j. RaptorRes. 34(3):187-195 ¸ 2000 The Raptor ResearchFoundation, Inc. POPULATION FLUCTUATIONS OF THE HARRIS' HAWK (PARABUTEO UNICINCTUS) AND ITS REAPPEARANCE IN CALIFORNIA MICHAEL A. PATTEN Departmentof Biology,University of California,Riverside, CA 92521 U.S.A. RICHARD A. ERICKSON LSA Associates,One Park Plaza, Suite 500, Irvine, CA 92714 U.S.A. ABSTRACT.--TheHarris' Hawk (Parabuteounicinctus) was consideredextirpated from California in the mid-1960s.Most sightingsin the past 30 yearswere, therefore, consideredto be escapedor released birds. The specieshas recently staged an incursioninto southernCalifornia and northern BajaCalifornia in the 1990s,involving nearly 50 individualsand local breeding.This incursionwas apparently another in a long-term seriesof population fluctuationsof the Harris' Hawk, each bringing large numbersto the north and west of its establishedrange in Arizona and Baja California. Although first recorded at the state border in the 1850s, the Harris' Hawk was not recorded as a breeder until an incursion in the late 1910s and 1920s brought hundreds to the state, including the first known breeders.Numbers declined again in the 1940s,built up again in the 1950s,and thereafterdrastically declined to the point of their absenceby the mid-1960s.Therefore, the recent incursionwas not anomalousbut rather follows historicalpatterns of occurrence,indicating that California is on the fringe of the natural range of the Harris' Hawk, with emigrationbringing birds into the stateand subsequentpopulation decreases leading again to "extirpation." KEYWORDS: Harris'Hawk; Parabuteo unicinctus; Baja California;California; population fluctuations. Fluctuacionespoblacionales de Parabuteounicinctus y su reaparicitn en California RESUMEN.--E1gavilfin de harris (Parabuteounicinctus) fue consideradocomo extirpado de California a mediados de 1960. La mayoria de los avistamientosde los filtimos 30 aftosfueron consideradoscomo avesescapadas o liberadas.La especieha incursionadoen el sur de Californiay norte de BajaCalifornia en los aftos90, incluyendounos 50 individuosy algunoseventos de reproduccitn locales.Esta incursitn es aparentementeuna mils de las ocurridasa largo plazo por esta especie.Cada una trayendograndes nfimeros de individuos al norte y oeste de su rango establecidoen Arizona y Baja California. Aunque por primera vez fue registradoen el borde del estadoen 1850, el gavilfinde harris no fue reportado en reproduccitn hastasu incursitn en 1910 y 1920 con cientosde individuosincluyendo los primeros registrosde reproduccitn. Los nfimeros de individuosdeclinaron otra vez en 1940, aumentaron en 1950, y declinaron drfisticamentehasta considerarlos ausentes en 1960. Por lo tanto, la reciente incur- sitn no es antmala, al contrario, siguelos patronesde ocurrencia indicando que California estfien el limite del rango natural del gavilfin de harris, con su emigraci0n trayendo avesdentro del estado y la subsecuentedeclinacitn la cual conllevaa su extirpacitn. [Traducci6n de Ctsar Mfirquez] The Harris' Hawk (Parabuteounidnctus) ranges as has been expandingnorthward in recent years from the southwestern United States southward (Bednarz et al. 1988, Bednarz 1995, Dawson 1998). through Central America to central Chile and cen- In California, the Harris' Hawk wasfound formerly tral Argentina, with a geographicallydisjunct pop- throughout the lower Colorado River Valley and in ulation on the Baja California peninsula. In the the Imperial Valley south of the Salton Sea (Grin- United States, it occurs from southern Arizona, nell and Miller 1944). By the mid-1950s,it wasex- southeastern New Mexico, and central Texas south- tirpated from California as a breeder (Remsen ward (Fig. 1; American Ornithologists' Union 1978, Walton et al. 1988), with the last definite wild 1998). Its range in Arizona, New Mexico, and Tex- bird recorded north of Blythe on 28 November 187 188 PATTENAND ERICKSON VOL. 34, NO. 3 LakeTahoe Colorad/e•'River California Arizona Salton Sea Figure1. The northwesternportion of the currentrange (the shadedarea) of the Harris'Hawk (Parabuteo un2- cznctus),modified from Bednarz (1995) and Dawson(1998). The dashedline signifiesthe formerwesterly limits of its rangein southwesternArizona and southeasternCalifornia. SEPTEMBER2000 HArems' HAwI• IN CALIFOm'4IA 189 1964 (Garrett and Dunn 1981, Rosenberg et al. mate of the magnitude of the incursion and to examine 1991). its geographicextent. A reintroduction project for the Harris' Hawk THE 1994 INCURSION wasinitiated in California in 1979 by variousstate, federal, and private groups (Stewart1979, 1982, Despitean annual "background"escape/release Walton et al. 1988). Eight birds were releasedthat rate throughout California of >3 Harris' Hawks year and severalmore were releasedeach year un- (Bloom pers. comm., Walton pers. comm.), a dif- til 1989, for a total of 222 releases (Linthicum ferent phenomenon began 15 April 1994, when J. 1989, Linthicum pers. comm.). The first pair nest- Rudley, P. Jorgensen,and M. Jorgensenobserved ed successfullyin 1983, three pairs bred success- three adults together in Borrego Valley. Between fully in 1986 (Walton et al. 1988, Rosenberget al. 1994-96, at least 34 individuals had been found in 1991), and five nested in 1989 (Linthicum 1989, southern California (Table 1; McCaskie 1995). The Bednarz 1995). However, it is unlikely that this largest groups of birds consistedof at least eight population is viable, as birds are now infrequently individuals in the Borrego Springs region of the noted (Rosenberget al. 1991, Patten pers. obs.). Anza-Borrego Desert and up to five individuals Since the mid-1960s,virtually all recent records both at the former George Air Force Base near of the Harris' Hawk in California are of birds con- Victorville and at Boulevard (Table 1). During this sidered to have escapedfrom falconers (Garrett apparentlynatural incursion (Bednarz 1995, Mas- and Dunn 1981, Unitt 1984). In some cases,birds sey1997, Walton pers.comm.), Harris' Hawkswere have been observedwith jessesand clearly came found north of their historical range as far as Vic- from this source. In other cases,there appearsto torville in the Mojave Desert, with scatteredindi- be some tendency for natural occurrence such as vidualsreported around the Salton Sea and else- sightingsof immatures along the Colorado River where (Fig. 2). Additional birds in cismontane near Blythe in September (Roberson 1980) and valleysat Riversideand in centralSan Diego Coun- December1978 (Rosenberget al. 1991) and at the ty may or may not have been naturally occurring, south end of the Salton Sea on 25 June 1989 with individualsfar westin coastalOrange County (McCaskie 1989). Nevertheless,records of individ- and in the Antelope Valley being particularlysus- ual birds are perhapsalways suspect given that the pect given the apparent geographicextent of the speciesremains popular with falconersand reha- influx (Fig. 2). Indeed, the Orange County bird bilitated birds are occasionallyreleased, as were a showed signs of being in captivity (Bloom pers. few around the Salton Sea in the 1970s and 1980s comm., Daniels pers. comm.). This influx into southern California was con- (Walton et al. 1988). Herein, we document a major increasein sightingsbeginning in April 1994 that comitant with at least 22 individuals well north of wasapparently a natural influx involvingnearly 50 the species'normal range in northern Baja Cali- individuals throughout southern California and fornia (Table 2; Radamakerpers. comm., Wurster northern Baja California. Further, we hypothesize pers. comm.) and in adjacentnorthwestern Sonora that such incursions are the rule rather than the (Russell and Monson 1998). During this period, exceptionfor the occurrenceof this speciesin Cal- Harris' Hawks bred in California at Borrego ifornia. Springs (Massey 1997), Boulevard (Unitt pers. comm.) and Laguna Dam (McCaskie 1996, Massey METHODS 1997), and in northern Baja Californiaat Valle San For the recent incursion,we gatheredrecords and doc- Telmo (Bloom pers. comm.). Small numbershave umentation from various field observers (see Acknowl- persistedin BorregoValley as recentlyas 7 March edgments)and from files of the California Bird Records 1999 (Jorgensenpers. comm.) and in Valle San Committee. All specificdata gathered are on file at the Telmo on 31 January 1999 (Patten pers. obs.). Western Foundation of Vertebrate Zoology (WFVZ), Camarillo, California U.S.A. Recent and historical data HISTORICAL TRENDS AND CURRENT STATUS were gathered from seasonalreports for the Southern Pacific Coast Region published in FieldNotes (now North The historical distribution of the Harris' Hawk AmericanBirds), Christmas Bird Countsand specimensat in California is not clear. The specieswas first re- the San Diego Natural History Museum (SDNHM), San Diego, California U.S.A., National Museum of Natural corded along the Colorado River on the Arizona History (USNM), Washington, D.C.U.S.A. and WFVZ. side in February 1854 (Kennerly 1859, Swarth We tabulatedand mapped these data to obtain an esti- 1914), but Elliot Couesnever recorded the species 190 PATTEN AND ERICKSON VOL. 34, NO. 3 Table 1. California records of the Harris' Hawk (Parabuteounicinctus) from spring 1994 through winter 1996-97 (Fig. 2), arranged chronologically.Birds nested at Boulevard (1994) and Laguna Dam (1996), and exhibited nesting behavior (copulations,carrying sticksand food) at Borrego Valley (1994-95), with immatures observedin 1995. Data are on file with the CBRC. DATE (S) LOCATION MAXIMUM