Wing feathers are so light and buoyant that they float. For loons to dive and chase fish underwater without popping back up like a cork, their wings are as small as possible. But feathers get worn out, frayed and weakened, and so 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, and then carry them back to the ocean again next fall. It takes a lot of energy and body resources to grow feathers.
Loons grow a lot of large, stiff feathers all at once.
They can become severely stressed if they are already weakened from some disease, for example. Or, if they have toxic chemicals stored in their body fat, when the fat is used for feather growth, these toxins can suddenly flood their system. Sometimes they even may die. A few times, there has been a large loon die-off in the ocean. When this tragic event happens, it's usually February, right when loons are molting and most vulnerable.