SHERMAN, TEXAS—During the summer months, highway overpasses throughout the western United States are swarming with cliff swallows. The small birds build mud nests in colonies on cliff ledges or under bridges, mostly living in large groups. The diversity of those groups and how the birds select their colonies is something Dr. Charles Brown, professor of biology at University of Tulsa and a 1981 graduate of Austin College, has been studying for nearly 30 years. . . . More . . .