Ptarmigan and the cost of being white
Rock ptarmigan moult three times a year rather than the usual two, which is unusual enough to be worth asking why. The answer is that they are tracking a background that changes on a schedule the bird cannot control.
White plumage in snow is obvious camouflage. The complication is timing. Moult is triggered largely by photoperiod, which is reliable but fixed, while snow cover is variable. In a year when snow arrives late or melts early, birds spend weeks conspicuously mismatched against their background, and predation goes up measurably.
There is a second benefit to white feathers that is easy to miss. White feathers lack melanin, and the resulting internal structure holds more air, so they insulate somewhat better. The bird is not only hiding; it is also dressed for the temperature. This may be part of why the winter plumage is retained a little longer than pure camouflage logic would suggest.
Behaviour compensates where plumage cannot. Mismatched birds select patches that match them, sitting on remaining snow when white and on bare ground when brown. It works imperfectly, which is the usual finding when an organism tuned to a historical climate meets a shifting one.