Peregrine falcons typically mate for life, although survivors will seek a replacement after a mate dies.
Grinnell was attacked by other falcons last fall and spent nearly three weeks in a wildlife hospital recuperating, while other rivals courted Annie. But he returned and observers felt that the couple was bonding again.
Then in February, Annie vanished from her gravel nest and was briefly presumed injured or dead before returning nearly a week later. Her disappearance made local headlines.
The falcon researchers said they had never seen a female vanish suddenly during peak breeding season and then suddenly return.
Peregrine falcons are considered the world’s fastest birds. They can reach speeds of 200 mph during a hunting dive known as a stoop. The American birds were declared endangered in 1970 because of ingesting prey poisoned by DDT and other pesticides; the chemical caused the falcons to produce thin-shelled eggs that couldn’t survive until hatching. However, recovery programs brought the bird back from potential extinction.
KQED's María Fernanda Bernal and Kevin Stark contributed to this report.