Operation Terror by Murray Leinster

It was nine o'clock at night when Lockley killed the porcupine, and ten by the time Jill had gone back to sleep huddled between the projecting roots of a giant tree. Shortly after midnight Lockley had been awakened when a skunk defeated a hungry predator within a hundred yards of their bivouac. But some time in between, there was another happening of much greater importance elsewhere.

Something came out of Boulder Lake National Park. All humans had supposedly fled from it. It was abandoned to the creatures of the thing from the sky. But something came out of it.