WELLFLEET, Massachusetts — Audubon wildlife biologist James Junda cushioned a catbird’s head between his index and middle finger. He put a band on its leg, gently spread a wing to note its colors and condition, measured its length, then turned it over. He blew on its breast. The feathers parted to reveal a brood patch, a fluid-filled sack that breeding females develop to incubate their eggs. Had it been a breeding male, the bird’s cloaca, an external reproductive structure, would have been enlarged.
