In late summer most song birds shed old feathers and grow new ones. This molting process greatly changes the birds appearance. What many people think is a disease on the birds is actually a natural process of replacing worn with healthy feathers in preparation for winter cold or the long flights of migration.
Some birds actually change feather color from bright to drab colors like the male American Goldfinch and Scarlet Tanager. It’s very exciting to see this transformation taking place in my backyard.