For centuries, the arrival of thousands of cliff swallows transformed the skies above a small California town into one of America's most celebrated wildlife spectacles. Every spring, the agile birds returned almost like clockwork after migrating from South America, filling the air with their distinctive chatter as they rebuilt their mud nests around the historic mission. Their homecoming became so famous that it inspired poems, songs and an annual festival. Then, over several decades, their numbers dwindled dramatically. As the birds abandoned the place that had made them famous, residents and scientists began searching for the reasons behind the decline and set out to restore the conditions that had once made the town a perfect seasonal home.Thousands of cliff swallows returned to this Ca