Double-crested Cormorants may look clumsy, even comical out of the water, but in the water they are swift and agile swimmers. They are a big bird with a solid build and a 4 foot wingspan. Unlike most birds, they have solid, dense, heavy bones and feathers that are not water proof. This makes them set very low in the water, with nearly their entire body underwater. These are the characteristics however, that make them such good swimmers. They can dive up to 30 feet deep, swim 38 miles per hour, and stay under for 30 seconds. They will typically fish for an hour, then they must sit on a perch to dry out their feathers.
An interesting side note: Ancient oriental fishermen would fish with these birds. They put a leash around their neck, let them dive for fish and then would take the fish from them when they surfaced.
They are efficient fishing machines with fish making up 99% of their diet. They prefer small fish but can swallow a quite large fish. All of this is what causes the controversy that surrounds them. They have been called Black Death and Devil Bird. They are hated by some fishermen. Although there are conflicting studies on the subject, they are seen by some as decimating the sport fish populations. An adult Cormorant will eat 1 to 1.5 pounds of fish per day. In 2005 the Cormorant population at Leech Lake in Minnesota was about 5000. That adds up to about 900,000 pounds of fish consumed each season. In 2008 that population was up to 10,000. These birds have come under the protection of the Migratory Bird Treaty Act of 1918. They have no natural predators except for the Bald Eagle—whose population is down. And, because they are absolutely no good to eat, they aren’t hunted. All this has allowed their population to skyrocket. If they do indeed cause problems with fish populations, this is probably why.