On sunny days, it’s common to see cormorants standing on land with their wings spread wide. But what do they do on cold, overcast days? This Double-crested Cormorant standing on a rock on the north shore of the park demonstrated the answer: flap the wings. It flapped its wings regularly, sometimes at high speed, for several minutes.
The interesting thing is that it could flap like this without taking off and flying. I’ve seen propeller planes race their engines on the ground without moving. They can do that because the pilot can adjust the pitch on the propeller blades to take a minimal bite. Jets can do it with their turbine blades. Obviously this bird can somehow do the same. When it flaps its wings to rise and to fly, it somehow shapes its wing so that it sheds air as the wing moves upward, and then reshapes it to push air as the wing comes down. If the bird could not let air slip past its wing on the upstroke, the upstroke would generate downward thrust, same as the downstroke generates upward thrust, and flight would be difficult if not impossible. Here the Cormorant demonstrates that it can shape its wing to let air pass by on both upstroke and downstroke, even when flapping vigorously.
Cormorants need to let their wings dry because their wings get soaked while diving. They don’t have the same wealth of preen oil as some other waterbirds that keep their wings oiled to shed water. The upside is that Cormorants are superb divers; one species has been filmed catching a fish at a depth of 150 feet.