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POPULATION ECOLOGY of the HARRIS&Apos; HAWK in ARIZONA

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POPULATION ECOLOGY of the HARRIS&Apos; HAWK in ARIZONA RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. VOL. 20 SPRING 1986 NO. 1 POPULATION ECOLOGY OF THE HARRIS' HAWK IN ARIZONA WAYNE H. WHALEY ABSTRACT- The Harris' Hawk (Parabuteounicinctus) was studied in Arizona during 1976-1977.Where 2 separate populationsonce resided in Arizona,1 nowremains and occupies 3,880 km 2 of the ArizonaUpland subdivision, Sonoran Desert.The former populationalong the ColoradoRiver is extirpated. Nestswere built or old nestsrepaired from Januaryto Augustand eggswere laid from mid-Januaryto mid-August. Subsequentclutches were documented in 50 nestingranges. Occasionally second clutches were laid beforeyoung of first broodswere fledged. Fledging dates ranged from April to October.At 9 locationsactive nests were 0.8 km apartand at 2 locationsnests were 0.5 km apart.Within 2 studyareas nesting density was 2.5 km2/activenest. In 1977 Harris' Hawks reoccupied91% of the nestingranges used in 1976. More than 2 adultswere observedat 46% of 227 nestingranges. Productivityfor 396 nestingattempts averaged 3.16 eggs/clutchand ! .62young fledged/nesting attempt. Seventy-four percentof the nestingattempts were successful. Foodconsisted mainly of rabbitsand groundsquirrels. Mortality occurred mainly during the egg laying and incubation period.Early nest failures resulted in secondbroods, but successfulpairs also had secondbroods. Habitat loss is the major causeof declineof the populationfollowed by excessivehuman disturbance. Researchon raptors hasbecome increasingly im- apparentextinction in southernCalifornia (R. Guy portant as a result of the marked declinein several McCaskie pers. comm.), and with the recent (late speciesover the past 3 decades(Arnold 1954; Cot- 1960) increaseduse for falconry, knowledge of the tam et al. 1961; Berger, Sindelarand Gamble 1969; Harris' Hawk's status in Arizona is critical. Peterson 1969; Sprunt 1969; Henny and Wight The primary purposeof this studywas to estab- 1972). lish base-line data on nesting distribution and In the southwest,particularly in Arizona, the abundance of the Harris' Hawk in Arizona and to rangesof somespecies ofraptors extendmarginally compare thesedata with the historicrecord to de- into the United States,and here populationstudies termine nestingsuccess of the present population. are usuallymost informative. Most of theseborder specieshave received little investigation.For exam- STUDY AREA AND METHODS ple, the Aplomado Falcon (Falcofemoralis)showed Arizona falls into a southwesternbi-seasonal climatic pattern of signsof declineas early as 1890 (Phillips,Marshall winter precipitation,spring dry period, summer precipitation, and Monson 1964). Its range and statusin Arizona and fall dry period.The springdry period(May -June) hashigher waspoorly documented(Phillips et al. 1964) until temp and the greatestinfluence on the plant and animal com- munity (Lowe 1976). the study of Hector (1975). Where it was once Sellers(1960) dividedthe stateinto homogeneoussections with thought to be locally fairly common, it is extinct respectto climate,topography, and vegetation.From the south- (Hector 1975).Because of habitatdestruction, Gray westsection (the area of importancein this study)to the plateau Hawk (Buteonitidus) populations have declined sectionthere are extreme changesin climaticconditions. The plateausection has an averageannual temp 20 to 25ø F lower and during the last century (Richard Glinski pers. annualprecipitation 38 to 51 cmhigher than the flat desertsof the comm.). southwest section. The southwest section contains the lowest, hot- The Harris' Hawk (Parabuteounicinctus superior) is test and driest areas of the state (Sellers 1960). another relatively unstudied southwesternraptor. Of the 6 life-zonesin Arizona, only the Lower Sonoran, con- Studies by Hensley (1959) and Mader (1975a, tainingportions of the Sonoran,Mojave and ChihuahuanDesert (Fig. 1), wasimportant to this study.The SonoranDesert has 2 1975b, 1977) are the only major works on this subdivisionsin Arizona-- the Lower Coloradoand Arizona Up- speciesin Arizona. Becauseof sudden decline and land subdivisions(Fig. 1). The Lower Coloradosubdivision (ele- 1 RAPTORRESEARCH 20 (1): 1-15 2 WAYNEH. W•AI•EY VOI•. 20, NO. 1 LOWER SONORAN LIFE-ZONE Sonoran Desert 50-1200 m. (a)Ar•zona Upland • (b)Lower Colorado •"• MohaveDesert / Ch•huahuanDesert .'.• H•stor•cald•str•but•on • Presentnesting dJstributmn • •Ciarkdale (Havesu Wlldl Refuge) Blythe0 Caclus Natl 50 0 50 I00 kilometers Figure1. Mapof thehistorical and present distribution of theHarris' Hawk in Arizonain relationto vegetational zones. vation 30-90 m) is primarily a creosotebush-bursage (Larrea- The Chihuahuan Desert enters Arizona in a small southeastern Franseria)community and includesthe hottestand mostarid re- sectionof the state,where it liesmostly above an elevationof 1070 gionsin Arizona.This subdivisionis characterized by sandyand m. This relativelycomplex desert is essentially shrubby with many gravellyplains and mesas, sand dunes, lava flows, silty valleys, salt grasses,several small species of cacti,and few deserttrees. basins,rocky hills, and desertpavement. The ArizonaUpland Field work was conductedin 1976 and 1977 from January subdivision(elevation 150-1220 m) is typified by a paloverde- throughOctober of eachyear. The Harris' Hawk in Arizonahas saguarocactus (Cericidium-Cereus) association. It attainsgreatest never been observednesting outside the Lower SonoranLife- developmenton therocky soils of desertmountain slopes and on zone, so I eliminated the northern half of the state from the thecoarse soils of upperbajadas that flank mountain ranges. The survey.After a broadgeneral search of the LowerColorado sub- ArizonaUpland subdivisionhas far greaternumbers of plant division,I determined that the Arizona Upland subdivisionwas speciesthan the Lower Coloradosubdivision. themost important area to nestingHarris' Hawks. Hills, windmill The MojaveDesert (elevation 240-1580 m) scarcelyreaches into towers,and otherelevated points were used to locatenests. In flat northwesternArizona. The higheraridity, lack of summerrains countryI usedan 8.5 m extensionladder mounted on thebed and andlonger periods of belowfreezing winter temp cause this desert cabof a pickuptruck which allowed visual clearance above the to be poorer in plant and animallife than the SonoranDesert vegetation. (Lowe and Brown 1973). All nest siteswere plotted on USGS 1:250,000topographic SPRING 1986 HARRIS' HAWK IN ARIZONA 3 quadranglesand seriesmaps. Throughout the populationcon- young/nestwere reported with only 2 parent birds t•nuumthere were local concentration points and from 2 of these caringfor them. Nestswere often built on top of old populousareas I attemptedto find all activenests to determine totalnesting density. The meandistance between nests was deter- Great Blue Heron (Ardea herodias) nests and were mined by measuringthe distancefrom eachnest and its nearest commonlyonly 3 to 5 m abovewater (Gale Monson neighborusing the distancebetween any 2 nestsonly once. By pets. comm.). A population decline started in the halving the mean distance,a value r (radius)was obtainedand late 1950's and by 1969 the Harris' Hawk disap- usedto determineaverage nesting range sizeby the formula A = r 2 under the assumptionthat nestingranges were circular. I used peared from the refuge. A wild populationhas not only thosenesting ranges that were activein a givenyear. sincebeen observedalong the Colorado River. A Each nest was visited a minimum of twice in order to determine concurrent decline and extinction was noted at Im- numberof eggslaid and numberof youngfledged (in 1976 nests perial National Wildlife Refuge to the south. An werevisited 3 to 5 times).When a clutchappeared small or incom- extensivelist of sourcesconcerning Harris' Hawks plete,a later visitwas scheduled in order to obtaindata for full clutch size. Young were aged by comparisonwith color photo- along the Colorado River is included in Whaley graphsof knownage birdstaken at 5 d intervals. (1979, AppendicesA and B). The Present Population (1976-1977). The RESULTS present Harris' Hawk population in Arizona oc- Distribution and Habitat Characteristics cupies3,880 km2 in Arizona Upland habitat with elevationsranging from 396 to 1,036 meters (X = Past Populations.-- Historically, there were 2 701 m). Lowe and Brown (1973) delineateprime localizedpopulations of Harris' Hawk in Arizona. Arizona upland habitatas the region "... eastthen One populationwas resident in westernArizona in north of a line drawn from Ajo to Tucson to Flor- Lower Coloradohabitat along the ColoradoRiver enceJunction, then northwestto Wickenburg"and from Yuma northward to Topock (Fig. 1). The the Harris' Hawk followedthis distributionclosely Lower Colorado population was extirpated by (see Fig. 1). Nearly all nests were placed in 1969. A secondpopulation approximately 280 km paloverde-saguarocactus habitat or in the more eastward in Arizona Upland habitat (Fig. 1) re- local narrow strips of blue paloverde-ironwood mainstoday. The 2 localitiesare separatedby a dry, (Circidiumfloridum-Olneya)habitat of the large ar- barren expanseof the Lower ColoradoDesert. royos.Three exceptionswere nestsplaced in large The earliest record for Harris' Hawk in Arizona cottonwoodtrees in riparian communitiesthat were wasreported near Topock by Kennerly(1859). The juxtapose to the aforementioned habitat. No nests Lower Colorado populationlikely originatedfrom were found, nor birds seen, in riparian com- Baja California, extendingits range
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