J. Field Ornithol., 54(4):401-406 NEST SITE RELATIONSHIP BETWEEN THE FERRUGINOUS HAWK AND SWAINSON'S HAWK BY THOMAS L. THUROW AND CLAYTON M. WHITE The objectiveof thisreport is to documentand explain an interspecific nestingrelationship we observedbetween sympatric buteos: the Fer- ruglnousHawk (Buteoregalis) and the Swainson'sHawk (Buteoswainsoni). Mutual defensebehavior by pairsfrom closelyassociated nests became strikinglyapparent in the courseof conductingan intensivestudy on the FerruginousHawk and its toleranceto humanactivity (Thurow et al. 1980, White and Thurow unpubl. data). METHODS Studyarea.--The studyarea wasin Raft River Valley of CassiaCounty near the town of Malta, southcentralIdaho. The region'sphysiography and climate are typical of the Great Basincold desert(Odum 1971). The vegetationof the valley is characteristicof the northern desert shrub-biomeas delineated by Cronquistet al. (1972).Juniper (Juniperus osteosperma)grows along the sidesof the mountainsand in intermittently moistdrainages which extend into the valley.The valleyvegetation was dominatedby big sagebrush(Artemisia tridentata). Our researcheffort wasconcentrated along the continuoussagebrush/juniper ecotone. Both Ferruginousand Swainson'shawks were commonin the valley and favored large junipers located along the rangeland ecotone for nesting.The tree chosenfor a nest site typicallywas near the ecotone interfaceand, in the caseof the FerruginousHawk, had a characteristic flat-toppedappearance. This wasin contrastwith the relatively scarce Red-tailedHawk (Buteojamaicensis) which primarily chosenest sitesin lombardypoplar (Populusnigra). Procedure.--Thearea within a .8 km (.5 mi wasoriginally chosen as the area to check)radius around 15 randomlychosen (from a table of random numbers) active Ferruginous Hawk nests was examined for possibleSwainson's Hawk nestingactivity during the 1978 and 1979 breedingseasons. Ferruginous Hawk nestswere usedas the focal point of searchesbecause their nestsare bulky, conspicuousstructures that are usedannually and easilyfound. In contrast,Swainson's Hawks build smallernests that are not necessarilyreused and are often concealed within the tree canopy.Therefore, to locateSwainsoh's Hawk nests,we inspectedeach individual tree. As a control, the sameprocedure was used around 15 FerruginousHawk nest sitesthat were not currently active, but may have been 1 to 2 years previously.Land use and the relative degree of human disturbanceremained constantat these sites and wasnot the causefor occupancychange. 401 402] T. L. Thurowand C. M. White j. FieldOrnithol. Autumn 1983 RESULTS AND DISCUSSION In 1978, 14 out of 15 (93%) active FerruginousHawk nestshad an active Swainsoh'sHawk nest within .8 km (.• = .6 km, range .32-.80 km). In 1979, searchareas were againrandomly selected and 11 out of 15 (73%) activeFerruginous Hawk nestshad an activeSwainsoh's Hawk nestwithin .8 km (.• = .57 km, range .32-.80 kin). In contrast,only 3 of 15 (20%) active Swainsoh'sHawk nestswere located within .8 km (.• = .42 km, range .35-1.7 km) of an inactiveFerruginous Hawk nest in 1978 and 4 of 15 (27%) (.• = .53 kin, range .4-.8 kin) in 1979. Nesting Swainsoh'sHawks were significantlyassociated (x 2 16.4, 1 dr, P -• .005) with activeFerruginous Hawk nestsites within a sampleradius of .8 kin. Most of the censusareas around inactiveFerruginous Hawk nests contained a cluster of inactive Swainsoh's Hawk nests. As a further example of this closeassociation, 2 active Swainsoh'sHawk nestswere located in the routine courseof traveling within the valley. Near one, a previouslyunknown active Ferruginous Hawk nestwas found .8 km away. At the other, an activeFerruginous Hawk nestwas found in the nearestsuitable tree which happenedto be 1.3 km away. In this latter case, trees closer to the Swainsoh's Hawk nest neither contained old FerruginousHawk nestsnor were they of the characteristicsize and shapetypically selected by this species. While a nestsite relationship between the Ferruginousand Swainsoh's hawk is statisticallyimplicit, the reasonsfor suchan associationare not as apparent.As summarizedby Newton (1976) at least9 factors,some environmentaland somepopulational, may act to control nest-siteplace- ment in raptors;not all of thesecan be closelyexamined because of the difficultyof obtainingreliable data. The environmentalfactors include: (1) foodavailability near potential nest sites; (2) qualityof nest;(3) overall scarcityof sitesin area, and (4) absenceof disturbance,predation, or competitionat nestsites. Population factors include: (1) populationden- sityin area; (2) degreeof sociality;(3) distanceto nearestoccupied nest; (4) historical successat nest site, and (5) percentageof older adults in population.Several possible reasons that are testableinclude: (1) place- ment of trees in a clumpedor other nonuniform pattern that influence nest placement;(2) a function of distancebetween nearest neighbor specieswhether conspecificor not; (3) food availabilityin poor or good prey years; (4) timing of arrival of the respectivespecies; (5) simple responseto local patterns of severalvariables in concert, and (6) some mutualisticrelationship. The problem of tree availability and distribution can be dismissed readily, sincetrees formed a more or lesscontinuous band along the foothills. Although trees of certain shapesor structureswere charac- teristically selected, Ferruginous Hawk nests were regularly spaced around the valleyat an averageof 4.3 ___1.0 km (n = 19) in 1978 and 3.8 ___.8 km (n = 25) in 1979 (Thurow et al., 1980). Swainsoh'sHawks arrived about a month after FerruginousHawks had establishedterri- tories. The Swainsoh'sHawks appearedto cue in on the Ferruginous Vol.:•4, yo. 4 HawkNest Sites [403 Hawks, not the reverse. Schmutz et al., (1980) also noted that Ferru- ginousand Swainson'shawks nested in closeproximity despiteavailable unoccupiednests in trees farther away. Such associationsare not un- precedentedin caseswhere one speciesis larger and establishesits breed- ing site prior to the smaller species.This hasbeen documentedfor the Little Gull (Larus minutus)that nested adjacent to the larger, earlier nestingBlack-headed Gull (L. ridibundus)(Veen 1980). While it is dif: ficult to documentwhat advantagethe Little Gull gainedby seekingout other gullcolonies and moving colony areas coincidentally with the other species,it nonethelessoccurred. Both the Ferruginousand Swainsoh'shawk generallytolerated each other's presenceproviding the actual nest sites were not closelyap- proached.On 19 occasionswhen the investigatorapproached the nests of one species,a member of the other specieswas first to appear and give the alarm calls.This action may have alerted the absentpair to impendingdanger earlier than they would have otherwisedetected it. In 13 instancesthis cooperativedefense was extended to the point that Swainsoh'sHawks stooped at an investigatorclimbing to a Ferruginous Hawk nest.We did not encounterthis stoopingbehavior in the reverse situation,although Ferruginous Hawks often circled low and screamed at the investigatorsnear Swainsoh'sHawk nests.Often during these instancesboth pairseventually arrived and, while soaringtogether, con- centrated their attention on the terrestrial intruder. Such a combined attackwas observed 8 timeswhen directedagainst an intruding Golden Eagle (Aquila cbrysaetos).In these instancesmembers of both pairs took successivestoops at the eagle, but never against each other, though sometimesflying close together. The Ferruginous Hawk can be ag- gressiveagainst mammalian predators other than man, and we are aware of caseswhen canidsapproaching too closeto a FerruginousHawk nest havebeen injured by the adult attacks.It appearsplausible that defense of the commonnesting territory by both pairs affordsa greater protec- tion of the nest than could be provided by only one pair. It shouldbe expectedthat the benefit of suchprotection be quan- tiffable in such variablesas reproduction. There was, however, no dif- ference in fledging rates of those speciespairs nestingnear each other versussolitary nesters,although the samplesize for the latter is small. Schmutzet al. (1980) showedthat for the Canadianprairies the thresh- old distancebetween the nearestnesting congener below which repro- duction was reduced was <.2 km for Swainsoh's Hawk and <.3 km for FerruginousHawk. They suggestedthat competitionfor spaceproduced this lower reproduction. In no casecould we find neststhat were <.2 km apart, which might explain why we were not able to find lowered fledgingrates. With both speciesin the Canadianstudy having greater than 80c• diet overlap, we suggestthat diet, in addition to space,may be a contributingfactor to loweredreproduction. We alsosuggest that this might be testedwhere diet overlap is lessthan 80c/cas in our study area (seebeyond) and where nestsare <.2 km apart. 404] T. L. Thurowa*td C. M. White J.F,eld Ornithol. Autumn 1983 Another possiblereason for the nest site associationmay be due to uneven prey distribution. It is possiblethat Swainson'sHawks (which arrived much later at the breedingsite and consequentlyhad to quickly initiatebreeding activities) chose the area becauseof higherprey den- sitiesor the presenceof FerruginousHawks indicated a high prey den- sity.Such a theory is not tenablein this case.Prey densitieswere high on the entire studyarea. The black-tailedjackrabbit (Lepuscalifornicus) summerpopulation was estimated to be at a near peak densityof about 300 per km2 (Thurow et al. 1980) usingthe flushingdistance