|molting Blue Jay|
Birds constantly replenish their feathers when they become worn out. The head feathers, however, molt and regrow all together (so the head often looks much worse than normal). Also, birds often pick up microscopic, parasitic feather mites.
They're not a serious problem and birds usually get rid of them by preening their feathers with their beaks. But they can't physically get to the mites in their heads, so the feathers there get disheveled sooner. When cool weather arrives, however, the mites will die and the birds' feathers will have regrown.
IT'S JUST A THEORY Woodpeckers have huge brains, relative to other birds. A recent gathering of the American Ornithological Union backed this up with measurements.
Nobody's certain why their brain is so big, but a leading theory has to do with their trait of hammering on wood. With the constant hammering, some of the woodpecker's brain cells are bound to be damaged, so woodpeckers are endowed with bigger brains to make sure they'll have enough left over in old age. (Sounds good to me!)