AN EVALUATION OF TECHNIQUES FOR CAPTURING RAPTORS IN EAST-CENTRAL MINNESOTA by Mark R. Fuller and Glenn S. Christenson Department of Ecologyand BehavioralBiology University of Minnesota 310 Biological SciencesCenter St. Paul, Minnesota 55108 ABSTRACT. To meet the objectivesof a study,several species of raptorshad to be trapped on a 9,880-hectare study area of heterogenoushabitat types. Bal-chatri,mist net, Swedish Goshawk,and automatic bow-net traps (and combinationsof these traps) were used in severalgeneral habitat situations.Mist nets combined with a baited bal-chatri or tethered bait were most successfulin capturingbirds, and the bal-chatrisalone and mist nets alone were next most effective. Trappingwas found to be most productivein deciduousupland habitats where an openingin the canopy or break in the understoryoccurred. Trapping along a woodlot-fieldedge was also effective. Strigiformeswere most often trappedjust before sunriseor just after sunset,while falconiformeswere most often capturedin the late morning and late afternoon. Trapping was least efficient from Decemberto February. A different trap type from that i•sedin the initial captureis often most effectivefor recaptur- ing raptors.Maintenance of healthybait animalsand frequent trap checksare emphasized. Introduction This paperpresents results from a combinationof methodsused to captureand recapture Great HornedOwls (Bubo virginianus), Barred Owls (Strix varia),Red-tailed Hawks (Buteo }amaicensis),and Broad-wingedHawks (Buteo platypterus) on a 9,880-hectarestudy area in east-central Minnesota. Additional information regarding the capture of Saw-whet Owls (Aegoliusacadicus), Long-eared Owls (Asio otus), Goshawks(Accipiter gentills), Red- shoulderedHawks (Buteo !ineatus), and a Harrier(Circus cyaneus) are included. We found no data quantifyingthe resultsof trappingthat employeda combinationof techniqueson a specificstudy area; however, there havebeen numerQuspapers describing varioustraps and techniquesfor capturingbirds of prey (Bergerand Mueller 1959; Ellis 1975; Groinroe 1937; Hamerstrom 1963; Meng 1963, 1971; Nicholls 1973; Robards 1967; Stewart et al. 1945; Tordoff 1954). Severalfalconry books also providehistorical informa- tion on techniquesused for catchingraptors (Beebe and Webster1964; Mavrogordato1974; Peetersand Jameson1970). Other papershave been concernedwith the effectivenessof particulartypes of trapsand methodsby improvingan old design(Henderson 1962, Kitsher 1958, Ward and Martin 1968, Whitman 1960). Data useful in assessingthe utility of a particulartrap for a particularspecies (Berger and Mueller 1959, Ellis 1975, Hamerstrom 1963, Henderson1962, Kitsher1958, Robards1967, Stewartet al. 1945) or for a particular situation (Clark 1971, Ellis 1975, Bergerand Hamerstrom1962, Berry 1971, Hamerstrom 1963, Meng1971, Nicholls1973, Stewartet al. 1945) canbe helpful to researchersdesigning studieswhich involvethe captureof birds of prey. Raptor Research10(1):9-19, 1976 10 RAPTOR RESEARCH Vol. 10, No. 1 The Study Area Thestudy was conducted on the Cedar Creek Natural History Area (93 ø 12'E,45ø24'N) in a variety of habitatsincluding oak uplands,mixed deciduousand coniferousuplands, white cedar (Thuja occidentalis)lowlands, tamarack (Larix laricina)lowlands, deciduous shrub lowlands,marshes, and open fields. Nicholls and Warner (1972) describedthese habitats in more detail and provided a generalphenology for the study area. For the purposeof analyzingtrapping data the followinghabitats were recognized:(1) deciduousopening (a break in the canopyof a deciduousupland woods at least6 m in diameter);(2) deciduous trail (a break in the understoryof a deciduousupland woods at least 3 m by 3 m that may be an actual trail through the woods); (3) field-woodsedge; (4) deciduous-conifertrail (openingor trail like 442but througha mixeddeciduous-conifer upland); (5) deciduous-conifer opening(an openingin the canopylike 4(1 except in a mixeddeciduous-conifer upland); and (6) openfield. Methods Severaltrap types and combinationof trapswere usedduring the study. Trap types included(1) two-shelf121-mm mesh, 12-m-longmist nets (Nicholls1973); (2) modified bal-chatritraps (bergerand Hamerstrom1962, Ward and Martin 1968) of either a 0.7- square-cmhardware cloth cage,10 cm x 20 cm, or a 2.5-cm chicken-wirecage, 25 cm x 35 cm, for mouseor pigeonbait, respectively;(3) SwedishGoshawk traps (Meng 1971); and(4) a modified automaticbow-net (Tordoff 1954). Trap combinationsincluded putting a bal- chatri or tethered bait or decoyin front of a line of one to three mist nets, or in a V formed by two mist nets or in the middle of a triangleof mist nets. The trianglewas formed by stringingone net lengthwiseand stakinganother in a V shape,with the open end of the V againstthe lengthwisenet. A baitedbal-chatri was placed under the triggermechanism of the automatic bow-net. Trap placementin 1971-1972 wasbased on the valuableadvice of Nicholls(pers. comm.) and Hamerstromand Hamerstrom(pers. comm.). During 1972-1973 we reducedthe total numberof trap sitesand alteredlocations because of our experiencefrom the previousfield season.Generally, traps were placed in areaswhere raptors had beenobserved in the habitats described.Nets were placedat the edgesof openingsor perpendicularto trails(see Nicholls 1973) and on field-woodsedges. Bal-chatris, Swedish Goshawk traps, and the bow-netwere placed in fields or on field-woodsedges. Bal-chatris were also droppedfrom vehiclesnear perchedor soaringraptors in the mannerdescribed by Bergerand Mueller (1959). Pigeons(Columba livia) were frequentlyused as bait becauseof easeof maintenanceand their heartiness(Berger and Hamerstrom1962). We alsoused brown and white laboratory mice (Mus musculus);hooded and white laboratoryrats (Ratms norvegicus);game-farm Ring-neckedPheasants (Phasianus colchicus); domestic rabbits (Oryctolaguscunniculus); gerbils (Meriones unguiculatus);and Starlings(Sturnus vulgaris)and Common Grackels (Quiscalusquiscula) obtained from pest-controlprograms. Decoy animals(Hamerstrom 1963, Andersonand Hamerstrom1967) includedGreat HornedOwls and Red-tailedHawks obtainedfrom a raptor rehabilitationprogram (Fuller et al. 1974). Tetheredpigeons and decoy animalswere securedby leather jessesabout their legs.Bait and decoy animalswere givenfood and water at the trap site and were returnedto holdingcages at leasttwo times eachday dependingon temperatureand precipitation conditions. All trapswere closeddown duringperiods of extendedor severeprecipitation or whenambient temperatures fell below 10øF.These conditions were judged too stressfulfor bait anddecoy animals, as well as for raptorsif they shouldbe trapped.Traps were checkedevery four hoursor morefrequently dependingon weatherconditions. Spring 1976 Fuller & Christenson - Capturing Techniques 1 1 Results and Discussion Trappingresults using the variousmethods are presentedin table 1. The combinationof mist nets plusbal-chatri was most successful in termsof numberscaptured and trap daysper capture.The mist net with tetheredbait was as efficient as mist nets plus bal-chatribut resultedin fewer total captures.The bal-chatrialone was next most productiveboth in terms of capturesand efficiency.The mist net alone rankedonly slightlybelow the bal-chatri.We captured only two birds in the SwedishGoshawk trap and were unsuccessfulwith limited useof the bow-net.Calculation of correlationcoefficients (Clarke 1969) for trap daysand capturesprovided no significantcorrelation between the two. Table 1 Capturesby Trap Type Days Tot. Capturesby Species Trap Used Capt. D/C GHO BO LEO SWO RT BW RS GH CH HA Mist 0279 11 25 5 2 1 2 1 % 20 12 24 67 5 15 6 Mist #191 17 11 1 1 4 5 1 4 1 Teth. % 13 18 5 100 18 38 20 25 100 Mist #327 30 11 11 1 9 3 2 2 2 B.C. % 23 32 52 33 41 23 40 50 13 Mist # 91 7 13 3 1 1 1 1 Dec. % 6 8 43 5 20 25 6 B.C. #324 14 23 1 2 3 3 1 1 3 % 23 15 14 10 14 23 20 25 19 Swed. #182 2 91 1 1 Gos. % 13 2 14 5 Bow- # 34 net % 2 Hand* # 12 2 2 3 5 % 13 29 10 14 31 Total 1428 93 7 21 1 3 22 13 5 4 16 1 *Hand-capture data not included in total trap days or days per capture calculations. GHO = Great Horned Owl BW = Broad-winged Hawk BO = Barred Owl RS = Red-shouldered Hawk LEO = Long-eared Owl GH = Goshawk SWO = Saw-whet Owl CH = Cooper's Hawk RT = Red-tailed Hawk HA: Harrier 12 RAPTOR RESEARCH Vol. 10, No. 1 The captureof eachspecies by trap type is alsopresented in table 1. Againthe combina- tions of mist net pluseither bal-chatrior tetheredbait gavethe bestresults. The bal-chatri aloneappeared to be moreefficient for falconiformesthan for strigiformeswhen compared to other methods. Great Horned Owls were used effectively as decoy animalsin efforts to trap other GreatHorned Owls. Hawks trapped by thismeans were caught incidentally to the efforts directed at Great Horned Owls. However, the attacks by hawks on Great Horned Owlsflushed during the day (Dunstanand Harrell 1973, Murphyet al. 1969) and the success of this owl asa decoy(Hamerstrom 1963) suggestthat thistechnique can be widely applied. Barred Owls would probably avoid a Great HornedOwl decoybecause of apparentinter- specificconflicts which occur between these two species(Fuller et al. 1974). No Barred Owls were availableas decoy animals. Mist nets with bal-chatrisand mist nets alone were most effective for capturingBarred Owls. These same two techniqueswere usedsuccessfully to captureRed-tailed and Broad-wingedHawks thoughthese species were trappedwith a variety of trap types. An important point to note here and to bear in mind throughoutthe