Behavior and Habitat Use of Breeding Northern Harriers in Southwestern Idaho

Home , Hen harrier

J RaptorRes. 21(2):57-66 ¸ 1987 The Raptor ResearchFoundation, Inc. BEHAVIOR AND HABITAT USE OF BREEDING NORTHERN HARRIERS IN SOUTHWESTERN IDAHO

JOHN W. MARTIN

ABSTR^CT.--Radiotelemetricand visual monitoring of fourbreeding pairs of the NorthernHarrier (Circus cyaneus)in predominantlyshrub-steppe habitat of the SnakeRiver Birdsof PreyArea, Idaho,indicated that harriersused riparian and cultivatedhabitats disproportionately. As alfalfa growthapproached 46 cm height,males shifted from predatingvoles (Microtus sp.) in alfalfa fieldsto the WesternWhiptail (Cnemidophorustigris) in openshrub-steppe habitat. Mean minimumhome ranges of maleswere estimated at 15.7 km2 while thoseof femaleswere estimatedat 1.13 km2. Males ranged up to distancesof 9.5 km from nests.Males huntedmost intensively in the secondweek after hatching.Females did not hunt until the third weekafter hatching.During mostof the day, bothsexes rested or preened<0.5 km from nests. Varyinghome range sizes in thisand other studies may be a functionof harrierresponses to differing habitatstructures and prey availabilityor vulnerabilityto capture.

Breedingbehavior of theHolarctic Northern (Hen) low (Salixsp.), and tamarisk(Tamarix pentandra). Grease- Harrier (Circuscyaneus) has been studiedand dis- wood (Sarcobatusvermiculatus) formed a transitional zone betweenriparian and shrub-steppehabitats and was in- cussedby Breckenridge(1935), Bent (1937), Brown cludedin the riparian habitat category.Shrub-steppe was and Amadon (1968), Hamerstrom (1969) and Wat- the dominantupland habitat, consistingprimarily of big son (1977). These and other studies showed that sagebrush(Artemisia tridentata), cheatgrass (Bromus rec- male participation in the breeding effort is limited torurn),and shadscale(Atriplex confertifolia)associations. primarily to provisioningprey for their matesand A variety of cropsmade up cultivatedfields, primarily alfalfa (Medicagosatira), wheat (Triticumaestivum), and young, with incubation or care of young rarely re- sugarbeets (Beta vulgaris).Cultivated fieldswere depen- corded (Bildstein 1979; Thompson and Cornely denton water throughsprinkle or floodirrigation. Aban- 1982). Male abandonmentof the breeding effort donedfields were dry fallow cultivatedfields invadedby prior to fiedging of young is not uncommonand is halogeton(Halogeton glomeratus), cheatgrass, and Russian thistle (Salsolakali). Barren soilswere a variety of mine usuallyassociated with increasedhunting activityby tailings,gravel pits, cliffs, and lava beds. females (Hamerstrom 1969; Watson 1977). Habitat analyseswere made by overlaying 1:24000 Few studieshave attemptedto quantify male and USGS topographicmaps with a grid. Each square size female harrier behavior and activities related to hab- represented0.405 ha and was assigneda habitat type itat utilizationin upland areas.This studywas con- determinedfrom aerial photographs,vegetation maps pro- videdby the BoiseDistrict Officeof the Bureau of Land ducted from March through July 1981 with the Management,and from field observations.A circulararea objectiveof determiningbreeding activities in a hab- of approximately200 km2 was analyzedsurrounding each itat type not typically associatedwith harriers. of the studynests. On this basis,habitat percentageswere calculated in order to detail harrier habitat utilization. Selectedalfalfa fieldsin the studyarea where harriers STUDY AREA AND METHODS were observedhunting were measuredfor height from the Four nestingpairs of harriers were selectedin the Snake groundalong linear transectsof 10 points.Cutting sched- River Birdsof Prey Area (BOPA) in southwesternIdaho, ules were observed and recorded. a 338778 ha sanctuarywithin the Upper Sonoran life Six harrierswere capturedusing a variety of techniques. zone of the Great Basin Region characterizedby a cold Two malesand two femaleswere capturedusing a noose- desertphysiognomy and climate (Odum 1971;USD I 1979). halo (Scharf 1968) and a mounted Great Horned Owl Nestingor pair-bondedharriers were locatedusing meth- (Bubo virginianus)on a portable one m high, placed odsdiscussed by Hamerstrom(1969, 1986). Emphasiswas 3-10 m from the suspectednest. One male was captured placedon determiningbehavior and activitiesof breeding usingan octagonalbal-chatri (Ericksonand Hoppe 1979) male harriers occurring within the BOPA. Precautions with a Montane Vole (Microtusmontanus) as bait. Another were taken to avoid investigatordisturbances near nests male was captured using a mist net set ->1 m behind a [i.e., tramplingvegetation and creatingpaths for predators mountedowl. A dho-gazaand mountedowl (Hamerstrom to follow (Fyfe and Olendorff 1976)]. 1963) attracted several adult harriers but failed to result Habitats were categorizedinto six types:water, ripar- in any captures.Harriers may havedetected dho-gazas or ian, shrub-steppe,cultivated fields, abandoned fields, and mist netsdue to motioncaused by prevalentwinds. A wire barren soils.Water habitatswere openwaters of the Snake dome with monofilament nooses(Burke 1979) or bownet River, reservoirs,and irrigation systems.Riparian habitat (Hamerstrom 1969) placedover the nestwas not useddue consistedprimarily of pampasgrass (Phragmites cornmu- to possibleabandonment of eggsor young (Hamerstrom ms),bulrush (Scirpus sp.), stinging nettle (Urtica sp.), wil- 1969).

57 58 JOHN W. MARTIN VOL. 21, NO. 2

Each individual capturedwas measured,weighed, and length of primary feathers(Munoff 1963; Hamerstrom aged (Hamerstrom 1968). Colored plastic and USFWS 1968; Scharf and Balfour 1971). bandswere placedon the legsof eachharrier, and a radio Activities and habitat utilization of individuals were transmitter was attachedto a central rectrix using tech- combinedafter synchronizingnesting chronologiesinto niques describedby Dunstan (1973), Beske(1978), and similar compositecategories for statisticalanalyses. Sta- modifiedby Martin (unpubl.). Transmittersused were tistical methods used are from Snedecor and Cochran AVM SM-1 units operatedat 164 MHz weighing about (1967). A breedingeffort was assumedto commencewith five g when encasedin dental acrylic. Transmitter whip the laying of eggs(through incubation of about 30-32 d), antennas were trimmed to about 200 mm. Receivers were and endingabout 35 d after hatching(with the young AVM LA-12 units with 10 subchannels,used alternately capableof flight), or about 70 d total (Hammond and with three-elementYagi or Adcock antennas. Henry 1949; Hamerstrom 1969). After hatchingof eggs, Prior to capture efforts,monitoring of harrier activities activities and behavior were broken into seven-d units for beganin March during courtshipand nestbuilding. Dis- analysis.For someanalyses, sample sizes were inadequate tinctivemarkings and plumages(such as moulting or color to list in weeklyincrements and were groupedinto three patterns)were usedto distinguishindividuals. After radio- major phasesof breeding:incubation, brooding (from hatch taggingharriers, behaviorand habitat use were sampled to the end of the secondweek), and post-brooding(from instantaneously(Altmann 1974) at 10-minintervals from the start of the third week to the end of the fourth week 0500-2200 H (MST). Monitoring was conductedin 17- or as youngfledged) (Watson 1977). hr blocksrotating once every seven d to eachof the nests. The sampling effort was equal through all time periods RESULTS of the day for the lengthof the study.Emphasis was placed on trackingmales of a givennest as I assumedthey would Activities and Behavior--Males. Males were be far ranging, hunting for themselvesand their mates, observedhunting at first light (+30 min prior to while femalesremained at the nests(Watson 1977). When sunrise).No prey deliverieswere recordedbefore a repeateddirection of travel to and from the nest could be determined,alternate tracking days were spentin the 0530 H (MST) and only one prey delivery was suspectedhunting areasand the nest.When possible,con- observedafter 1900 H (MST). Prey deliveries/d tact between two investigatorswas maintained by two- peakedfrom 0900-1200 H (MST) and this pattern way radios.One investigatoralways remained near a given remained constantthroughout the breeding season nest in the event that visual or radio contact was lost. In (Fig. 1). Copulationsassociated with prey given to this manner, lost contact could be regained as the male harrier returned to a nest with prey and female behavior femaleswere observedfor 15.6% of the total prey and activitiescould be moni-toredconcurrently with males. deliveriesby males(N = 96), 48.0% occurringprior Home rangeswere determinedusing the non-statistical to 0900 H (MST), 35.0% from 0900-1200 H (MST), minimum polygon method (Mohr 1947; Jennrich and and 17.0% from 1200-1500 H (MST). All copu- Turner 1969). Radio telemetry was used primarily to assistin visually locatinga harrier within a given habitat lations observedwere in associationwith prey de- type, then observeits behavioralactivities as suggestedby liveriesand continueduntil day 15 of incubation. Craighead et al. (1963) and Mech (1983). Radio trian- The numberof prey itemsdelivered/d by male gulatedpositions (Mech 1983) were usedto clarify harrier harriers to the nest or females peaked during the locationswhen necessary.All missed data points were secondweek post-hatching.Concurrently, intervals ehminatedfrom statisticalanalyses. Harrier activities were placed into classesbased on a between prey deliveries shortenedand observed schemesimilar to thoseused by Linner (1980), Schipper hunting activitiesincreased (Fig. 2). For the entire (1977), and Bildstein (1982) [i.e., loafing (perching,rest- season,the males deliveredan averageof 3.5 prey ing, or preening),feeding, agonistic interactions (both in- items/dto the femaleor nest.Prey deliveriesranged tra- and interspecific),soaring or gliding, hunting flights (either border following or quartering), and other flights from one to three items/d during courtshipand in- 0ncludingpowered transit or high flights)].Starting, end- cubation,to a maximum of sevenitems/d during •ng, and duration of flightsor activities,prey deliveries, brooding,and decreasingto two to three items/d as prey items identified, and compassdirections of travel to chicksapproached fledging. Male harriers were in and from the nests were recorded. flight during 52% of total observations.Only 20% Nesting chronologyof each nest was determinedby backdatingthe first prey taken into the nest, indicating of male flight activitiescould be classedas hunting hatchof eggs(Hamerstrom 1969). Due to the closeprox- behavior (Table 1). imity of two of the studynests, it was possibleto observe Early in the seasonmales spent most of their time harrier behaviorand activitiesconcurrently, noting initial loafing near nests(Tables 1 and 2). If monitoring prey deliveriesto newly hatchedyoung, without interfer- ing with the establishedmonitoring schedule. In addition, had been conductedon a 24-hr basis,it is probable two of the nestsoccupied by radio-taggedharriers were that the number of observations of males near nests examined to determine the age of young by weight and would have beengreatly reduced.At leastone male SUMMER 1987 NORTHERN HARRIERS IN IDAHO 59

INITS 6

2O -5 15- //• FemalesMales....

1 PREYITEM S---'-•---- c • / 0500 1000• 1500• ...... 20;00 240 INCUBATION I 7 S 14 15 21 22 28 29+ HOUR OF THE DAY DAYS DAYS DAYS DAYS DAYS

Figure 1. Numberof prey deliveries/hourof the day for Figure2. Frequencyand number of preyitems delivered malesand females. to nestor femaleby male Northern Harriers did not roost within two km of the nest from the marily with other male harriers near nests(73.1%, onset of incubation. After eggs hatched, the time N = 82). Most of theseinteractions occurred early malesspent near nestsdecreased more rapidly than in the seasonas territories were being established. females.Foraging sortiesby male harriers of 4.5- Interactionswith neighboringmales and femalesde- 9.5 km were routinely observed.As the breeding clined through the breeding season.However, re- seasonprogressed, harriers tendedto range farther, action to other harriers approachingthe nest terri- but theseobservations could not be quantifiedbefore tory, even from nearestneighbors, always resulted the study terminated. in vigorouspursuit. Males defendedan area of about Flightsto andfrom nests followed certain compass 0.78 ha with the nest at the center. Borders between bearingsthat were traveled repeatedlythroughout territorieswere often definedby vegetationchanges the nestingseason at at leasttwo of the studynests. or heightdifferences, fence lines, or topographicfea- Flight patternswere establishedduring the courtship tures.Aerial talon-grapplingdescribed previously by and nestbuilding period, prior to radio-tagging.Due Craig et al. (1982) was observedthree times and to nestingfailures, observations were not completed these were the only intraspecificinteractions ob- for the other two nests. servedaway from nests. Male agonisticinteractions observed were pri- Interspecificencounters included the pirating of

Table 1. Activities of male harriers in relation to nesting events.

PERCENT TIME a

OTHER NESTINGEVENTS LOAFING HUNTINGb FEEDING AGGRESSION SOARING FLIGHTSc

Incubation 53 15 2 2 10 18 Brooding 30 34 2 2 8 24 Post-brooding 55 12 2 _2 _3 28 Total for season 46 20 2 2 7 23

Basedon 688 visualor radio contactlocations at 10-min intervalsfrom a total of 2256 trackingintervals spent in the field (29.6% actual) and excludes observations of male activities between intervals. Huntingactivities included edge following and quartering flights. Other flightswere all flightsdetermined not to be huntingor soaring(i.e., gatheringnest materials, courtship, and highor transitflights). 60 JOHN W. MARTIN VOL. 21, No. 2

Table 2. Harrier activities in relation to distance from nestsfor the total breedingseason. lOO- ...... 90• PERCENT ACTIVITIES a 80-' MALES FEMALES .... DISTANCE FROM NESTS MALES FEMALES 7o-'

. <0.5 km 42.3 85 0.5-1.0 km 23.9 11 ß z• 50-

1.0-2.0 km 7.7 4 . 2.0-3.0 km 11.3 0 • 40- $2,8 3.0-6.0 km 8.5 0 >6.0 km 6.2 0 30 '"•'"''-.2..5..0 a All activitycategories combined, based on 688 maleand 250 female instantaneoussamples at 10-min intervals,plotted on topographic .I 15-•.•__•_10•• maps. i 15-21 22-28" 29+ i , 1_178_114 , i i INCUBATION DAYS SINCE HATCHING a large unidentified prey item from a flying Red- Figure 3. Time maleand female harriers spent <0.5 km tailed Hawk (Buteojamaicensis). Males appeared from nest site. mostaggressive towards intruders early in the breeding season,just prior to or just after start of incu- deliveriesby femalescould not be determinedquan- bation. titatively due to emphasison monitoringmale ac- Activities and Behavior--Females. Hunting by tivities.Additionally, female activities during the in- female harriers was not observedprior to the third cubationperiod were not determined(Fig. 3). wk post-hatching(Table 3). Femaleswere observed From the beginningof the third wk post-hatching, routinelyfor 10-15 min from 0700-0730 H (MST) femalesranged progressively farther from nestsbut in preeningbouts or low flights near nests(<100 never as extensivelyas males (Fig. 3). The longest m) from the start of incubationthrough the second flight observedby a female harrier was 600 m. wk post-hatching.The only other time femaleswere Flights to and from nestsfollowed specificcom- observedaway from the nestswas for foodtransfers passbearing, in patterns similar to males;however, from matesor to assistmates or neighboringharriers female direction of travel differed from that of males. in attacking intruders (Powers et al. 1984). After One harrier pair was observedflying to and from the beginningof the third wk post-hatching,females the nest in oppositedirections. Females continued were observedhunting from 0600-2120 H (MST). theseflight patternsduring courtshipand nestbuild- Female prey deliveriestended to follow the male ing, and while hunting for themselvesand young. prey delivery pattern from mid-morning to mid- Intraspecific interactionsinvolving females were afternoon(Fig. 1). Frequencyor number of prey oriented more towards other females (43.3%, N =

Table 3. Activitiesof female harriers in relation to nestingevents.

PERCENT TIME a

OTHER NESTING EVENTS LOAFING HUNTING b FEEDING AGGRESSION SOARING FLIGHTS c

Incubation ...... Brooding 79 0 2 1 1 17 Post-brooding 64 8 3 1 2 22 Total for season 46 4 3 1 2 19 Basedon 250 visualor radio contactlocations at 10-min intervalsfrom a total of 264 trackingintervals spent in the field (94.7% actual) and excludes observations of females' activities between intervals. Hunting flightsincluded edge following and quarteringflights. Otherflights were all flightsdetermined not to be huntingor soaring(i.e., gatheringnest material, courtship, and high or transitflights) SUMMER1987 NORTHERNHARRIERS IN IDAHO 61

Table4. Habitattypes surrounding study nests in the SnakeRiver Birds of PreyArea, and male harrier habitat utilization.

HABITAT TYPES a

CULTI- ABANDONED BARREN DISTANCE FROM NESTS WATERb RIPARIAN SAGE VATED FIELD SOILS

% Habitat Types Nest-0.5 km 26.6 22.6 46.7 1.0 3.1 -- 0.5-1.0 km 11.3 27.1 42.2 13.7 2.9 2.8 1.0-2.0 km 37.7 16.2 25.3 20.8 -- -- 2.0-3.0 km 16.1 12.7 45.5 20.8 2.6 2.3 3.0-6.0 km 2.5 3.1 74.9 17.2 1.2 1.1 77.9 18.0 1.0 1.0 >6.0 km 1.1 1.0 -- Total habitattypes 6.9 5.9 67.5 17.9 1.4 0.3 % Male Habitat Use by Distancec Nest-0.5 km 5.5 81.8 -- 1.8 10.9 -- 0.5-1.0 km 2.0 25.8 6.5 36.7 29.0 -- 1.0-2.0 km 20.0 10.0 50.0 20.0 -- -- 2.0-3.0 km 6.6 20.0 66.7 6.7 -- -- 3.0-6.0 km -- -- 45.5 54.5 -- -- >6.0 km -- -- 12.5 87.5 -- -- Total habitat used 4.5 43.9 17.7 21.3 11.5 --

Male Habitat Use Related to Nest Events Incubation 3.0 48.0 7.0 21.0 21.0 Brooding 4.0 18.0 42.0 24.0 12.0 Post-brooding 4.0 27.0 38.0 23.0 4.0 Total activitiesper habitat typefor seasona 4.9 33.3 28.5 22.8 10.6 Percentagebased on 200 km 2 areasurrounding study nests determined from aerial photos, vegetation maps, and on-site inspections. Harrier useof water habitatlimited to overwater flightsto and from terrestrialhabitats. Male harrier habitatuse determined from 688 instantaneoussamples, using radio triangulationor visualcontacts. Error in totalmale harrier habitatuse and activitiesdue to differencesin computationand is not statisticallysignificant.

90) than males(21.7%, N = 90). Most intraspecific as prolonged)as femalesafter eggshatched. Deer encountersoccurred early in the seasonand females were suspectedof tramplingone nest in the study defended identical areas as males. Male-female in- area. teractionsof mated pairs were excludedfrom con- Habitat Use--Males. Due to problemsin visual sideration. and radio-tracking,home rangesfor male harriers Femaleswere most aggressivetowards interspe- couldonly be approximated.Enderson and Kirven cificintruders, such as the Great Blue Heron (Ardea (1983) discussproblems in radio-trackingPeregrine herodias)(6.5%, N = 90), Canada Goose (Branta Falcons(Falco peregrinus), similar to thoseencoun- canadensis)(4.3%, N = 90), and American Coot tered during this study. The estimatedminimum (Fulicaamericana) (2.1%, N = 90). Encounterswith mean homerange for malesof the four studynests Coyotes(Canis latrans) (10.8%, N = 90) havebeen was 15.7 km2 (range 9.7-17.7 km2). previouslyreported (Powers et al. 1984). Females Chi-square(X 2) analyseswere made for eachhab- of severalclosely grouped nests (seven nests within itat typeoccurring at variousdistances from the nests 100 m rad) were observedto defensivelydive at in order to compareexpected habitat useby males Coyotesand Mule Deer (Odocoileushemionus). to availablehabitat in the studyarea (after Nicholls Males,when present, would join in suchdefense but and Warner 1972). Male harriers used riparian were neveras vigorous(i.e., did not dive as low or habitatssignificantly more (P = 0.01) and shrub- 62 JOHN W. MARTIN VOL. 21, NO. 2 steppehabitat significantly less (P = 0.01) than ex- a vole, and a Red-wingedBlackbird caughtin mid- pected.The dominanthabitat type in the studyarea air from a mobbing flock as it passedbelow the was shrub-steppe(67.5%) with riparian habitat rel- female'splane of flight. A similarresponse by female atively uncommon(5.9%), but males frequentedri- harriers to mobbinghas beenreported by England parian habitat mostoften (43.9%) (Table 4). If ir- (1986). rigated cultivatedfields are consideredsimilar to riparian habitatsbased on availabilityof free water DISCUSSION (Thomas et al. 1979), male activities in riparian- Habitat Use and Foraging Behavior. Habitat like habitats increases to 66.2% of total observations. useby harriershas been discussed by Nieboer(1973), Male activity in shrub-steppehabitat were limited Schipper (1973, 1977) and Watson (1977), mostly to 17.7% of total observations(Table 4). in referenceto sympatricoccurrence with other har- A total of 20 observations of males with identi- rier species.Northern Harriers in North America fiable prey items were recorded.An apparent shift are typically associatedwith lowlands,brackish or from mammalianprey to reptilian prey appearedto fresh water marshes, and mesic grasslands(Bent be associatedwith alfalfa growth. All volesdelivered 1937; Brown and Amadon 1968; Apfelbaum and by males to the nest or female (30% of total prey Seelbach1983). Additionally, harriers are found in items) were observedprior to alfalfa growth of 46 upland habitats distant from water sources(Rees cm. Following alfalfa growth to 46 cm in height, 1976; Duebbert and Lokemoen 1977; Call 1978; males delivered mostly Western Whiptails (Cne- Thurow et al. 1980; Thompson-Hanson 1984; Ry- midophorustigris) (35% of total prey items). Other ser 1985). In the Palearcticthe Hen Harrier usually preyitems identified included the Kangaroo Rat (Di- occupiesdrier open grasslands(Cramp 1908; Har- podomys sp., 15%), Deer Mouse (Perornyscus rison 1982). Watson (1977) suggeststhat harriers rnaniculatus,5%), juvenileNuttall's Cottontail(Syl- secondarilyoccupy lowlands in North America due vilagusnuttallii, 10%), Red-wingedBlackbird (Age- to a lack of competitionwith other harrier species. laiusphoeniceus, 5%) and showedno patterns asso- Althoughthe few prey itemsobserved in this study ciated with changesin vegetationheight. are peripheral to the initial objectiveof determining Male harrier activitiesin alfalfa fields were neg- habitat utilization, they collaboratewith previous ativelycorrelated with vegetationheight (r -- -0.857, studiesshowing breeding harriers to be highly de- P < 0.05). However, activitiesin shrub-steppehab- pendent on certain prey species,principally voles itat were positivelycorrelated with alfalfa height (Craigheadand Craighead1956; Hamerstrom1969, (r = 0.914, P < 0.05). These relationshipssuggest 1979, 1986; Barnard 1983; MacWhirter 1985), and a shift in habitat preferenceby huntingmale harriers indirectlyindicate foraging patterns and habitat use. to moreopen shrub-steppe habitats as alfalfa growth When temporal changesoccur in prey densities, approachedan averageheight of 46 cm. Further, a predatorshave beenobserved shifting to the second distinct increase can be observed in harrier activities highestprey density (Taylor 1984). MacWhirter in shrub-steppehabitat after hatchingof young(Ta- (1985) found nestingharriers at Tantramar Marsh, ble 4). Male harriers from three of the four study New Brunswick,principally predatingvoles but ob- nestswere observedin this apparenthabitat and prey serveda significantshift to fledging passerinesin shift.The first cuttingof alfalfa fieldsoccurred about late-Juneand early-July. Barnard (1983) alsoob- 1 June whenalfalfa growth reached an averageheight serveda significantprey shift, from volesto passer~ of 61 cm. Following cutting of alfalfa, males ap- ines. Errington (1933) and Errington and Breck- pearedto return to the fieldsto hunt, but this could enridge (1936) observedprey shifts, resulting in a not be quantified beforethe study terminated.No successionof prey speciesappearing in the diet of other significant patterns were observedbetween harriers through the breeding season. However, harrier activitiesand habitat types. Watson (1977) implies that any abundant popula- Habitat Use--Females. Minimum home range tion readily accessibleto harriersmay be substituted sizefor the two studyfemales was estimatedat 1.13 as prey. If volesare rare or absent,harriers prey on km2. Femaleswere observedonly in riparian and birds, primarily passerines(Schipper 1973, 1977; cultivatedhabitats directly adjacentto nests. Watson 1977; Picozzi 1978; Barnard 1983; Femaleswere observedwith only two prey items: MacWhirter 1985). Rees (1976) observednesting SUMMER 1987 NORTHERN HARRIERS IN IDAHO 63

harriers in shrub-steppehabitats of easternWash- may be at a disadvantagein locatingor capturing ington preying exclusivelyon vole populations. prey in high, densevegetation similar to alfalfa )46 Data collectedby Diller and Johnson(1982) in- cm. Difficulty in plunging through densevegetation dicate that riparian habitats support the greatest to capture prey may also be a major disadvantage, number of vertebrate species.Montan (1977) ob- allowing escapetime for potential prey. Both vole servedthe greatestrodent numbersin the BOPA in and lizard movementswere highly audiblein natural riparian zones,with microtine speciesfound only in habitatsto 2.0 m (pers. observ.). wet sitessuch as riparian or irrigatedcultivated hab- Harriers appearedto prey uponhigh density,vul- itats. It may be assumedthat volesare limited in the nerable vole populations in riparian-like habitats. BOPA due to the predominantlyxeric environment. When volevulnerability to predationdecreased, har- I found evidenceof voles (i.e., runways, cuttings, riers shifted to low density,relatively high biomass and actual observations)only in riparian habitats Western Whiptail populationsvulnerable to pre- includingadjacent greasewood understory. Diller and dation in open shrub-steppehabitats. Johnson(1982) observedonly moderatenumbers of Temeles(1986) observedpartitioning of foraging WesternWhiptails in greasewoodand riparian hab- behaviors and habitat utilization between sexes in itats, and low numbers in big sagebrushhabitats. wintering harriers. Observationsof males and fe- However, they predicted that whiptails had the malesforaging in differentdirections from nestssug- greatestpotential as a prey speciesdue to their ubiq- geststhat foraging behavior and habitat differences uity and relativelyhigh mean biomass(g/ha). I ob- between sexesmay continue during the breeding servedWestern Whiptails only in shrub-steppehab- season,except when femalesare dependenton mates itats, but no effort was made to determine numbers provisioning prey. in this or other habitat types. Home Range. Previousstudies have observed male Craighead and Craighead (1956) found raptor harriers hunting 4.0-7.0 km from the nestand may numbers directly related to prey vulnerability but range even farther (Schipper 1973, 1977; Watson notrelated to preydensity. Wakeley (1979), Bechard 1977; Barnard 1983; Thompson-Hanson 1984). (1982) and Janes (1985) suggestthat factorsother Home range sizesobserved in the BOPA differ con- than prey availability or density,such as vegetative siderably from those observed by Breckenridge coveror structure,may influenceforaging patterns (1935), who estimated2.49 km2 to be an individual by raptors.Errington and Breckenridge(1936) not- home range for harriers in mesic habitats of Min- ed that alteration of vegetation structure, such as nesota.Craighead and Craighead (1956) estimated mowing,increased vulnerability of someprey species homerange for breedingharriers at 2.1 km2 (range to predationby harriers.Montan (1977) notedthat 0.6-6.3 km2). Hamerstrom and De La Ronde Wilde irrigated wheat fields attracted breedingdeer mice (1973) and Picozzi(1978) estimateda home range and voles, but wheat is harvestedonly once and for a breeding pair to be 8.8 km2 and 14.0 km2, growsto a heightwhich rendersprey lessvulnerable respectively.Schipper (1977) observedhome ranges to predation. In contrast alfalfa is harvestedseveral of malessympatric with other harrier speciesvarying timesduring its growingseason, continually altering from1.8-12.3 km 2 (.• = 4.97 km2).Balfour (1962) vegetationstructure and density.As indicatedin this estimatedharrier hunting rangesin Orkney with no study,voles were predatedin alfalfa fieldsprior to sympatric competitorsto be 66.4 km2. Thompson- vegetationheight of 46 cm and after cuttingof alfalfa Hanson (1984) foundmale homeranges in the shrub- fields. The shift to Western Whiptails is probably steppeof easternWashington varied from 72.1-366.0 a responseto the reducedvulnerability of voles.Dil- km 2. ler and Johnson(1982) observedthat habitatswith Minimum home rangesestimated for femalesin highlizard densitieshad relativelylow percentground this studyare similarto thoseobserved by Craighead coverwith many areasof openground or rocks.Such and Craighead(1956) and Schipper(1977). Watson habitat characteristicswould probably increasevul- (1977) noted that femalesusually hunt a smaller nerabilityof WesternWhiptails to predationby har- area than males, but Balfour (1957) observeda fe- riers. male at eight km from a nest with young. Rice (1982) demonstratedthat harriers depend Thompson-Hanson(1984) criticallyreviewed field heavily on auditory cuesto locate prey. Harriers methodsand variousanalyses of radiotelemetricdata 64 JOHN W. MARTIN VOL. 21, NO. 2 for determininghome range size for harriers, con- Management, and the Zoology Department and Associ- cluding that the minimum convexpolygon method ated Studentsof Brigham Young University. underestimateshome range sizes. Swihart and Slade (1985) determined that nonstatisticalmethods, such LITERATURE CITED as thoseused in estimatinghome ranges for harriers in this and other studies are valid, and that some ALTMANN,J. 1974. Observationalstudy of behavior' samplingmethods. Behaviour 49:227-267. statisticalanalyses may overestimatehome range de- APFELBAUM,S. I. AND P. SEELBACH.1983. Nest tree, pendingon the samplinginterval of radio locations. habitat selection and productivity of seven North However, large home rangesobserved by Balfour American raptor speciesbased on the Cornell Univer- (1962), Thompson-Hanson(1984) and this study sity nest record card program. Raptor Res. 17:97-113. may resultfrom differences in habitattype and struc- BALFOUR,E. 1957. Observationson the breedingbiology ture, or prey availabilityor vulnerabilityto capture. of the Hen Harrier in Orkney. Bird Notes27:177-183, MacArthur and Pianka (1966), Schoener (1971, 216-224. 1983) and Hixon (1980) predicted that the most 1962. The nest and eggsof the Hen Harrier probableresponse to a decreasein prey densityis in Orkney. Bird Notes 30:69-73. BARNARD,P. E. 1983. Foraging behaviour and ener- an increasein the foraging area. geticsof breeding Northern Harriers Circuscyaneus Conclusion. Presumingthe rationaleof optimal (L.). Unpubl. honrs.thesis, Acadia Univ., Nova Scotia, foragingtheory (Krebs et al. 1983), harriersmay Canada. have visited relatively uncommonriparian and ri- BEGHARD,m. J. 1982. Effect of vegetativecover on parian-like (cultivated)habitats as long as foraging foraging site selectionby Swainsoffs Hawk. Condor effortswere successful.When foragingefficiency de- 84:153-159. clined,harriers shifted to commonshrub-steppe hab- BENT, t. C. 1937. Life histories of North American itat. After alterationof vegetationstructure, increas- birdsof prey. Pt. 1. U.S. Nat. Mus. Bull. 167. ing foragingefficiency of voles,harriers returned to BESKE,A. E. 1978. Harrier radio-taggingtechniques riparian-like habitats.Rather than shifting specific and local and migratory movementsof radio-tagged juvenile harriers. Unpubl. M.S. thesis,Univ. Wiscon- searchimages of prey species(voles to lizards), har- sin, Stevens Point. riers may have shifted foraging locations(riparian BILDSTEIN,K.L. 1979. Behavior of a pair of Northern to shrub-steppe),each with differing prey species Harriers on the day of nest abandonment.Inland Bzrd compositions.Such shifts in foraging locationhave Band. News 51:63-65. been discussedby Royama (1970, 1971), Krebs et 1982. Responsesof Northern Harriers to mob- al. (1983) and Taylor (1984), or as a predatorre- bing passerines.J. Field Ornith. 53:7-14. sponseto patchy or intermittent resources(Mac- BRECKENRIDGE,W.J. 1935. An ecologicalstudy of some Arthur and Pianka 1966). Male harriersselectively Minnesota Marsh Hawks. Condor 37:268-276. foragingin habitat types with an associatedprey BROWN,L. ANDD. AMADON. 1968. Eagles,hawks, and specieseasily captured may havebeen observed dur- falcons of the world. McGraw-Hill Co., New York. BURKE,C.J. 1979. Effect of prey and land useon mating ing this study.Additionally, with male harrierspref- systemsof harriers. Unpubl. M.S. thesis,Univ. Wis- erentially foraging a selectpercentage of available consin, Stevens Point. habitat types,large homeranges were observed. CALL,m.W. 1978. Nestinghabitats and surveying tech- niquesfor commonwestern raptors. Bur. Land Man- ACKNOWLEDGMENTS age. Tech. Note TN-316. Serv. Cent., Denver, CO. I am grateful for the field assistanceof W. V. Sykora. CRAIG, T. H., E. H. CRAIG AND J. S. MARKS. 1982 I thank K. L. Bildstein,S. H. Broadbent,A. and J. Comer, Aerial talon-grappling in Northern Harriers. Condor F Hamerstrom, M. N. Kochert, C. D. Marti, B. and J. 84:239. Martin, and K. Steenhoffor their assistance,encourage- CRAIGHE•D, J. j. AND F. C. C•tAIGHEAD,JR. 1956 ment, and support. Also thanks to F. E. Andersen, P. E. Hawks, owls, and wildlife. StackpoleCo., Harrisburg, Barnard, J. T. Flinders, M. N. Kochert, J. S. Marks, PA. J R. Parrish, L. R. Powers, C. L. Pritchett, R. E. Sim- CRAIGHEAD,F. C., JR., J. CRAIGHEADAND R. S. DAVIES mons, K. Steenhof,and C. M. White for reviewing and commentingon drafts. Special thanks go to M. Martin 1963. Radio tracking grizzly bears. Pages 133-148. who patiently followed this study through to its comple- In L. E. Slater,ED. Biotelemetry:the useof telemetry tion, and E. Abram and J. R. Parrish who typed or edited in animal behaviorand physiologyin relation to eco- earlier drafts.Research was supportedby the SnakeRiver logical problems.Macmillan Co., New York. Birds of Prey ResearchProject, U.S. Bureau of Land CR•M?, S. 1980. Handbook of the birds of Europe, the SUMMER 1987 NORTHERN HARRIERS IN IDAHO 65

Middle East, and North Africa. The birds of the west- Pages159-188. In M. L. Cody, ED. Habitat selection ern Palearctic. Vol. 2. Oxford Press, New York. in birds. Academic Press, New York. DILLER, L. V. AND O. R. JOHNSON. 1982. Ecologyof JENNRICH,R. I. aND F. B. TURNER. 1969. Measurement reptilesin the SnakeRiver Birds of Prey Area. Con- of non-circularhome range.J. Theor.Biol. 22:227-237. tract Final Report, USDI, Bur. Land Manage., Boise, KREBS,J. R., D. W. STEPHENSAND W. J. SUTHERLAND. ID. 1983. Perspectivesin optimal foraging theory. Pages DUEBBERT,H. F. ANDJ. T. LOKEMOEN.1977. Upland 165-215. In G. A. Clark and A. H. Bruch, EDS. Per- nestingof AmericanBitterns, Marsh Hawks, andShort- spectivesin ornithology.Cambridge Univ. Press,Cam- eared Owls. Prairie Nat. 9:33-40. bridge. DUNSTaN,T.C. 1973. A tail feather packagefor radio LINNER,S. 1980. Resourcepartitioning in breedingpop- tagging raptorial birds. Inland Bird Band. News 45: ulations of Marsh Hawks and Short-eared Owls. Un- 3-6. publ. M.S. thesis,Utah State Univ., Logan. ENDERSON,J. H. AND M. N. KIRVEN. 1983. Flights of MACARTHUR,R. H. AND E. R. PIANKA. 1966. On op- nestingPeregrine Falcons recorded by telemetry.Rap- timal use of a patchy environment.Amer. Nat. 100: tor Res. 17:33-37. 603-609. ENGLAND,M. E. 1986. Harrier kills mobbing Willet. MACWHIRTER,R.B. 1985. Breedingecology, prey se- Raptor Res. 20:78-79. lection and provisioningof Northern Harriers Circus ERICKSON,M. G. AND D. HOPPE. 1979. An octagonal cyaneus(L.). Unpubl. honrs.thesis, Mt. Allison Univ. bal-chatri trap for small raptors. Raptor Res. 13:36- and Acadia Univ., Nova Scotia, Canada. 38. MECH, L. D. 1983. Handbook of radio-tracking.Univ. ERRINGTON,P.L. 1933. Food habits of southern Wis- Minnesota Press, Minneapolis. consinraptors: Part 2, hawks. Condor35:19-29. MOHR, C.O. 1947. Table of equivalentpopulations of -- AND W. J. BRECKENRIDGE.1936. Food habits North American small mammals. Amer. Midl. Nat. 37: of Marsh Hawks in the glaciatedprairie region of 223-249. north-central United States. Amer. Midl. Nat. 17:831- MONTAN,J. R., JR. 1977. Rodent densityand species 848. compositionin the SnakeRiver Birds of Prey Natural FYFE, R. W. aND R. R. OLENDORFF. 1976. Minimizing Area, Idaho. Unpubl. M.S. thesis, Utah State Univ, the dangersof nesting studiesto raptors and other Logan. sensitivespecies. Canad. Wildl. Serv. Occas. Papers MUNOFF,J.A. 1963. Foodhabits, growth, and mortality 23, Ottawa, Canada. in nestingMarsh Hawks. Kingbird 13:67-74. HAMERSTROM,F. 1963. The use of the Great-horned NICHOLLS, T. H. AND D. W. WARNER. 1972. Barred Owl in catchingMarsh Hawks. Proc.Internat. Ornithol. Owl habitat use as determinedby radiotelemetry.J. Congr. 13:866-869. Wildl. Manage. 36:213-224. 1968. Aging and sexingharriers. Inland Bird NIEBOER,E. 1973. Geographicaland ecologicaldiffer- Band. News 40:43-46. entiationin the genusCircus. Ph.D. dissertation,Vrijie 1969. A harrier populationstudy. Pages367- Univ. te Amsterdam. 383. In J. j. Hickey, ED. Peregrine Falcon popula- ODUM, E.O. 1971. Fundamentalsof ecology.Saunders, tions;their biologyand decline.Univ. WisconsinPress, Philadelphia. Madison. PIcozzI, N. 1978. Dispersion,breeding and prey of the 1979. Effect of prey on predator: voles and Hen Harrier Circuscyaneus in Glen Dye, Kincard- harriers. Auk 96:370-374. shirre. Ibis 120:498-509. 1986. Harrier, hawk of the marsh: the hawk POWERS,L. R., T. H. CRAIGAND J. W. MARTIN. 1984. that is ruled by a mouse.Smithsonian Institute Press, Nest defenseby Northern Harriers againstthe coyote Baltimore. in southwesternIdaho. Raptor Res. 18:78-79. REES,J. R. 1976. Behavioral ecologyof Marsh Hawks • aND D. DE La RONDEWILDE. 1973. Cruising in dissimilarhabitats. Unpubl. M.S. thesis, Eastern range and roostof adult harriers. Inland Bird Band. News 45:123-127. WashingtonState Univ., Cheney. RICE, W. R. 1982. Acousticallocation of prey by the HAMMOND,M. C. AND C. J. HENRY. 1949. Successof Marsh Hawk: adaptation to concealedprey. Auk 99. Marsh Hawk nests in North Dakota. Auk 66:271-274. 403-413. HARRISON,C. 1982. An atlas of the birds of the western ROYAM& T. 1970. Factors governingthe hunting be- Palearctic. Collins Ltd., London. haviour and selectionof food by the Great Tit (Parus HIXON, M. A. 1980. Food productionand competitor major L.). J. Anita. Behar. 39:610-668. densityas the determinantsof feedingterritory size. 1971. Evolutionary significanceof predators' Amer. Nat. 115:510-530. responseto local differencesin prey density: a theo- JANES,S.W. 1985. Habitat selectionin raptorial birds. retical study. Pages344-357. In P. J. Den Boer and 66 JOHNW. MARTIN VOL. 21, NO. 2

G. R. Gradwell, EDS.Proc. Adv. StudyInst. Dynamics Wildlife habitatsin managedrangelands -- the Great Numbers Popul. Oosterbeek,Netherlands. Basinof southeasternOregon: riparian zones.For. Serv RYSER,F. A., JR. 1985. Birds of the Great Basin.Univ. Gen. Tech. Rept. PNW-80. Pacific Northwest For of Nevada Press, Reno. and Range Exp. Stat., Portland, OR. SCHARF,W. C. 1968. Improvedtechnique for trapping THOMPSON,S. P. ANDJ. E. CORNELY. 1982. Nest pro- harriers. Inland Bird Band. News 40:163-165. visioningbehavior by a male Northern Harrier on the • AND E. BALFOUR. 1971. Growth and develop- death of his mate. Wilson Bull. 94:564-566. ment of nestlingHen Harriers. Ibis 113:323-329. THOMPSON-HANSON,P. A. 1984. Nesting ecologyof SCHIPPER,W. J.A. 1973. A comparisonof prey selection Northern Harriers on the Hanford Site, south-central in sympatricharriers (Circus) in WesternEurope. Le Washington.Unpubl. M.S. thesis,Washington State Gerfaut 63:17-120. Univ., Pullman. 1977. Hunting in threeEuropean harriers (Cir- THUROW,T. L., C. M. WHITE, R. P. HOWARDAND J. F. cus) during the breeding season.Ardea 65:53-72. SULLIVAN. 1980. Raptor ecology of Raft River SCHOENER,T. W. 1971. Theory of feeding strategies. Valley,Idaho. u.S. Dept.Energy, Idaho Falls, ID. Ann. Rev. Ecol. Syst.2:369-404. U.S.D.I. 1979. SnakeRiver birdsof prey specialreport. 1983. Simple modelsof optimal feedingterri- Bur. Land Manage., BoiseDist., Boise,ID. tory size:a reconciliation.Amer. Nat. 121:608-629. WAKELEY,J. S. 1979. Use of hunting methodsby Fer- SNEDECOR,G. W. AND W. G. COCHRAN. 1967. Statis- ruginousHawks in relationto vegetationdensity. Rap- tical methods. Iowa State Univ. Press, Ames. tor Res. 13:116-118. SWIHART, R. K. AND N. A. SLADE. 1985. Influence of WATSON, D. 1977. The Hen Harrier. T. and A. D samplinginterval on estimatesof home-rangesize. J. Poyser,Berkhamsted, England. Wildl. Manage. 49:1019-1025. TAYLOR,R.J. 1984. Predation. Hall and Chapman, 6702 East 950 South, Huntsville, UT 84317. Current New York. address:UC-772, Bureau of Reclamation, P.O. Box TEMELES,E.J. 1986. Reversedsexual dimorphism: ef- 11568, Salt Lake City, UT 84147. fecton resourcedefense and foragingbehaviors of non- breedingNorthern Harriers. Auk 103:70-78. Receivedi January 1986; Accepted5 January 1987 THOMAS,J. w., c. MASER AND J. E. RODIER. 1979.

22nd Annual Meeting of The Raptor ResearchFoundation, Inc.--The Raptor ResearchFoundation, Inc., will hold its annual meeting28-30 October 1987 at the Red Lion RiversideHotel, Boise,Idaho. The meetingwill feature a symposiumon migrationof raptorsin westernNorth America.Karen Steenhof,Bureau of Land Management,3948 DevelopmentAvenue, Boise, Idaho 83705, is the ProgramCommittee Chairperson. Deadline for abstractsis 31 July. For moreinformation contact the ConferenceCommittee Chairman, Rich Howard, U.S. Fish and Wildlife Service, 4696 Overland Road, Room 576, Boise, ID 83705, telephone (208) 334-1888.