Feathers with dual function: the cormorant's secret of success
Cormorants' body feathers both absorb and repel water, improving the birds’ maneuverability while keeping them insulated in cold waters.
The cormorant was sitting in the uppermost top of a tree on the other side of the pond, its shimmering black wings spread wide and raising its head towards the sun. It was a cold morning in January, my hands felt numb from the cold winter air, but this bird looked like it was enjoying its sunbath to the fullest.
The plumage of cormorants is not fully waterproof and becomes wet when they dive through the water to catch their prey. That is why you can regularly observe these birds drying their feathers in the sun. By absorbing water, their plumage reduces the cormorants’ buoyancy and improves their maneuverability in shallow water, making this species one of the most successful diving predators.
The problem is, if you get wet, you also get cold. Cormorants hunt throughout the whole year, no matter how freezing cold the water is, and also at latitudes as high as Greenland, which is how far their breeding distribution ranges. How do they stay warm despite getting their plumage wet in arctic temperatures?
Today’s featured bird research1, published in the Journal of Avian Biology in 2005, revealed the answer: cormorant feathers have a unique dual function. The researchers observed that the body feathers of great cormorants (Phalacrocorax carbo) are wettable and water-resistant at the same time, both traits joined in each individual feather!
The inner area, around the central shaft (rachis), is extremely water-resistant. It shows the ‘standard’ regular structure of feathers, with the interlocked barbs orderly running in parallel alignment. This region of the feather is even more water-resistant than the feathers of other diving birds, such as ducks. It ensures that an insulating layer of air stays trapped within the plumage during a dive, preventing the cold water from making direct contact with the cormorant’s skin.
The outer area of the feathers is wettable. The microstructure is much more irregular, the barbs arranged rather loosely in a more random fashion. This part of the feather absorbs water like a sponge, which is the reason cormorants need to dry their plumage after dives—hence the iconic wing-spreading posture.
This dual function of feathers is rather unique among diving birds. The researchers tested the body feathers of 15 more waterbirds, most of them divers as well, and only one species besides the great cormorant had a similar dual function feather structure—the European shag, which is (surprise!) just another species of cormorant.
With the feathers being partly soaked with water, how much insulating air can the plumage still hold? To test this, the researchers submerged dead cormorants (ethically sourced, no birds were killed for this study) in water with and without feathers to measure the volume of the feathered and plucked body. The difference in volume, minus the volume of the feathers themselves, is the volume of the insulating air. The researchers observed that, for a cormorant of about 2.3 kg, the total volume of trapped air is approximately 400 ml—about three times less than would be expected in a ‘standard’ diving bird of similar size. But still, this volume of air is what keeps them warm enough to dive through the coastal waters of Greenland. Impressive!
I used to think that the wettable plumage allows cormorants to dive particularly deep, but that is not necessarily the case2. Great cormorants are shallow-water specialists, usually hunting within the first six meters below the water surface. Deeper dives are possible, but would compress the air volume in their plumage and diminish the insulation to a critically low level. In the meters just below the water surface, however, the cormorant is one of the most successful predators, having the highest capture rates so far recorded in any diving bird.
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Have a wonderful rest of the week!
All the best,
An imperial cormorant, however, has been observed to dive as deep as 46 meters, but that is not the norm.
Fascinating! Thankyou. 🙂
What fascinating adaptations! Thank you so much for sharing this, Maja—you’re such a skilled science communicator.