The American cliff swallow (Petrochelidon pyrrhonota) is a migratory bird that spends its winters in Goya, Argentina, but makes the 6,000-mile (10,000 km) trek north to the warmer climes of the American Southwest in springtime.

According to legend, the birds, who have visited the San Juan Capistrano area every summer for centuries, first took refuge at the Mission when an irate innkeeper began destroying their mud nests. The Mission’s location near two rivers made it an ideal location for the swallows to nest, as there was a constant supply of the insects on which they feed, and the young birds are well-protected inside the ruins of the old stone church.

Tradition has it that the main flock arrives on March 19, Saint Joseph’s Day and flies south on Saint John’s Day, October 23.