By August, many birds have just completed the intense rigors of nesting and raising young and now undergo a complete molt. Molt is a cyclic process of feather growth. As new feathers grow in, they push the old ones out. Why molt? Because feathers wear out. Songbirds that migrate long distances need to complete this process on a tight schedule, to be ready when it's time to strike out in September. You might not even recognize this American Goldfinch in its winter plumage. 

More info and transcript at BirdNote.org. 

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