As they do each year on March 19, hordes of people have flocked to San Juan Capistrano, necks craned hopefully to the sky.
They're eager to witness the annual return of the cliff swallows to the seaside city's historic mission after the birds' migration from South America.
But the swallows – the subject of a yearly celebration and a huge tourist attraction for the south Orange County city – haven't come around in recent years.
The mission is trying to change that, saying it's taken on a goal matching the title of a 1940 classic song associated with the city: "When the Swallows Come Back to Capistrano."
Last year, cliff swallows expert Charles Brown, a biology professor at University of Tulsa, helped the Mission San Juan Capistrano install speakers that play a recording of the birds' courtship calls. The speakers are placed behind the statue of Father Junipero Serra, who founded the mission in 1776.
The mission began playing the sharp, squeaky vocalizations this year on March 1.
Listen: Recording of cliff swallows being played at Mission San Juan Capistrano
"I think that eventually we'll get them back," Brown told NBC News, speaking about the project. "It may not be this year; it may not even be next year. But if we keep trying long enough, some individuals will come by, they'll see the mission and they will realize that it's a good place to nest, as they did in the past."
Last year, a number of swallows flew in to investigate the sounds, and the birds were spotted feeding overhead. Still, they nested on nearby freeway overpasses – and as far away as a high school in Palm Desert. The birds also made a home at Xavier College Preparatory High School, nearly 100 miles east of San Juan Capistrano, in 2011.
In 2010, the birds flocked inland as well, to the Vellano Country Club in Chino Hills.
Yet each year, the birds are still expected to darken San Juan Capistrano skies on March 19, or St. Joseph's Day. The mission rings its historical bells, and hundreds of schoolchildren and tourists learn about the history of the grounds and, of course, the swallows.
This year, the city's 55th Annual Swallows' Day Parade, which attracts thousands of tourists, is on Saturday. 
A number of changes have prompted the swallows to alter their centuries-long destination for spring and summer months, according to Brown.
The birds are attracted to grasslands and prairies – the kind of landscape that once surrounded the mission, he said.
The mission's Great Stone Church, itself likely appeared to swallows as a huge cliff – perfect for making mud nests, as shown above.
But now the mission is just one of many big structures in the urbanized area, and the birds have been known to nest elsewhere nearby – under freeway overpasses and the like – or go much farther away.
Surprisingly, the planting and growth of trees in the area has also contributed to the swallows' failure to return to the mission, Brown said. Swallows tend to avoid trees, preferring open spaces.
They birds have been seen in increasing numbers in Great Plains states, while coming in lower populations to Southern California, Brown said.
He said it's possible urbanization and the changing features of the region may affect the birds' possible return.
"Eventually, the Southern California landscape may be just inappropriate for this particular species," Brown said.
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At the same time, preservation work done in recent years to stabilize the church's crumbling walls required old swallows' nests to be removed. Bird sightings have been rare since.
Nonetheless, St. Joseph's Day and the idea of the swallows remain a big draw for tourists coming to Mission San Juan Capistrano, considered the "jewel" of the state's 21 Spanish colonial-era missions.
Rafael Gutierrez, the mission's bellringer and an employee for 47 years, said the day is about history, not just about hoping the swallows return.
"Siempre viene, no muchos, pero viene," he said, meaning "they always come, not many, but they come."
When they've come in past, the swallows are said to take flight again for their return to the Southern Hemisphere on the Day of San Juan, Oct. 23.
NBC4's Vikki Vargas contributed to this story.
Photo Credit: Photograph courtesy of Mission San Juan Capistrano