Even many waterfowl tend to avoid actually getting wet. Like many birds, ducks and geese have feathers that 'zip' together and trap air, forming a water-resistant coat of sorts. They coat their feathers with oil from their preen glands, further helping repel water. Dabbling ducks (like the mallard and wood ducks at Eaglecrest) spend most of their time on the water's surface, tipping butt-up every once in a while to nibble passing food. Wet feathers would weigh them down, causing them to ride lower in the water. This in turn would lead them to expend more energy while searching for food - a bad trade off all the way around. Dabbling ducks want buoyancy.
But the cormorant has a different hunting strategy. A cormorant's outer feathers are water permeable, which weighs the cormorant down and helps it remain underwater. There is some debate about whether this has to do with feather structure or its preen gland. Some sources claim that cormorants produce less preen oil, aiding feather permeability, while others believe that feather structure plays a more dominant role. Whatever the cause of permeability, dry feathers next to the cormorant's skin trap air. Its unique feathers reduce buoyancy while retaining warmth. For all practical purposes, the cormorant is wearing a sort of home-grown dry suit. Unlike its cousin the Anhinga, which soaks all the way through, cormorants can handle cold water. I've seen them in shallow water on the south side of Lake Superior, near the Apostle Islands, where summer surface water temperatures may not climb much above the mid-sixties - and that is in the warm part of the Big Lake! The cormorant's long tail and cyclical paddling also help reduce buoyancy. All of these things
keep the cormorant underwater longer at a lower metabolic cost, which pays off in more food opportunities and reduced energy loss.
Dabbling ducks spend a lot of time floating on the surface of the water. But once a cormorant has secured dinner, it tends to leave. It is a diver and swimmer, not a leisurely floater. It spreads its wet wings to dry until the next time it goes hunting underwater.
Some cultures fish with trained cormorants. Click here for a video that shows cormorant fishing: http://youtu.be/Me3J0K1Qd5o. Note how much lower they ride in the water even when floating. Scuba divers should check this link out - at least scuba divers that also enjoy reading about birds. This is the first time I've found a reference to Boyle's Law in a bird book. And finally, some people believe that the cormorant's wing spreading may serve social or thermoregulatory purposes as well.
Some more things that helped me learn about this topic:
- Spread wing postures: http://www.stanford.edu/group/stanfordbirds/text/essays/Spread-Wing_Postures.html
- Dabbling versus diving ducks: http://blogs.sandiegozoo.org/2012/07/18/dabbling-vs-diving-ducks/
- Cormorants and submerged swimming: http://jeb.biologists.org/content/207/12/2101.long
- Cormorants: http://www.allaboutbirds.org/guide/double-crested_cormorant/lifehistory
