Winter Roost Sites of Northern Harriers and Short-Eared Owls on Illinois Grasslands

j RaptorRes.32(2):116-119 ¸ 1998 The Raptor ResearchFoundation, Inc.

WINTER ROOST SITES OF NORTHERN HARRIERS AND SHORT-EARED OWLS ON ILLINOIS GRASSLANDS

JEFFERYW. WALK1 Departmentof Zoology,Eastern Illinois University,Charleston, IL 61920 U.S.A.

A•sTP•CT.--Characteristicsof the dietsand roostinghabitats used by Northern Harriers (Circuscyaneus) and Short-earedOwls (Asioflammeus) on grasslandsanctuaries in Jasper and Marion counties,south- eastern Illinois, were describedfor December 1993 and January 1994. Northern Harrier communal roosts contained from 21-43 individuals and were only observedin undisturbed cool-seasongrasses. Short-earedOwls also roosted communally (3-17 individuals)but the two specieswere not observedto roost within the same fields. Short-eared Owls roosted in shorter cover than Northern Harriers, but there wasno evidenceof selectionfor any grassor managementtype by Short-earedOwls. Both species frequentlyfed on southernbog lemmings (Synaptomyscooperi) and avian remainswere found in less than 5% of all pellets. KEYWORDS: NorthernHarriet;, Circus cyaneus;Short-eared Owl; Asio flammeus; communalroosts;, roost habitat;, diet.

Perchasde invierno de Circuscyaneus y Asioflammeusen las praderasde Illinois RESUMEN.--Lascaracteristicas de las dietasy de las perchasutilizadas por Circuscyaneus y Asioflammeus en los santuariosde praderasde los condadosde Jaspery Marion en el sur de Illinois fueron descritas en Diciembre de 1993 y Enero de 1994. Las perchascomunales de Circuscyaneus estaban compuestas por 21-43 individuosy fueron detectadasen pastizalesestacionales no perturbados.Asioflammeus utiliz6 perchascomunales (3-17 individuos),las dos especiesno utilizaronperchas en el mismocampo. Asio flammeusutiliz6 perchas en sitios de cobertura menos altos que los utilizados por Circuscyaneus. No hubo evidenciaque Asioflammeustuviera alguna preferencia por algfin tipo de pastizalo fireade manejo. Las dos especiesse alimentaronfrecuentemente de Synaptomyscooperi. Restos de avesfueron encontrados en menos del 5% del total de egagr6pilas. [Traducci6n de C•sar Mfirquez]

Both the Northern Harrier (Circus cyaneus)and competition for food may influence their habitat Short-earedOwl (Asioflammeus)are noted for their use. The purpose of this study was to document communal winter roostsin the Midwest (e.g., Wel- roost site characteristics of Northern Harriers and ler et al. 1955, Mumford and Danner 1974). Little Short-earedOwls on a 800-ha grasslandsanctuary has been reported, however,on the characteristics systemin southeasternIllinois. I alsocollected pel- of habitats that are selected for roosting. Because lets to examine the diets and possiblecompetition both speciesare listed as Endangered in Illinois between these two species. (Herkert 1992), information regarding the specific METHODS habitats these speciesuse would help in their con- servation. The study was conducted at the Prairie Ridge State Natural Area in Jasper and Marion counties, approxi- Most studiesof these two raptor specieshave re- mately 800 ha of restoredgrassland tracts managed par- ported that they capture similar prey, particularly ticularly for Threatened and Endangered wildlife. The small rodents (Colvin and Spaulding 1983, Clark Jasper and Marion county units are approximately60 km and Ward 1974, Craigheadand Craighead 1956). apart. A total of 13 tracts (sevenin Jaspercounty and six in Marion county),ranging in sizefrom 7-120 ha, were Therefore, they may overlapin diet and, asa result, examined. Within each county, tracts averaged0.8 km from their nearest-neighbortract. The maximum dis- tancebetween any two tractswas about 7 kin. Tractswere • Present address:Department of Natural Resourcesand further subdivided into management units averaging Environmental Sciences,University of Illinois at Urbana- about 3 ha (range 0.5-15 ha). The areas studied were Champaign, Urbana, Illinois 61801 U.S.A. about 75% introducedcool-season grasses such as redtop

116 JUNE 1998 WINTERROOSTING OF GP,ASSLAND RAPTORS 117

bentgrass (Agrostisalba), timothy (Phleum pratense), tion estimatesby more than about 5%. A site man- smooth brome (Bromusinermis), and bluegrass(Poa spp.), and 25% native warm-seasongrasses such as switchgrass ager observedone roost site (which I had previ- ( Panicurnvirgatum), big bluestem (Andropogongerardii), ously surveyed) on the same evening I surveyed indiangrass(Sorgastrum nutans), and little bluestem (Schi- the secondJasper county roost. Only 2 more har- zachyrium scoparium).Management measures include riers were counted than I had previouslycounted mowing, haying, limited grazing, and undisturbedfields at the roost. Short-eared Owl communal roosts (Simpsonand Esker 1997). Northern Harriers and Short-eared Owls were ob- contained from 3-17 individuals (i = 6.8 owls). servedfrom 26 December 1993-12 January 1994 to de- One Short-earedOwl was observedroosting indi- termine habitats used for roosting. Observations were vidually. Flush counts of Short-eared Owl commu- made from a stationary vehicle and were continuous nal roosts were the same (four of five roosts) or from 1.5 hr before sunset until dark. Each tract was sur- larger (observation estimate = 15 owls and flush veyed for one evening. Two small (<20 ha), adjacent tractswere surveyedsimultaneously once and three large count -- 17 owls) than counts derived from ob- tracts were divided and observed from two locations servingowls leaving the roostsin the evenings. when topographyand size preventeda complete survey Northern Harrier communal roostswere only from a singlelocation. A total of 13 tractswere surveyed observedin undisturbedareas of cool-seasongrass- from 15 points, each with an observationradius of--<500 m. Every available tract was observed for the same es. Due to the small samplesof these roosts, the amount of time regardlessof the level of raptor activity resultswere not significant (Table 1). Short-eared in that tract. The locationsof Northern Harriers landing Owls were not found to select a particular cover in communalroosts and of Short-earedOwls comingoff type (Table 1). However, 32 of 34 total roost ob- of communalroosts were recorded on detailed sitemaps. Population estimatesof Short-eared Owls were obtained servationswere in shorter vegetation of mowed by flushing birds off of all known communal roost sites grasses(15 observations)and new grassseedings on 30 December (Jaspercounty) and 6January (Marion (17 observations).Average visual obstruction county) at midday (1000-1400 H). The population of height (Robel et al. 1970) at harrier roosts (i = 25 Northern Harriers wasestimated by summingthe num- cm) was significantlyhigher than at owl roosts (i ber of birds observedroosting on each tract. Visual obstruction measurements (Robel et al. 1970) were made = 12 cm; t = 2.64, df = 5, P < 0.025, separate to characterizevegetation height and densityat 10 North- variance t-test). Short-eared Owls tended to roost ern Harrier and six Short-eared Owl communal roost at the baseof a denseclump of grasswithin a field sitesduring the day in January1994. Vegetation type and of much thinner cover, while Northern Harriers management history were provided for each manage- roosted in relatively even standsof grass. ment unit within grasslandtracts by the grasslandman- agers, confirmed in the field, and recorded for the roost- Rodents accounted for 100% of the mammalian ing site of every individual observedof both species. remains found in 65 Northern Harrier and 52 Pelletswere collectedfrom the roostsites of eachspe- Short-earedOwl pellets. The only rodent species cies. Pellet identification was further confirmed based on identified was the southernbog lemming (Synap- charactersdescribed by Holt et al. (1987). Pelletswere carefully separated,teeth and/or skullswere identified tomyscooperi). Every pellet analyzedcontained ro- when possibleaccording to Hoffmeister (1989), and non- dent remains. A few also contained unidentified mammalian remains were noted. passerinebird remains(4.6% of the harrier pellets and 3.8% of the owl pellets). RESULTS The wintering population of Northern Harriers DISCUSSION was estimated to be at least 102 birds, or 12.3 har- Northern Harriers and Short-eared Owls have riers/100 ha. The Short-earedOwl populationwas used the Prairie Ridge State Natural Area for at estimated to be at least 34 birds, or 4.1 owls/100 least 25 years as a wintering ground (R. Weste- ha. meier, pers. comm.). Since1990, winter raptor cen- Three Northern Harrier and five Short-eared suseshave reported 100-150 harriers and 25-50 Owl communal roosts were documented. The owlsannually (S. Simpsonpers. comm.). This con- number of harriers at each communal roost sistentlevel of winter activityseems unique to the ranged from 21-43 birds (i = 34 harriers). Indi- region, as Bildstein (1979) showshigh variationin vidually roostingNorthern Harriers were recorded harrier activity in Ohio and Weller et al. (1955) five times.Only a singleroost site occurredin Mar- remarked on the ephemeral occurrence of both ion county.Double-counting of Northern Harriers speciesin Missouri. between dayson the twoJasper county communal Habitat of harrier communal roosts has been de- roost siteswas possible,but did not alter popula- scribed as often-dampwheat stubble fields over- 118 W•mK VOL. 32, No. 2

Table 1. Relative availabilityof cover typesand numbers of Northern Harrier and Short-earedOwl winter communal roostslocated within each cover type in Illinois.

# OF NORTHERN HARRIER # OF SHORT-CARED OWL COMMUNAL ROOST SITES COMMUNAL ROOST SITES % OF AVAILABLE COVER TYPE a GRASSLAND OBSERVED EXPECTED a OBSERVED EXPECTED

Undisturbed cool-seasongrass 42.5 3 roosts 1.3 roosts 2 roosts 2.1 roosts (99 harriers) (19 owlsb) Mowed cool-seasongrass 33.7 0 1.0 roost 2 roosts 1.7 roosts (1 harrier) (10 owls) Undisturbed warm-seasongrass 15.4 0 0.5 roost 0 0.8 roost Mowed warm-seasongrass 8.4 0 0.3 roost I roost 0.4 roost (2 harriers) (5 owls) X2 = 4.19 X2 = 4.28 P = 0.24 P = 0.23 a Expectedvalues for number of roostsites per covertype were calculatedas the total number of observedcommunal roost sites for a speciesmultiplied by the proportion of each availablecover type. b 17 of 19 of theseobservations were from one young (<1.5 yr) Bromusinermis seeding. This field wasatypical of undisturbedcool- seasongrasses, being shorterwith grassin scatteredclumps.

grown with ragweed (Ambrosiaartemisaefolia) to a a foraging area. I likewiseobserved birds leaveand height of 60-90 cm (Weller et al. 1955), stubble arrive at roost sites from all directions. Individual fields, prairie grasses,and fescue 60-110 cm tall harrierswere followedfor over 7 km from foraging with up to "severalinches" of water (Mumford and areas in the surrounding agriculture landscapeto Danner 1974), and weedy old fields (Colvin and the roosting area, and birds were frequently seen Spaulding1983). In this study,harriers roostedin foraging up to 16 km from the roost site. No other fields with dense cover up to a height of about 45 roostswere found or reported locally. Short-eared cm and thinner screeningcover to a height of 1.0- Owlswere observedforaging on grasslandsand ad- 1.2 m. Fields of undisturbed redtop bentgrassand jacent agricultural lands, but the distance they trav- timothy and undisturbed smooth brome fit these eled to foraging areaswas not known. characteristics. One communal roost was situated Other studies of the winter dietary habits of in a wet field, with the others in well-drained areas. Northern Harriers and Short-eared Owls have Short-eared Owl communal roosts have been shown that they have similar diets and utilize any even less frequently described,but Weller et al. abundant mid-sized prey available. Rodents are (1955) found the roostsin densegrass less than 30 normally the primary prey (e.g., Weller et al. 1955, cm high, frequently in a tuft of grass.My observa- Craighead and Craighead 1956, and Colvin and tions closelyagree, with communal roostsites often Spaulding 1983, Holt and Leasure 1993). Only oc- found in fields mowed to 30-40 cm. Seventeen of casionally are birds taken in large numbers(e.g., the 19 observationsof Short-eared Owls roosting Collopy and Bildstein 1987, Holt and Leasure in undisturbed cool-seasongrasses came from a 1993). My resultswere similar to thesestudies, with single field which had been sowed to smooth rodents being the primary winter food used by brome 15-16 mo previousto this study.That field both Northern Harriers and Short-eared Owls. wasatypical of undisturbedcool-season grass fields, Long-term small mammal data are unavailablefor being quite short (under 50 cm versusthe typical this site, but current researchindicates rodent pop- 100-120 cm) and the grass in isolated clumps as ulations are relatively high (E. Heske pers. opposed to an even stand. This type of structure comm.). was more typical of mowed areas.Available fields This site is the last in Illinois to host the Endan- of undisturbed warm-seasongrass may have been gered Greater Prairie-Chicken (Tympanuchuscupi- too tall or too dense to be suitablefor roosting for do). No evidence was found indicating that these either species. raptorskill prairie-chickensdespite the high raptor Bildstein (1979) showed that Northern Harrier density. Northern Harriers do, however, harass communal roost siteswere centrally locatedwithin prairie-chickensdisplaying on leks in both the JUNE1998 WINTERROOSTING OF GRASSLAN•)RAPTORS 119

springand autumn (S. Simpsonand R. Westemeier Undergraduate Research Council, Eastern Illinois Um- versity. pets. comm.; J. Walk pets. obs.), although the largely-migrant harrier population is much re- LITERATURE CITED ducedby the time prairie-chickenbreeding begins. BILDSTEIN, K.L. 1979. Fluctuations in the number of Therefore, it appears that maintaining prairie- Northern Harriers Circus cyaneushudsonius at com- chickens and grassland raptors are compatible munal roostsin south central Ohio. RaptorRes.13:40- 46. management objectives. These two open country raptors exhibit a great CLARK,RJ. 1975. A field study of the Short-eared Owl (Asioflammeus) in North America. Wildl. Monog•.47' deal of sympatryand habitat and dietary overlap 1-67. (Clark and Ward 1974), potentiallycontributing to -- ANDJ.G. WARD.1974. Interspecific competition in interspecificcompetition. Schoener (1983) dem- two speciesof open country raptors Circuscyaneus and onstratedthat overlapof macrohabitatusually does Asioflammeus. Proc. PA. Acad. Sci.48:79-87. not lead to competition, whereasoverlap of micro- COLLOPY,M.W. AND K.L. BILDSTEIN.1987. Foraging be- habitat and food usuallydoes. This studysuggests havior of Northern Harriers wintering in southeast- that Northern Harriers and Short-eared Owls uti- ern salt and freshwater marshes. Auk 104:11-16. lize separate roosting habitat. The diets of these COLVIN,B.A. ANDS.R. SP^ULDING.1983. Winter foraging behavior of Short-eared Owls (Asioflammeus)in Ohio. specieswere essentiallyidentical. However, niche Am. Midl. Nat. 110:124-128. overlap does not necessarilytranslate into compe- CRAIGHEAD,JJ. AND F.C. CRAIGHEAD.1956. Hawks, owls tition (Holt 1987). Rodentsmay have provideda and wildlife. Stackpole Co., Harrisburg, PA U.S.A. superabundantfood source,foraging habitat may HERKERT,J.R. [ED.]. 1992. Endangered and threatened have differed, or foraging densitiesmay have been speciesof Illinois: statusand distribution. Vol. 2. Ilh- so low as to make exploitation competition levels nois Endangered Species Protection Board, Spring- insignificant.Further isolating the specieswith re- field, IL U.S.A. HOFFMEISTER, D.F. 1989. Mammals of Illinois. Univ. Ilh- spect to foraging behavior is timing. Harriers are nois, Urbana, IL U.S.A. apparently restricted to hunting during daylight HOLT, D.W. AND S.M. LEASURE. 1993. Short-eared Owl hours. Short-eared Owls are not limited in this (Asioflammeus). In the Birds of North America, No sense, but are primarily crepuscular (Clark and 62, A. Poole and F. Gill lEDs.]. Acad. Nat. Sci., Phil- Ward 1974, Holt and Leasure 1993). Some level of adelphia, PA U.S.A. and Am. Ornithol. Union, Wash- interference competition exists between the spe- ington, DC U.S.A. cies, which manifested itself during this study in --, LJ. LYONAND R. HALE.1987. Techniquesfor dif- the form of aerial sparring in the mornings and ferentiating pellets of Short-eared Owls and Northern Harriers. Condor 89:929-931. eveningsas one speciesbecame activeand the oth- HOLT, R.D. 1987. On the relationship between niche er returned to communal roost sites.In part due overlap and competition: the effect of incommensu- to occasionalkleptoparasifism by Northern Harri- rable niche overlap. Oikos48:110-114. ers, Clark (1975) concluded that the Short-eared MUMFORD, R.E. AND C.R. DANNER. 1974. An Indiana Owl is a fugitive species,which accountsfor occa- marsh hawk roost. Indiana AudubonQuarterly 52:96- sional,temporary food and habitat overlap. 98. ROBEL,RJ., J.N. BRIGGS,A.D. DAYTONAND L.C. HURL- ACKNOWLEDGMENTS BERT.1970. Relationshipsbetween visual obstruction measurementsand weight of grasslandvegetation. J. This paper is based on an undergraduate internship RangeManage. 23:295-297. completed through the Environmental Biology program of Eastern Illinois University's Zoology Department. I SCHOENER,T.W. 1983. Field experiments on interspecific thank T.L. Esker and S.A. Simpson for field assistance competition. Am. Nat. 122:240-285. and sharing site information. J.R. Herkerr, K.C. Kruse, SIMPSON,S.A. ANDT.L. ESKER.1997. Prairie Ridge State and R.E. Warner provided adviseand review of manu- Natural Area habitat plan. Illinois Department of Nat- scripts and J.M. Fair, D.W. Holt, N. Jenks-Jay,and an ural Resources,Springfield, IL U.S.A anonymousreferee reviewedthis manuscript.I gratefully WELLER,M.W., I.C. ADAMS,JR. ANDBJ. ROSE.1955. Win- acknowledge the Illinois Department of Natural Re- ter roosts of marsh hawks and Short-eared Owls in sources, Illinois Nature Preserves Commission, and The central Missouri. Wilson Bull. 67:189-193. Nature Conservancyfor accessto properties and field equipment. Fieldwork was supportedby funds from the Received8 March 1997; accepted2 February 1998