You may not have thought about it, but feathers get worn out, frayed and weakened. That's why loons grow new ones every year. These new feathers will carry them hundreds, or even thousands, of miles to their breeding grounds in a month or two, as well as back to the ocean again next fall. Unlike eagles or hawks, which often fly about while missing two or three wing feathers, a loon wouldn't be able to fly while missing three wing feathers. This has as much to do with the size of loons' wings as the feathers themselves.
Wing feathers are so light and buoyant that they float.
For loons to dive and chase fish underwater without popping back up like corks, their wings are as small as possible. As it is, to get up and stay up, loons must beat their wings fast and steady. That's not the only reason why this is a testy time for loons. Loons can become severely stressed if they are already weakened from disease, for example. Or, any toxic chemicals stored in their body fat can suddenly flood their system with toxins when the fat is used for feather growth.