We observed a Yellow-billed Cuckoo (Coccyzus americanus) remaining in southern San Diego County from 20 to 28 July 2012. The bird made extensive use of upland habitats as well as the adjacent riparian area. Away from the two known breeding populations, the 82 Yellow-billed Cuckoo observations in southern California since 2000 peak from 15 June to 3 July. Observations made later in summer likely consisted of migrants perhaps in poor condition, lingering individuals prospecting for breeding habitat, or possibly scattered breeding pairs. Eight locations have had multiple occurrences since 2000, and these sites may support breeding birds. Though a relatively large number of cuckoos was detected in the region in 2011, no trend is apparent in numbers of detections since 2000. Annual variation in cuckoo numbers regressed on El Niño–Southern Oscillation climate data produced a strongly predictive model (r² = 0.54, P = 0.004). Given the Yellow-billed Cuckoo’s urgent conservation needs in the western United States in general, and in California in particular, focused attention is needed, including systematic surveys to determine if there are additional breeding pairs in the region.