Charles Brown, an expert consulting with the mission, blames urban sprawl for the vanishing birds, which once arrived in large numbers about this time of year from wintering grounds in southern Argentina.
After efforts to lure the birds with man-made nests and some of their favorite snacks failed, Brown started playing the recording.
Now, hidden speakers play swallow courtship calls for up to six hours each day in a last-ditch effort to woo the cliff swallows back.
Curious swallows have been observed flitting around the speakers and dipping and weaving over the gardens. But none have taken up residence.
The mission, made famous by the love ballad “When the Swallows Come Back to Capistrano,” was founded by Franciscan missionaries in 1776.
The padres soon noticed that large numbers of the birds would arrive each spring, and even nested on the ruins after the mission was wrecked by an earthquake in 1812.