RaptorRes. 29(3):158-165 1995 The Raptor ResearchFoundation, Inc.

PRODUCTIVITY, FOOD HABITS, AND BEHAVIOR OF SWAINSON'S HAWKS BREEDING IN SOUTHEAST COLORADO

DAVID E. ANDERSEN 1 U.S. Fish and Wildlife Service,Colorado Fish and Wildlife AssistanceOffice, 730 SimmsStreet, No. 290, Golden,CO 80401 U.S.A.and Departmentof WildlifeEcology, Universityof Wisconsin,Madison, WI 53706 U.S.A.

ABSTRACT.--From1984 through 1988, I studiedSwainson's hawk (Buteoswainsoni) ecology during the breedingseason on the Pition Canyon Maneuver Site (PCMS) in southeastColorado. The numberof nestingattempts located and monitoredannually rangedfrom four in 1984, to 22 in 1987. Nestsused by Swainson'shawks were locatedpredominantly in one-seedjuniper (Juniperusmonosperma) or cotton- wood (Populusspp.) trees.Traditional nestingsuccess estimates averaged 0.64 and rangedfrom 0.42 in 1985 to 1.00 in 1984. Mayfield estimatesof nestingsuccess ranged from 0.27 (1988) to 1.00 (1984). Basedon prey remainscollected at nestsites, food deliveries to nestlingsconsisted primarily of smallbirds (50%) and mammals(45%), and diet breadth over the 5-yr studyperiod was high. Minimum-convex- polygonhome-range size of radio-markedadults during the late-nestling and post-fledging period averaged 21.2 km2 in 1985 and 27.3 km2 in 1986, with males exhibiting larger home rangesthan females(P = 0.15) acrossyears. Compared with other breedingSwainson's hawk populations,breeding area reoccu- pancy amongyears on the PCMS was moderatelyhigh, home rangesduring the late-nestlingand post- fledgingperiods were large, and ground-nestingbirds were importantin the breeding-seasondiet. KEY WORDS: Buteo swainsoni;Colorado;food habits; home range; reproduction; Swainsoh's hawk.

Productividad,hfibitos alimentarios y conductade nidificaci6nde Buteoswainsoni en el surestede Colorado R•.sUMl•N.--Desde1984 a 1988, estudi• la ecologlade Buteoswainsoni durante la estaci6nreproductiva en el Pition Canyon Maneuver Site (PCMS), al surestede Colorado. E1 nfimero de nidificaciones localizadasy monitoreadasanualmente van desdecuatro en 1984 a 22 en 1987. Los nidosde B. swainsoni se 1ocalizaronpredominatemente en firbolesde las especiesJuniperus monosperma y Populus sp. E1 •xito tradicionaldel nido fue estimadoen un promediode 0.64 y conun rangode 0.42 (1984) a 1.00 en 1984. Basadoen losrestos de presacolectados en lossitios de nidificaci6n,el alimentoentregado a lospolluelos consisti6primariamente en pequefiasaves (50%) y mamlferos(45%); la amplitudde la dietaen un perlodo de cincoaftos de estudiofue alto. Durante losperlodos polluelo-tardlo y post-volant6n(juvenil), el tamafio del rangode hogar(pollgono convexo mlnimo) de adultosradio-marcados fue en promediode 21.2 km2 en 1985 y 27.3 km2 en 1986; los machosexhibieron mayores rangos de hogar que las hembras(P = 0.15) a travis de los aftos.Camparando la reocupaci6ndel firea de nidificaci6nentre aftosen el PCMS, con otras poblacionesreproductivas de B. swainsoni,fue moderadamentealta; el rango de hogar durantelos perlodospolludelo-tardlo y post-volant6nfue extensoy las aves que nidificaronen el suelo fueron importanteselementos de la dieta en la estaci6nreproductiva. [Traducci6n de Ivan Lazo]

Swainson'shawks (Buteo swainsoni)breed pri- those studies were conducted in areas that contained marily in grasslandand other openhabitats (Johns- a significant proportion of habitat that had been gard 1990, Andersen 1991). Several studies of convertedto crop production(e.g., irrigated mead- Swainson'shawk breedingseason ecology have been ows[Dunkle 1977], cultivatedlands [Olendorff 1972], conductedin grassland habitats (Olendorff 1972, and pasture and hay fields [Gilmer and Stewart Dunkle 1977, Gilmer and Stewart 1984). However, 1984]). Few studieshave beenconducted in largely unaltered habitats (Schmutz et al. 1980, Bednarz • Current address:Minnesota Cooperative Fish and Wild- and Hoffman 1988), and none has been conducted life ResearchUnit, U.S. National BiologicalService, De- in shortgrassprairie habitat lacking significanthu- partment of Fisheriesand Wildlife, University of Min- man disturbancesthat include crop production. nesota, St. Paul, MN 55108 U.S.A. Recently, concernhas been expressedregarding

158 SEPTEMBER 1995 COLORADO SWAINSON'S HAWKS 159 the populationstatus of Swainson'shawks in several portionsof their breedingrange (Littlefield et al. 1984, Woffinden 1986, Janes 1987, Risebrough et al. 1989, Estep and Tersa 1992). The causesof populationdeclines are not clear (Risebroughet al. 1989), although disagreementexists regarding the influenceof crop productionin grasslandecosystems on Swainson's hawks (Bechard 1983, Gilmer and Stewart 1984, Schmutz 1984, 1987, 1989, Schmutz and Hungle 1989, Bechard et al. 1990). To place current population levels in historical perspective and to understandthe potentialimpact of conversion of grasslandsto agriculturalcultivation on breeding Swainson'shawks, information from breedingpop- ulationsin relativelyunaltered habitats may be use- Figure 1. Locationof the Pition Canyon Maneuver Site ful. Herein, I describethe nestingecology of Swain- in southeasternColorado. Shaded areas representthe ap- son'shawks in shortgrassprairie habitats in south- proximate extent of pinyon-juniper woodland habitats. eastColorado, where the predominantland use since Unshadedareas representshortgrass prairie habitats. the late 1880s has beenlivestock grazing.

STUDY AREA AND METHODS stick nestsand observationsof Swainson'shawks during The studywas conductedon the 1040-km2 Pition Can- the breedingseason were plottedon 1:24000 U.S. Geo- yon Maneuver Site (PCMS) in southeastColorado (Fig. logicalSurvey topographic maps. Old nestsand the im- 1) from 1984 through 1988. Elevation on the PCMS, mediatevicinity around where adults were sightedwere located adjacentto the northwestrim of the Purgatoire searchedfor evidenceof breeding attempts. Each year I River Canyon in Las Animas County, rangedfrom 1310- searchedthe immediate vicinity of all Swainson'shawk 1740m (U.S. Departmentof theArmy 1980). Topography nests that had been located in previous years. Nesting consistedof broad, moderatelysloping uplands bordered attemptsand potential nest sites were also identified during by the Purgatoire River Canyon on the southeast,lime- productivitysurveys conducted from helicoptersduring stonehills on the west, and a basalthogback on the south. June and July for nestingred-tailed (B. jamaicensis)and Averageannual precipitationon the semi-aridPCMS was ferruginoushawks (B. regalis). 32 cm, but it fluctuated widely from year to year and Swainson's hawk territories on the PCMS were defined amongsections of the studyarea (U.S. Departmentof the basedon the presenceof nestingattempts. Individual nests Army 1980). Mean monthlytemperature ranged from -1 ø that were usedfor nestingin > 1 yr were includedin the C in January to 23ø C in July. same territory. In addition, different nestsbetween and Vegetationon the PCMS was dominatedby shortgrass amongyears were includedin the sameterritory when the prairie and pinyon (Pinusedulis)-juniper (Juniperus mon- distancebetween nest siteswas smaller than the average osperma)woodland (Costello 1954, Kendeigh 1961). Dom- minimum distancebetween nesting attempts among ter- inant perennial grassspecies included blue grama (Bou- ritories from 1984 through 1988 (N -- 6 territories), or telouagracilis), sideoatsgrama (B. curtipendula),western when individualsequipped with radiotransmittersnested wheatgrass(Agropyron smithii), galleta (Hilaria jamesii), at different nestsbetween years (N = 3). and needle-and-thread(Stipa comata). Dominant trees and Accessiblenests were climbed to at least once during shrubsincluded pinyon pine,juniper, cholla (Opuntiaim- the nestlingperiod, except in 1987 and 1988, when only bricata),yucca (Yucca glauca), fourwing saltbush (Atriplex a portionof nestswere visited.At eachvisit, nestlingswere canescens),broom snakeweed (Gutierrezia sarothrae), Big- weighedand the ageof nestlings(days since hatching) was elow sagebrush(Artemisia bigelovii), mountain mahogany estimatedbased on fourth primary measurements(Peter- (Cercocarpusmontanus), winterfat (Ceratoidesspp.), and sen and Thompson1977, D.E. Andersenunpubl. data) rabbitbrush (Chrysothamnussp.). For a more complete Hatching dateswere estimatedfrom nestlingage, based descriptionof vegetationon the PCMS see Shaw and ona 34-d incubationperiod (Bednarz and Hoffman 1988). Diersing (1990). Predominant land use on the PCMS Terminologyand definitionsrelated to reproductionfol- since settlementby people of European descentin the low those of Steenhof (1987). A breeding territory was 1880s (Friedman 1985, Knight et al. 1989) has been live- identified when young were raised or eggswere laid, or stockgrazing. an adult was observedin incubating posture on a nest. I locatedSwainson's hawk nestsby searchingpotential Nestingsuccess rates were calculatedusing both the May- nestinghabitat (e.g., isolatedcottonwood [Populus spp.] field and traditional methods(May field 1961, 1975, John- trees and the ecotonebetween grassland and pinyon-ju- son 1979, Steenhof and Kochert 1982)--I used a 34-d niper woodland)on foot or horseback,from a vehicleor incubationperiod and a 45-d nestlingperiod (Bednarz and all-terrain cycle,and from a helicopter.Locations of old Hoffman 1988) in calculatingMayfield nesting success 160 DAVID E. ANDERSEN VOL. 29, NO. 3

Table 1. Number of breedingpairs, nestingsuccess, and productivityof Swainson'shawks on the Pition Canyon Maneuver Site, Colorado, 1984-88.

YOUNGFLEDGED/ SUCCESSFUL NUMBER OF PAIRS NESTING SUCCESSa YOUNGFLEDGED/ BREEDING YEAR BREEDING SUCCESSFUL TRADITIONAL MAYFIELD BREEDING PAIR ATTEMPT

1984 4 4 1.00 (4) b 1.00 (3) 1.75 (4) 1.75 (4) 1985 12 5 0.42 (12) 0.33 (11) 0.75 (12) 1.80 (5) 1986 15 8 0.53 (15) 0.31 (14) 0.93 (15) 1.75 (8) 1987 22 12 0.60 (20) -- 1.00 (18) 1.64 (11) 1988 7 4 0.67 (6) 0.27 (5) 0.83 (6) 1.25 (4) Total 60 33 Mean 0.64 0.48 1.05 1.64 Terminologyfor nestingsuccess is after Steenhofand Kochert(1982) and Steenhof(1987). Number of nests from which estimate was derived. estimates.A nesting attempt was classifiedas successful cordedfrom 1:24000 topographicmaps of the studyarea when young were >-50% of averagefledging age when the to the nearest 100 m for each radio fix, and the behavior status of the nest was last known (based on feather de- (perchedor flying) of the bird and whether the fix was velopmentand behavior)or were observedfree-flying in basedon visualobservation or telemetry signalwere noted the vicinity of the nest. Minimum convexpolygon (MCP) homeranges were cal- At eachnest visit, prey remainswere removedfrom the culatedfor the late-nestlingand post-fledgingperiod using nest for identification and measurement.Prey remains the computerprogram SEAS (J.R. Cary, University of were identified using guides to local fauna (Armstrong Wisconsin,Madison). All sequential locationswere in- 1972, Burt and Grossenheider 1976, Hammerson 1982, cludedin MCP analyses(Andersen and Rongstad1989) National GeographicSociety 1983) or by comparisonto Descriptivestatistics, pairwise statistical tests, and anal- reference material collected on the PCMS. Diet breadth ysis of variance (ANOVA) proceduresfollow thoseout- was calculatedusing Levins' (1968) formula based on lined in Snedecorand Cochran (1980). Chi-square tests frequenciesacross years of individual prey speciesin the for independenceare after Gibbons (1985). diet, followingthe suggestionof Greeneand Jaksi• (1983). Pocketgophers (Thomomys, Geomys) were combinedinto a singlegroup without identificationto species.Birds not RESULTS •dentifiedto specieswere excludedfrom diet-breadthanal- yses. Reproduction. The number of breedingterrito- In 1985 and 1986, adult Swainson's hawks that were ries on the PCMS ranged from four in 1984 to 22 membersof breedingpairs were capturedusing a variation in 1987 (Table 1). Traditional nestingsuccess aver- of the techniquedescribed by Bloom et al. (1992). Cap- aged0.64 (coefficientof variation [c.v.]= 0.34), and tured birds were fitted with battery-powered,solar-as- sistedradiotransmitters attached as a backpack(Andersen the averageage of youngin the nestat the last time 1994).Radio-equipped Swainson's hawks were sexed based when the nest statuswas known ranged from 65- on observationof relative sizeand behaviorsubsequent to 80% of fiedgingage. Mayfield estimatesof nesting capture. successaveraged 0.48 (c.v. -- 0.73). Young fledged During the periodfrom capturethrough migration from the study area (approximatelymid-September), Swain- per successfulbreeding attempt averaged1.64 and son'shawks equipped with transmitterswere locatedand exhibitedlittle variation (c.v. = 0.14) amongyears. followed for 3-4-hr tracking periods,with locationsre- Estimatedhatching dates were concentratedin mid- cordedat 0.5 hr intervals (Andersenand Rongstad1989). to late June, and extendedinto July in 1984, 1986, Individual birds were trackedin either the morningor the and 1988. No differencesin hatching dates were afternoon at approximately 7-10-d intervals systemati- cally through the study period (Andersenand Rongstad evidentamong years (1-way ANOVA, F3,•9= 0.51, 1989). Fixes were obtainedby a singleobserver during a P = 0.680). trackingperiod and were eitherbased on visualobservation A total of 34 territories and 60 nests were iden- or triangulationfrom telemetrysignals. Fixes basedonly tified on the PCMS from 1984 through 1988. Swain- on telemetry signalswere obtained by a single observer receivingsignals from morethan two locationsin sequence son'shawk nestswere primarily locatedin junipers and plottingsignal direction on 1:24000 topographicmaps. (76% of 60 nests)and cottonwoods(15%), with four Universal transversemercator grid coordinateswere re- of the remainingnests in elms (Ulmussp.) planted SEPTEMBER 1995 COLORADO SWAINSON'S HAWKS 161

Table 2. Number and frequencyof occurrenceof prey remains collectedat 20 Swainson'shawk nest siteson the Pition Canyon Maneuver Site, Colorado,1984-87.

YEAR TOTAL

SPECIES 1984 1985 1986 1987 NUMBER %

Birds Western meadowlark (Sturnellaneglecta) 2 1 1 3 7 11.7 Horned lark (Ereraophilaalpestris ) 6 0 0 0 6 10.0 Mourning dove (Zenaidaraacroura) 1 0 0 2 3 5.0 Scaledquail ( Callipeplasquaraata) 0 1 0 0 1 1.7 Lark bunting (Calaraospizaraelanocorys) 0 0 0 1 1 1.7 Unidentified birds 1 8 1 2 12 20.0 Total Birds 10 10 2 8 30 50.0

Mammals Pocketgophers (Geomyidae) 3 9 9 0 21 35.0 Spottedground squirrel (S•oerrnophilusspilosoma) 1 0 1 0 2 3.3 Ord's kangaroorat (Dipodoraysordii) 0 2 0 0 2 3.3 Desert cottontail(S•vlvilagus audubonii) 0 1 0 1 2 3.3 Total Mammals 4 12 10 1 27 45.0

Reptiles Eastern fence lizard (Sceloporusundulatus ) 0 0 1 0 1 1.7 Texas hornedlizard (Phrynosoraacornutura ) 0 0 1 0 1 1.7 Unidentified snake 0 0 1 0 1 1.7 Total Reptiles 0 0 3 0 3 5.0 Total 14 22 15 9 60 100.0 as windbreaks,and one in a pinyon pine. When I and diet breadth for all years combinedwas rela- comparedthe proportionof territorieswhere differ- tively high (B = 6.52). ent nest structures were used, Swainson's hawks Home Range and Movements. Six Swainson's predominantlyused territories with junipers (73% hawks were captured and fitted with radio trans- of 34 territories) and cottonwoods(12%) as nest sites mitters. Two of those individuals returned to the (2 [6%] additional territories had nestsin both ju- study area the year following capture with func- nipersand cottonwoodsin differentyears). Nearest- tioning radios, and were radiotracked during two neighbor distancesbetween nests ranged from 3.4 breeding seasons.The remaining individuals were km in 1987 to 9.2 km in 1984, and averaged5.6 km tracked during one breeding season(Table 3). In- overthe 5-yr studyperiod. On average,nesting at- dividual hawks were monitored for an average of temptswere locatedin territories55% of the years 6.5 tracking periodsper season,resulting in an av- that they were monitoredwhen includingthe year erage of 57 locationsper bird. Locationswere ob- that territorieswere first identified,and 31% of years tained based on direct visual observation (30.4%), subsequentto the year they were first identified. extrapolationfrom an immediatelypreceding or sub- Food Habits. A total of 60 prey remains was sequentdirect visualobservation (18.7%), extrapo- collectedfrom 20 nestsites on 12 breedingterritories lation within a tracking period where the bird was from 1984 through1987 (Table 2). No prey remains observedat least once(28.3%) but not immediately were encounteredat nestsin 1988. Fifty percentof precedingor subsequentto the fix, or only on re- prey itemsencountered were birds,45% were mam- ceptionof a telemetrysignal (22.6%). mals,and the remaining5% werelizards and snakes. CombiningMCP estimatesfrom both malesand Excluding reptiles (N = 3), the relative frequencies females,home range size averaged21.3 km2 in 1985 of birds and mammals in prey remains were not and 27.3 km2 in 1986 (t = -0.74, df = 5, P = 0.49). independentof year (X2 = 13.41, df = 3, P < 0.005) Males (• = 31.7 km2) tended to have larger home 162 DAVID E. ANDERSEN VOL. 29, NO. 3

6O Table 3. Tracking history, behavior, and post-fledõinõ seasonhome range size of adult radio-equippedSwain- son'shawks on the Pition Canyon Maneuver Site, Colo- rado 1985-86.

NO. OF TRACK- TOTAL % OF MCP IDENTIFICATION INC NO. OF LOCA- HOME 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1900 1900 NUM- PERI- LOGA- TIONS RANGE TIME OF DAY YEAR BER SEX ODS TIONSFLYING (km2)

Figure 2. Radio-telemetrylocations for six Swainson's 1985 8 F 8 64 78 24.4 hawks capturedand monitoredin southeasternColorado 17 M 7 67 87 23.8 from 1986-87 as a function of time of day. 10 F 4 31 84 6.8 18 M 7 62 92 30.0

1986 8 F 7 69 55 12.1 10 F 7 63 81 34.2 26 M 6 52 71 41.3 rangesthan females()7 = 19.9 km2),both when MCP 30 F 6 52 63 21.7 area for each breedingseason and each year were treated as independentobservations (t = 1.69, df = 6, P = 0.15), and when I calculatedan averageMCP homerange size for the two femalesthat weretracked (e.g.,tawny owls [Strix aluco; Southern 1970]; great •n both 1985 and 1986 (t = 2.21, df = 4, P = 0.16). horned owls [Bubovirginianus; Rusch et al. 1972, The proportionsof locationsobtained in each1-hr Mclnvaille and Keith 1974]; ferruginoushawks time interval from 0800-1800 H when all locations [Smith et al. 1981]), the most highly variablere- from all tracking periodswere combined,were not productiveparameter for Swainson'shawks on the distributed significantly differently from random PCMS appearedto be the proportionof pairs that (x2= 14.08, df = 9, P > 0.05). Samplingintensity attemptednesting. prior to 0800 and after 1800 was not comparableto It is not clear from this study,however, whether intensityduring that time interval (Fig. 2). During all territorieswere occupiedannually, evenin the the periodsthat radio-markedbirds were monitored, absenceof a nestingattempt, or whetherthe PCMS both males ()7 = 83% of locations)and females ()7 = populationof Swainson'shawks trackedlocal prey 72% when calculatedas independentobservations populations, as has been suggestedelsewhere and )7= 71% when basedon between-yearaverages), (Schmutzand Hungle 1989). Similar to other tem- spentthe majority of their time flying. perate-zoneraptors, the number of young fledged per successfulnesting attempt was relativelystable overthe 5-yr period(Newton 1979), suggestingthat DISCUSSION breedingconditions in territoriesthat fledgedyoung Reproduction. Swainson'shawks nesting in were relativelyconstant among years, or that suc- shortgrassprairie habitatin southeastColorado from cessfulbreeders adjusted to changing conditions. 1984 through 1988 exhibitedmoderate nesting suc- There was no evidencefor brood reduction,observed cessand stableproductivity. The numberof nesting in other areasin responseto low prey availability attemptslocated annually was highlyvariable. These (Bechard 1983). resultsare comparableto descriptionsof reproduc- Food Habits. Basedon frequencyof preyremains tive parametersfrom other portionsof the breeding collectedat nest sites from 1984 through 1987, rangeof Swainson'shawks, where the mean number Swainson'shawks on the PCMS preyedheavily on of young fledged per successfulnest ranged from ground-nestingbirds and smallmammals (Table 2). 1.19-2.00, and the mean number of young fledged Thesefood habits differ from thosereported in most per breedingpair rangedfrom 1.11-1.85 (Olendorff other publishedstudies in that birds compriseda 1972, Dunkle 1977, Fitzner 1978, Bednarz and highproportion of the diet, comparedto the reported Hoffman 1988). As has been notedin other raptors predominanceof small and medium-sizedmammals SEPTEMBER1995 COLORADOSWAINSON'S HAWKS 163 in other locations.In Wyoming, Dunkle (1977) re- quentialtriangulation from the groundby one ob- portedthat 68% of prey itemswere small mammals server. I was unable to estimate the magnitude of and lagomorphsand in Utah, Smith and Murphy the error associated with either of these two sources. (1973) reportedthat only 17% of prey items were However, neither of these sourcesof error likely birds. In North Dakota, Gilmer and Stewart (1984) introducedbias into estimatesof averagehome range foundthat groundsquirrels and pocketgophers con- size of Swainson's hawks on the PCMS. Rather, stitutedthe majorityof prey itemsof nestingSwain- thesesources of error probably increasedthe vari- son'shawks. Similarly, Schmutzet al. (1980) re- ance associatedwith averagehome range size esti- portedthat prey items of nestingSwainson's hawks mates,reducing the power of statisticalcomparisons. in Alberta consistedof 85% mammals,67% of prey BreedingSeason Ecology. Ecology of Swainson's items removed from Swainson's hawk nests in Mon- hawks breeding in shortgrassprairie habitat in tana were mammals (Restani 1991), and small southeastColorado may be characterizedas inter- mammalswere the predominantprey of Swainson's mediate between raptors that are territorial year hawks in Washington(Bechard 1983). In Mexico, round, and those that are nomadic and exhibit nu- Thiollay (1981) observedlizards and small rodents merical responsesto temporary prey abundancein as the primary prey speciesof nesting Swainson's localized areas. Swainson's hawks return to the hawks, and in New Mexico, nesting Swainson's breedinggrounds after potential competitorsfor nest hawks preyed predominantlyon insectsand lago- sites(e.g., red-tailed and ferruginoushawks and great morphs(Bednarz 1988, Bednarzand Hoffman 1988). horned owls) have already initiated nesting.They In contrast, in California, Swainson's hawks have establishterritories that are defendedagainst con- been observedto include birds as the predominant specificsand may defend these territories against prey in the breeding-seasondiet (Estep 1989). other speciesof raptors (Rothfels and Lein 1983, Although samplesize (60 items from 20 nestsites Janes 1984, Bechardet al. 1990, Restani 1991), or over4 yr) wassmall, Swainson's hawks on the PCMS alternatively,nest in associationwith other raptors appearedto havea relativelybroad diet. Diet breadth (Schmutz et al. 1980, Thurow and White 1983). On of Swainson'shawks on the PCMS compareswith the PCMS, late-nestling and post-fledging period that of red-tailed hawks in Idaho, the specieswith home range (and possiblybreeding territory) size the mostgeneral diet of three large breedingraptors appearsto berelatively large, and there is a moderate studiedby Steenhofand Kocherr(1988). High diet rate of breedingterritory reoccupancyand high vari- breadth of Swainson's hawks on the PCMS is in ability in the number of nestingattempts initiated large part attributable to high variability in food among years. Reproductivesuccess is moderately itemscollected among years (Table 2). variable, and productivityof successfulnests is rel- Home Range and Movements. Estimatedhome atively high and stable. range sizesof adult Swainson'shawks during the This reproductivestrategy may in large part be late-nestlingand post-fledgingperiods on the PCMS explainedin terms of annual variability in prey re- were similar to home ranges reported from other sources.Prey availabilitythat is unpredictableand comparablestudies. Radio-equippedmale Swain- highly variable may result in birds establishingor son'shawks in Washington exhibited an average reoccupyingterritories annually, but only breeding homerange sizeof 8.9 km2 (Bechard 1982). In Cal- in yearsin which prey availabilityis abovea mini- ifornia, Estep (1989) observedhome rangesaver- mum threshold(Southern 1970). Abovethis thresh- aging 27.6 km2 for 12 radio-marked Swainson's old, nest successmay be influencedby density-in- hawks during the breedingseason. On the PCMS, dependentfactors (e.g., weather), which causefail- Swainson'shawks had relativelylarge home ranges, ure of the nestingattempt rather than reducingbrood similar in size to thosereported in California, and size.Whether this reproductivestrategy was typical spentthe majorityof their time budgetflying. of Swainson'shawks acrossthe breedingrange prior Two potentialsources of error in calculatinghome to extensivehuman-induced changes in landscape rangesize were present in datacollected in thisstudy. patternsis not clear. However, reproductiveecology First, becauseSwainson's hawks on the PCMS spent of Swainson's hawks on the PCMS can serve as a the majority of their time flying, estimatingfixes basisfor comparisonfor other populationsof Swain- precisely,even when birds were observed,was dif- son'shawks in areas where grasslandshave in part ficult. Second, 22.6% of fixes were obtained via se- or in whole been convertedto crop production. 164 DAVID E. ANDERSEN VOL. 29, NO. 3

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