For most birds, wet feathers are highly undesirable because they impede their ability to fly and don’t provide insulation. But cormorants dive underwater to catch food. They have feathers that become easily waterlogged, which allows them to dive deeper by preventing air bubbles from getting trapped underneath their feathers. This is one reason you often see cormorants standing with their wings spread, drying their wet wings after diving.
This video accompanies Chapter 4, Feathers and Plumages, Handbook of Bird Biology, 3rd Edition from the Cornell Lab of Ornithology and Wiley Publishing.
Recorded by Timothy Barksdale, Macaulay Library