Imagine you are a bird who makes its living in the water. Or more precisely, underwater. You want to be as efficient as possible conducting day-to-day activities such as moving about, finding food, and staying warm. These objectives create a serious challenge in life. Such is the cormorant’s dilemma.
Cormorants are fish eaters and pursue their prey by giving chase underwater using their powerful webbed feet for propulsion. Some species can dive up to 150 feet deep. There are about 40 species of cormorants (a few are known as shags) around the world. Here in Lake Arrowhead, the only species we see is the double-crested cormorant.
So what are the details of the cormorant’s dilemma and how are they addressed?
To stay warm, birds have feathers that trap air and provide insulation from heat loss (birds are warm-blooded and maintain a body core temperature of about 107ᵒ F) by preventing cold water from being in contact with the body. The covering of feathers essentially makes the bird waterproof! But a lot of trapped air among the feathers would make a cormorant very buoyant and require a lot of extra energy to dive underwater and pursue the fish that make up its diet. This buoyancy would not only require more energy expenditure, it would also make the cormorant less effective at catching fish because it would be less maneuverable underwater when chasing elusive prey.
However, if the cormorant gives up its ability to be waterproof to be a better underwater hunter, it may quickly suffer from hypothermia. Cormorants can be seen here in Lake Arrowhead from March through late December, where they can experience water temperatures of 47ᵒF. Getting soaked to the skin in cold water is a recipe for disaster for a bird. A further disadvantage to having drenched plumage is that significantly more energy is needed for flying due to the added weight of the water.
So it seems the cormorant can either stay dry and warm or get soaked to catch fish. A careful examination of these birds’ feathers reveals the clever solution evolution has created.
Two factors work together to help make (and keep) feathers waterproof. The first is their physical structure. The typical feather consists of a central shaft that has a series of paired branches (called barbs) on either side to form a flattened, usually curved surface known as the vane. Each barb has small branches called barbules that hook to the barbules of adjacent barbs that stiffen the vane and give it the appearance of a solid surface and causes it to shed water. The second factor relies on an oil-secreting structure located at the base of the tail known as the uropygial gland (also called the preen gland). Like other birds, the cormorant rubs its beak on this gland to pick up the oil and transfer it to the feathers. This oil keeps the feathers supple, inhibits bacteria, and enhances the feathers waterproofing qualities.
Evolution has provided cormorants with a brilliant compromise. Detailed studies have revealed the portion of the feather furthest from the body lacks barbules so that it doesn’t trap air and actually gets wet. The part of the feather closest to the body still has the ability to trap air, so while the outer section gets wet, the part of the feathers close to then body still form a protective layer of air and keep out the water.
Here’s where it gets really interesting. A chemical property unique to the cormorant’s preen oil may allow the feathers to get wet but causes the feathers to eject the water when the bird comes back up from its dive. The researchers who discovered this property refer to it as “spontaneous de-wetting.” It is not uncommon to see cormorants standing on a rock or a log with their wings spread, drying in the sun after feeding. The de-wetting property of their preen oil may help the feathers dry more quickly and spend less time posed in a vulnerable position.
As a side note, while it is widely believed that cormorants produce less preening oil than other birds, there have been no studies (to my knowledge) that support this idea. Interestingly, a study published in the Journal of Ornithology in January 2023 appears to be the first effort to compare the urpoygial gland of cormorants to other birds. While it found some structural differences, there was nothing to suggest they produce less preening oil than other birds.
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