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“Hawks, though, and eagles,” I said, “molt one primary feather at a time. Just one, maybe two. If raptors lost all of their primary feathers at once they’d be grounded. Running wouldn’t help. They couldn’t hunt. End of story.”
Molt, I said, is obligatory. Feathers, like clothes, wear out, fade, get torn and dirty. New feathers appear on a schedule each bird species has evolved to meet its particular needs.
Cardinals molt in the fall, I told Ashley. Males replace worn red with feathers of a gray cast, no longer needing courtship brightness. The off-color is on feather ends that wear away. Come spring, we see the bright red we expect.
Crows molt throughout the year. They’re among those species that show no change. New feathers look like the ones they replace.
Warblers are brightly colored in spring, duller in fall. Bright colors are needed for courtship. Birds pay a high price in energy for those bright colors. When bright is no longer necessary, the birds switch to less expensive plumage.
“Sort of like shopping for spring clothes at Talbot’s and fall clothes at Target,” I said.
Warblers molt here in the fall, before migrating to their Central and South American homes. They molt down there before moving north in the spring. They take advantage of abundant food supplies to support molt before travel.
Barn swallows, on the other hand, don’t molt in the fall until they’ve reached their Central American wintering locations.
“Why?” Ashley asked.
“I don’t know,” I said. “But there’s a reason. There’s always a reason.”
Lifelong birder Jim Williams can be reached at email@example.com. Join his conversation about birds at www.startribune.com/wingnut.