SAN LUIS VALLEY, ALAMOSA COUNTY — The sun was not up yet, but Rob Sparks and Colin Woolley were changing into their waders and rubber boots before heading on a short hike on a recent summer morning.

Chimney swifts are very fast fliers and spend most of their lives in midair. They eat, drink and mate while flying, only stopping to raise young. They even take their baths in flight by dipping in a ...