Tiny backpacks reveal the secret, sometimes circuitous migrations of small songbirds

Squinting through my binoculars, I scanned the dense branches of a black spruce, trying my best to catch a glimpse of the thin legs of a small black, white and yellow songbird. I was on the hunt for a yellow-rumped warbler—a bird affectionately called "butter-butt" by birdwatchers after the distinctive buttery yellow patch of feathers above its tail.

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