By James Blish.  1975.  A shorter version of this novel appeared in Fantasy and Science Fiction magazine, April 1972.

The year is 25,000 A.D. when the earth is in a tropical phase.  Its inhabitants are atavistic tribesmen who have become mystical, ritualistic, and death oriented – too obsessed with the afterlife, in fact, to defend themselves effectively against their very real enemies, the Birds.”

midsummer century