Why else would cormorants be all black if it wasn't to absorb heat through their plumage?"
He cites guillemots and the king and blue-eyed shags of the Antarctic as evidence that most diving birds have white-plumed bellies. "None of them have a wing-holding stance," he adds. But Dr Sellers quotes the white-breasted cormorant of South Africa which, perversely, does hold its wings out after diving.
So if plumage-drying is at least part of the explanation, how come more of the world's 150 or so species of diving bird do not stand like crucifixes after each sortie?
The answer, it seems, is the feather structure of cormorants and their close relations. Different diving birds adopt different strategies to enable them to get deep down under water. Like most creatures, birds are naturally buoyant. Cormorants are thought to swallow pebbles to increase their weight. Their main adaptation, though, is a much more open feather structure that does not trap buoyancy-increasing air but gets wet instead. Penguins, which dive to a remarkable 250 metres and can stay under for 15 minutes, do not get wet because their body feather structure is not as open. It is also designed to trap little air.
Why cormorants were not designed in the first place with penguin-like feathers is anyone's guess. If they had been, Dr Sellers would not have spent cold winter days with his binoculars on the Severn Estuary. And David Gremillet probably would not have thought to heat up some fish to feed to them, either.