Population fluctuations of the Harris' Hawk (Parabuteo unicinctus) and itsreappearance in California

The Harris' Hawk (Parabuteo unicinctus) was considered extirpated from Cali fornia in the mid-1960s. Most sightings in the past 30 years were, therefor e, considered to be escaped or released birds. The species has recently sta ged an incursion into southern California and northern Baja California in t he 1990s, involving nearly 50 individuals and local breeding. This incursio n was apparently another in a long-term series of population fluctuations o f the Harris' Hawk, each bringing large numbers to the north and west of it s established range in Arizona and Baja California. Although first recorded at the state border in the 1850s, the Harris' Hawk was not recorded as a b reeder until an incursion in the late 1910s and 1920s brought hundreds to t he state, including the first known breeders. Numbers declined again in the 1940s, built up again in the 1950s, and thereafter drastically declined to the point of their absence by the mid-1960s. Therefore, the recent incursi on was not anomalous but rather follows historical patterns of occurrence, indicating that California is on the fringe of the natural range of the Har ris' Hawk, with emigration bringing birds into the state and subsequent pop ulation decreases leading again to "extirpation."