Yellow-billed Cuckoos are a Federal candidate for endangered status, an Arizona Species of Special Concern, and have multiple listings indicating their status as a species of concern under Partners in Flight. Their regional population trend has shown a marked decline, they are of regional conservation concern, and there is a major threat in the region to their breeding status. Despite this, little is known of Yellow-billed Cuckoo long-term population trends or basic biology. This information is critical because a great deal of effort is currently spent on surveys and habitat restoration, with little knowledge of what these surveys tell us about population numbers, or if these restoration projects are providing the habitat that cuckoos require. This project will (1) monitor the declining Yellow-billed Cuckoo population on the San Pedro River in Arizona; (2) monitor cuckoo nests to determine prey base; (3) monitor cuckoo nestling mortality; and (4) search for banded cuckoos to determine site fidelity.