“Molt,” said my friend Ashley, “sounds dull.”

.We were talking about this column, and my effort to make bird molt interesting.

“Molt doesn’t sound interesting,” she said.

“Well, you don’t know anything about molt,” I said.

“Birds molting is sort of like women shopping for clothes,” I said. “Some women buy new clothes spring and fall. Some are always buying. On some women you notice the change. Other women always look the same.

“That roughly describes molting in its various forms.”

Ashley, who molts stylishly season-to-season, looked skeptical.

“You’ve seen a loon, right,” I asked, “watched them run across the water to take flight?”

“I think so,” Ashley said, who looks like a city girl.

Well, I said, loons molt their primary flight feathers all at once: Boom, they go flightless until new feathers are in place.

“Sounds extreme,” she said.

“Yes,” I said, “but there’s a reason.

“Loons are birds with unusually heavy bones, for birds. Loons eat fish, so they need weight to make diving efficient.

“The birds must accommodate both diving and flight. The extra weight creates a high wing-load ratio. But fly they must, so loons run across the water, slap, slap, slap, to acquire the air speed needed for liftoff.

“Take away a primary feather or two and flight becomes difficult, maybe impossible,” I said. ”So, loons make wholesale replacement of those feathers when they reach their ocean wintering sites.”

“Wait,” Ashley said. “What’s wing-load ratio?”

“Oh. It’s the amount of weight each square inch of wing surface must carry. Chickadees, tiny guys that weigh almost nothing, have a ratio that’s 10 percent that of loons,” I said.

“So, loons run like hell to get airborne while chickadees pop into the air.

“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 woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.