AN EVALUATION OF TECHNIQUES FOR CAPTURING RAPTORS IN ...
AN EVALUATION RAPTORS
OF TECHNIQUES
IN EAST-CENTRAL
FOR CAPTURING
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,severalspecies of raptorshad to be trapped on a 9,880-hectare study area of heterogenoushabitat types. Bal-chatri,mist net, Swedish Goshawk,and automaticbow-net traps (and combinationsof these traps) were used in severalgeneralhabitat 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-fieldedgewas also effective. Strigiformeswere most often trappedjust beforesunriseor just after sunset,while falconiformesweremostoften capturedin the late morning and late afternoon. Trappingwas least efficient from Decemberto February. A different trap type from that i•sedin the initial captureis often most effectivefor recapturing raptors.Maintenanceof healthybait animalsand frequenttrap checksare emphasized. Introduction
This paperpresentsresultsfrom a combinationof methodsusedto captureandrecapture GreatHornedOwls(Bubovirginianus), BarredOwls(Strix varia),Red-tailedHawks(Buteo }amaicensis), and Broad-winged Hawks(Buteoplatypterus)on a 9,880-hectarestudyareain east-central Minnesota. Additional information regarding the capture of Saw-whet Owls
(Aegoliusacadicus),Long-earedOwls (Asio otus), Goshawks(Accipitergentills), Redshouldered Hawks(Buteo!ineatus),anda Harrier(Circuscyaneus)areincluded. We found no data quantifyingthe resultsof trappingthat employeda combinationof techniqueson a specificstudyarea;however,there havebeen numerQuspapersdescribing varioustrapsand techniquesfor capturingbirds of prey (Bergerand Mueller 1959; Ellis 1975; Groinroe 1937; Hamerstrom 1963; Meng 1963, 1971; Nicholls 1973; Robards 1967; Stewartet al. 1945; Tordoff 1954). Severalfalconry books also providehistoricalinformation on techniquesusedfor catchingraptors(BeebeandWebster1964; Mavrogordato1974; Peetersand Jameson1970). Other papershave been concernedwith the effectivenessof particulartypesof trapsand methodsby improvingan old design(Henderson1962, Kitsher 1958, Ward and Martin 1968, Whitman 1960). Data useful in assessing the utility of a particulartrap for a particularspecies(Bergerand 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 helpfulto researchers designing studieswhich involvethe captureof birds of prey.
Raptor Research10(1):9-19, 1976
10
RAPTOR RESEARCH
Vol. 10, No. 1
The Study Area
Thestudywasconducted ontheCedar Creek NaturalHistory Area(93ø 12'E,45ø24'N) in a variety of habitatsincludingoak uplands,mixed deciduousand coniferousuplands,white
cedar (Thuja occidentalis)lowlands,tamarack(Larix laricina)lowlands,deciduousshrub
lowlands, marshes, andopenfields.Nicholls andWarner(1972)described thesehabitatsin more detail and provided a generalphenologyfor the study area. For the purposeof analyzingtrappingdata the followinghabitatswere recognized:(1) deciduousopening(a breakin the canopyof a deciduous uplandwoodsat least6 m in diameter);(2) deciduous trail (a break in the understoryof a deciduousuplandwoodsat least3 m by 3 m that may be an actual trail through the woods); (3) field-woodsedge;(4) deciduous-conifer trail (openingor trail like 442but throughamixeddeciduous-conifer upland);(5) deciduous-conifer opening(an openingin the canopylike4(1exceptin a mixeddeciduous-conifer upland);and (6) openfield. Methods
Severaltrap types and combinationof trapswere usedduringthe 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 0.7square-cm hardwarecloth cage,10 cm x 20 cm, or a 2.5-cm chicken-wirecage,25 cm x 35 cm, for mouseor pigeonbait, respectively; (3) SwedishGoshawktraps(Meng1971);and(4) a modified automaticbow-net (Tordoff 1954). Trap combinations includedputting a balchatri 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 stakinganotherin a V shape,with the openend of the V againstthe lengthwisenet. A baitedbal-chatriwasplacedunderthe triggermechanism of the automatic
bow-net.
Trap placementin 1971-1972 wasbasedon the valuableadviceof Nicholls(pers.comm.) and Hamerstromand Hamerstrom(pers. comm.). During 1972-1973 we reducedthe total numberof trap sitesand alteredlocationsbecauseof our experiencefrom the previousfield season.Generally,trapswereplacedin areaswhereraptorshad beenobserved in the habitats described.Netswere placedat the edgesof openingsor perpendicular to trails(seeNicholls 1973) and on field-woodsedges.Bal-chatris, SwedishGoshawktraps,andthe bow-netwere placedin fields or on field-woodsedges.Bal-chatriswere alsodroppedfrom vehiclesnear perchedor soaringraptorsin the mannerdescribed by BergerandMueller(1959). Pigeons(Columbalivia) were frequentlyusedasbait becauseof easeof maintenance and their heartiness(Bergerand Hamerstrom1962). We alsousedbrown and white laboratory mice (Mus musculus);hooded and white laboratoryrats (Ratms norvegicus);game-farm Ring-neckedPheasants(Phasianuscolchicus);domesticrabbits (Oryctolaguscunniculus); gerbils (Meriones unguiculatus);and Starlings(Sturnusvulgaris)and CommonGrackels (Quiscalusquiscula) obtained from pest-controlprograms.Decoy animals(Hamerstrom 1963, Andersonand Hamerstrom1967) includedGreat HornedOwls and Red-tailedHawks obtainedfrom a raptor rehabilitationprogram(Fuller et al. 1974). Tetheredpigeonsand decoy animalswere securedby leather jessesabout their legs.Bait and decoy animalswere givenfood and water at the trap site and were returnedto holdingcagesat leasttwo times eachday dependingon temperatureandprecipitationconditions.All trapswere closeddown duringperiodsof extendedor severeprecipitationor whenambienttemperatures fell below
10øF.Theseconditions werejudgedtoostressful for baitanddecoyanimals, aswellasfor raptorsif they shouldbe trapped.Trapswere checkedeveryfour hoursor morefrequently dependingon weatherconditions.
Spring 1976 Results
Fuller & Christenson- Capturing Techniques
11
and Discussion
Trappingresultsusingthe variousmethodsare presentedin table 1. The combinationof mistnets plusbal-chatriwasmostsuccessful in termsof numberscapturedandtrap daysper capture.The mist net with tetheredbait was as efficient as mist nets plusbal-chatribut resultedin fewertotal captures.The bal-chatrialonewasnext mostproductiveboth in terms of capturesand efficiency.The mist net alone rankedonly slightlybelow the bal-chatri.We capturedonly two birds in the SwedishGoshawktrap and were unsuccessful with limited useof the bow-net.Calculationof correlationcoefficients(Clarke 1969) for trap daysand capturesprovidedno significantcorrelationbetweenthe two.
Table 1
Capturesby Trap Type Days
Capturesby Species
Tot.
Trap
Used Capt.
D/C GHO BO LEO SWO RT
Mist
0279 % 20
11 12
25
5 24
Mist Teth.
#191 % 13
17 18
11
1 5
Mist B.C.
#327 % 23
30 32
11
11 52
Mist Dec.
# 91 % 6
7 8
13
3 43
B.C.
#324 % 23
14 15
23
1 14
Swed. Gos.
#182 % 13
2 2
91
1 14
Bownet
# 34 % 2
Hand*
# %
12 13
2 29
2 10
Total
1428
93
7
21
2 67
1 100 1 33
BW
Owl
4 18
5 38
1 20
9 41
3 23
2 40
2 50
2 13
1 20
1 25
1 6
1 20
1 25
3 19
3 14
3 23
1 6
1
3
22
13
BW = Broad-winged Hawk
CH = Cooper's Hawk
Hawk
1 100
5 31
SWO = Saw-whet
Owl
4 25
3 14
LEO = Long-eared Owl = Red-tailed
HA
1 5
RS = Red-shouldered GH = Goshawk
RT
CH
2 15
5
*Hand-capture data not included in total trap days or days per capture calculations. GHO = Great Horned BO = Barred Owl
GH
1 5
1 5
2 10
RS
HA:
Harrier
Hawk
4
16
1
12
RAPTOR RESEARCH
Vol. 10, No. 1
The captureof eachspecies by trap type is alsopresented in table 1. Againthe combinations of mist net pluseither bal-chatrior tetheredbait gavethe bestresults.The bal-chatri
aloneappeared to be moreefficientfor falconiformes thanfor strigiformes whencompared to other methods. Great Horned Owls were used effectively as decoy animalsin efforts to
trap otherGreatHornedOwls.Hawkstrappedby thismeanswerecaughtincidentallyto the efforts directed at Great Horned Owls. However, the attacks by hawks on Great Horned
Owlsflushedduringthe day(DunstanandHarrell1973, Murphyet al. 1969) andthe success of this owl asa decoy(Hamerstrom1963) suggest that thistechniquecanbe widely applied. BarredOwls would probably avoid a Great HornedOwl decoybecauseof apparentinter-
specificconflictswhichoccurbetweenthesetwo species (Fuller et al. 1974). No Barred Owls were availableas decoy animals.Mist nets with bal-chatrisand mist nets alone were most effective for capturingBarredOwls.Thesesametwo techniqueswere usedsuccessfully to captureRed-tailedand Broad-winged Hawks thoughthesespecies were trappedwith a variety of trap types. An important point to note here and to bear in mind throughoutthe discussion is that our trappingefforts were restrictedto a study area,and the densitiesof all specieson the area were not equal. Thesedifferencesin densityare reflectedin the "total captureby species"row of table 1. For example,BarredOwlsand Red-tailedHawksweremorenumerous on the area, and therefore more likely to be trapped, than Great Horned Owls or Broad-wingedHawks. Additionally, our efforts were concentratedon these four species. Thesedata cannot be viewed as the result of an experiment,or randomsample,designedto test the effectiveness of all trap typesin all habitat typeson all species. When trapping on a specificstudy area, trap placementis an important consideration. Table 2 presentsdata concerningcapture successin the six habitat types in which we trapped.There wasno significantcorrelationbetweenthe number of dayswe trappedin a habitat and the number of captures.The deciduousuplandsappearto be suitablehabitats for trappingnearly all species,and trapsplacedunderopeningsin the canopyof a deciduous uplandresultedin the greatestnumberof raptorscaught.Trapsalongtrailsor openingsin the understoryin deciduousupland producedthe most efficient trappingin termsof trap days per capture. Great Horned Owls, though not trapped in the uplands,were known to use these habitats and openings.These owls and two Goshawkswere trapped at field-woods edgesor in fields.The woodlots,in thesecases,were deciduousuplands.The trappingdata for three of the Great Horned Owls and three of the Goshawksreflect the logisticsof our winter trapping.It waseasierto set trapsnear accessroadsor trailswhen the snowwasdeep. Thesetrailsmost frequentlyran alongthe edgesof woodlots.In light of thisbiasit appears that all speciesusedeciduouswoodlots,except the Long-earedOwl, whichis known to use coniferousand field habitatsin the uppermidwest(Christenson and Fuller 1975, Nicholls 1962). The importanceof openingsandedgesis furtheremphasized by trappingresultsfrom the mixed deciduous-conifer openinghabitat. Raptorsapparentlyuseopeningsand trailsto their advantagein severalways. First, these breaksin the habitat provide pathwaysof unob-
structedflight-particularlyfor the BarredOwl (Nicholls1973). Secondly,edgesandopenings furnish effective hunting perchesfor the sit-and-waittype of predatorwith relatively unobstructedview and flight path towardthe prey. The raptor is alsoaffordedsomeprotection from inclement weather and predators. Though bait or decoy animalswould seem visiblein the open-fieldsituation,thesesetsdid not proveproductive.The mixed deciduousconiferoustrail habitatwasprobablynot trappedenoughto yield conclusiveresults.
Spring 1976
Fuller & Christenson- Capturing Techniques
13
Table 2
Capturesby Habitat Type
Hab.
Days Tot. Used Capt.
Decid.
#371
32
Open
% 26
40
Decid.
#140
19
Trail
% 10
23
Field/
#323
18
Woods
% 23
22
Decid. Conif.
#157 % 11
8 11
# 56 % 4
1
Field
#381 % 27
3 4
Total
#1428
81
Capturesby Species
D/C GHO BO LEO SWO RT 11 7
18
BW
RS
GH
CH
10
3
6
5
3
4
53
,100
32
38
60
36
5
5
5
1
3
26
26
38
25
27
4
2
5
2
1
2
2
80
11
26
15
20
50
18
2 11
2 11
1 8
1 20
17
HA 11
100
2 18
Open. Decid. Conif.
1 100
Trail
127
1 20 5
1 5 19
1
3
19
1 25 13
5
4
l1
1
*See Table i for key.
Long-earedOwls are uncommonon the CedarCreekNatural History Area;therefore,the singlecaptureis not unusual.Saw-whetOwlsare nestingresidents on the areaandgenerally use cedar and tamaracklowland-mixed/deciduous-coniferous upland edgesat that time (Forbes and Warner 1974, Nicholls pers. comm.). The three birds we capturedwere all trapped in deciduousuplandsin March. Theseobservations agreewith other reports on habitat useby Saw-whetOwlsduringspringmigration(Catling 1971). Goshawksarewinter visitorson the area,utilizing a variety of habitat types. Red-shouldered Hawksdid not nest
on the areaduringthisstudy,but successful Red-shouldered Hawknestlng doesoccuralong nearby dyers (Malone, Christenson,and Fuller unpublisheddata), and we have observed nestingattemptsadjacentto lakes.All Red-shoulderedHawks capturedon the CedarCreek Natural History Area were immature birds, either from the previousyear (and therefore probablynot attachedto a nestingterritory) or youngof the year. The Harrier is not an uncommonbird; two pair nestedon the area.The Harrier trapped, an incubatingfemale, was taken in deciduousopeningabout 200 m from her nest. The bird was flushedtwice from the triangularmistnet andbal-chatri(pigeonbait) set.Eachtime she "helicoptered"off the trap and up over the top of the nets. The third time we rushedthe net, and she flew into it and was captured.With the exception of a SwedishGoshawktrap set 30 m from her nest, no efforts were made to trap nestingHarriers.Observationson this femaleand anotherfemalewith youngshowedthat thesebirdswere fairly restrictedin their movements,often flying over and perchingin woodland habitats adjacent to the nesting marsh.Successful efforts to trap Harriersare describedby Hamerstrom(1963) and Berger andHamerstrom(1962).
14
RAPTOR RESEARCH
Vol. 10, No. 1
One of the objectsof the study at the CedarCreekNatural History Areawasto monitor the movementsand activity patternsof both diurnal and nocturnalraptors.Thereforeour traps were generallyset 24 hoursa day. Figure 1 presentsthe capturedistributionby time of day. Sinceour trapswere checkedasinfrequentlyasonceeachfour hours,a capturetime of 0600 may representa bird that actually enteredthe trap at 0200. As might be expected, most of the owl capturesoccurredduring the night, dawn, and dusk periods.Two owlcapturepeaksoccur:predawnandpostdusk.Hawksweretrappedduringthe day, exhibiting capturepeaksin late morningand late afternoon.Muellerand Berger(1973) foundfalconiformeswere trappedwith equal frequencythroughoutthe day duringfall migration.We have no comparativedata for hawksduringSeptember,October,and November.It is possiblethat seasonal differencesin behavioraccountfor the differenttrappingresults. The resultsof our trappingby seasonarepresentedin table3. As seenfrom the trap days per capture, our efficiency was low in January and December.Our effort was 'alsosubstantially reducedduringthesemonths,but calculationof a correlationcoefficientresultedin no significantcorrelationbetween the number of trap days per month and the number of captures. Table 3
TrappingEfforts and Success by Month
Species Jan Feb Mar Apr May Jun Jul GHO BO
1 2
1
2
1 1
1
2
3
LEO SWO
3
RT
2
4
RS
1
1
GH
1
CH
3
2
1
5
1
19
1
1
2
6
4
1
19
1
1
1
5
2 1
2
3
1 5
1
1 12
HA
3
1
4
4
2
1
1
10
17
11
7
3
4 11
1
BW Total
Aug Sep Oct Nov Dec T-C
1
2
2
12
1
13 1
1
3
81
Trap
Days 95 44 213 228 156 178 208 106 67 21 15 97 1428 TD/C
48
22
18
19
16
11
19
15
22
21
15
32
Nine raptorswere recapturedone or moretimesduringthe study(not includingcaptures by hand). Table 4 illustratesthat the time of capturevariesconsiderablyfrom captureto recapture,whereasmostbirdswere retrappedin the sameor similarhabitat associations. The type of trap involvedin recaptureswas often different from that successful in first-trapping the bird. The Great Horned Owl was initially capturedwith a pigeonin a bal-chatri.Tracks in the snowrevealedthat the sethad been struck and draggedprior to being checkedat 2200 and againat 2400. At 0100 the owl wasfound caughtin the nooses-the samebird that was
Spring 19 7 6
Fuller & Christenson - Capturing Techniques
15
recapturedthree monthslater. Radio-locationdata (Fuller and Tester 1973) showedthat this bird had had numerousopportunitiesto go after pigeonsin bal-chatrisetsduringthe three-monthinterval prior to recapture.It wasnot until a mist net with a Great Horned Owl decoywas set adjacentto the woodlot where this owl'snestwasthat he wasrecaptured. Similarly, BarredOwl g•725often perchedaround an openingin which a mist net was set. This bird hadbeencapturedin a mistnet but wasnot retrappeduntil the first nighta pigeon in a bal-chatriwasaddedto the set.Thesedatasuggest that attemptsto recaptureraptorsbe undertakenwith a different type or combinationof trap types. Table 4
Capture-RecaptureData
An # 719
717
720
725
831
813
Spa
Date
Habb
Trapc
GHO
12-10-71
0130
6
5
P
03-10-72
0330
3
4
GHO
BO
BO
BO
GH
RT
Time
B/Da
12-08-71
0615
3
5
P
05-25-72
0340
1
1
-
09-27-72
2100
1
3
P
06-28-72
1930
2
1
-
07-19-72 08-08-72
0530 2100
2 3
1 3
P
03-06-73
-
2
3
P
07-06-72
2215 0615
1
1
1 3
-
08-03-72 02-21-73 03-15-73
1630 0830
3 3
4 3
04-24-72
1745
2
5
P
07-07-72
1115
1
2
-
1800 1100
3 2
4 3
P P GHO
836
RT
04-04-73 07-10-73
808
CH
04-06-72
1730
3
5
06-29-72
2100
1
4
04-24-72
1015
2
5
P
04-26-73
-
1
3
P
812
CH
a See Table 1
GHO P P
GHO
c 1 = mist net
2 = mist net and tethered pigeon
b1 = deciduous open 2 = deciduous
trail
3 = field/woods 4 = deciduous/coniferouswoods 5 = deciduous/coniferousopen 6 = field
3 = mist net and bal-chatri
4 = mist net and decoy 5 = bal-chatri
dbait/decoy P = pigeon
16
RAPTOR RESEARCH
Vol. 10, No. 1
Our incompletedata for bait type usedduringsuccessful capturesand our biaswith the use of pigeonsmake analysisof success with differentbait animalsimpractical.All species capturedwere represented by caseswhenthe bait animalwasa pigeon.It is doubtfulthat the Saw-whetOwl was attemptingto capturethe pigeon,but it is not unreasonable to assumethat other specieswere attractedby pigeons.If oneis moreconcernedaboutcapturing specieslike Cooper'sHawks, more efficiencymight be obtainedwith smallerbait animals.
Somegeneralconsiderations regardinguseof trap typesareworth mentioninghere.There are positiveand negativeaspects of the useof any of thesetraps.The mainconsiderations in trapping for our study were ability to trap severalspecieswithin a limited area in severalhabitat types and the necessityto usetrapswhich did not requireconstantvigilance. The bal-chatriis fairly effectiveand easilytransportedand placed,but birdssometimesbreak the noosesor escapefrom them. Mist netsshouldbe set up in a situationprovidinga dark backgroundto camouflagethe net. Also, protectionfro the wind is desirablebecausewhen the "bag" of the net is blown by the wind, raptorsmay "bounce" off the net. The combination of mist net and baited bal-chatriappearsto increasethe captureefficiencyaboveeither
of thesetypesusedseparately(seetable 1). This efficiencymay be due to severalcircumstances:(1) birds may be attractedto an areaby the bait andtrappedin the net while flying by; (2) they may be trappedwhen goingin on the bait; or (3) they may be trappedin the net after an encounter with the baited bal-chatri.
The mist net plus tethered bait was as efficient as the mist net plus bal-chatri.The advantageof the bait method may be that birdsreluctantto go in on the "foreign" wire trap are attracted to the more natural-appearingtethered pigeon.A disadvantage is that if the bird is not netted, it may take the bait and escape.One might try addinga noosecarpet (Andersonand Hamerstrom1967) to this combinationfor increasedefficiency.The maintenanceof nooses,whether on a bal-chatrior noosecarpet,is time-consuming. One automatic bow-net was usedto a limited extent. This technique,usingeither tetheredbait or bait in a bal-chatri,couldbe very useful(seeMatray 1974) becauseit is easily transportedand set up, and, if camouflaged,it is inconspicuous.One would have to use a hoop largeenoughto capturethe largestbird likely to be caught.Also,the apparatuswould have to be stakeddown and a safetylatch usedso the raptor could not escapeunder the frame or hoop. The SwedishGoshawktrap is easilyset, and bait animalscan be left in it continually when food, water, and someshelterare provided.It was not very efficient for our study of residentbirds,however.Suchbirdsbecomefamiliarwith their surroundings and its contents(Southern1970, Nicholls1973). The frameworkof the SwedishGoshawktrap may dissuaderesidentbirds from attemptingto obtain the bait. Whereraptorsare attracted to unusualconcentrations of prey, suchas on gamefarms(Meng 1971), or duringperiods when winter visitantsor dispersingbirds are in an area, the SwedishGoshawktrap may be very effective.
Finally, we wishto makesomesuggestions regardingraptortrappingin general.Oncetrap typeshavebeenchosenand madereadyfor the field, onemusthavea supplyof bait and/or decoy animals and adequatefacilities for their maintenance.As Berger and Hamerstrom (1962) have emphasized,healthy bait animalsare essentialfor good trapping.The bait animalsshouldbe checkedfrequentlyin the field and replacedregularlydependingon the environmentalconditions.Similarly the trapsshouldbe checkedas frequently as possible. When trappedor bait birds are exposedto direct sunlight,wind, or precipitation,they may undergostressfulconditions.Added to this problemis the strugglecapturedraptorsor bait birdsput up in effortsto escape.
Spring 19 7 6
Fuller & Christenson - Capturing Techniques
17
Raptors may injure themselvesin mist nets. We had one Goshawk and severalBarred
Owls that upon releasewould not fly and showedsignsof wing injuries.Subsequent examinationrevealedno brokenbonesor other seriousinjuries,but the birdshad obviously strainedtheir wing muscles.Raptorsmay also pierce their bodieswith their talonswhile tryingto escape.In struggles to free themselves they may attract other raptorsto the trap.
We had severalmultiplecaptures; Bergerand Hamerstrom (1962) alsoreportmultiplecaptures.Thusthe potentialfor oneraptorto prey on anotherexists,and trappedraptorsare alsovulnerable to mammalian predators. All thispointsto the needfor frequenttrapchecks. Raptorscan be removedfrom the trap and held safelyfor sometime before processing (Fuller 1975). Whencapturingbirdsby handfrom the nestor roost,caremustbe taken not to leavea scenttrail for mammalianpredators,suchasraccoons(œrocyonlotor), to follow. Napthalene crystalscan be usedto cover one's scent. They may also be spreadaround the tetheredbait or bait animalin a bal-chatrito discourage mammalianpredation. Acknowledgments T. H. Nicholls, F. Hamerstrom, and F. H. Hamerstrom, Jr., gave helpful suggestions regardingtrap useand placement.We thank A. Peterson,the late B. Worleyand P.S. Stolen of the Cedar Creek Natural History Area for use of facilities and their cooperation. J. Malone, G. Erickson,R. E. Huempfner,and S.C. Piersonprovidedassistance with trap checks,careof bait anddecoyanimals,and raptor processing. J. R. Testerand T. H. Nicholls havemadehelpfulsuggestions on the manuscript.Our sincerethanksto L. M. Ruppertfor preparationof the manuscript.Financialsupportfor the studywasfrom NIH TrainingGrant 5 TOl
GMO1779
Literature
and ERDA
COO-1332-118
to J. R. Tester.
Cited
Anderson, R. K., and F. Hamerstrom. 1967. Hen decoys aid in trapping cock Prairie Chickenswith bow netsandnoosecarpets.J. Wildl. Manage.31:829~832. Beebe,F. L., and H. Webster.1964. North Americanfalconry and huntinghawks.Denver: WorldPress,Inc. 281pp. Berger,D. D., and F. Hamerstrom.1962. Protectinga trapping station from raptor predation. J. Wildl. Manage.26:203-206. Berger,D. D., and H. C. Mueller. 1959. The bal-chatri:a trap for the birds of prey. Bird Banding30:18-26. Berry, R. B. 1971. PeregrineFalcon populationsurvey,Assateague I•land, Maryland, Fall 1969. Raptor Res.News 5:31-43. Catling,P.M. 1971. Springmigrationof Saw-whetOwls at Toronto, Ontario. Bird Banding 42:110-114.
Christenson,G., and M. R. Fuller. 1975. Food habits of two Long-earedOwl families in
east-central Minnesota.Loon47(2) :58-61. Clark,W. S. 1971. Migrationtrappingof hawks(and owls) at CapeMay, N.J., fourth year. EBBA News 34:160-169.
Clarke, G. M. 1969. Statisticsand experimentaldesign.New York: American ElsevierPublishingCo., Inc. 161 pp. Dunstan,T. C., and B. E. Hartell. 1973. Spatio-temporalrelationshipbetweenbreeding
Red-tailedHawksand Great HornedOwlsin SouthDakota. Raptor Res.7(2):49-54.
18
RAPTOR RESEARCH
Vol. 10, No. 1
Ellis, D. H. 1975. First experimentswith capturingGoldenEaglesby helicopter.Bird Banding 46:217-219. Forbes, J. E., and D. W. Warner. 1974. Behaviorof a radio-taggedSaw-whetOwl. Auk 91:783-795.
Fisher,J. H., Jr. 1889. Goodway to trap hawks.Oologist6:192. Fuller, M. R. 1975. A technique for holding and handlingraptors. J. Wildl. Manage. 39:824-825.
Fuller, M. R., T. H. Nicholls,and J. R. Tester. 1974. Raptorconservationandmanagement
applications of bio-telemetrystudiesfroxnCedarCreekNaturalHistoryArea.Raptor Res. Rep. no. 2:33-44.
Fuller, M. R., and J. R. Tester.1973. An automaticradio-tracking systemfor biotelemetry. Raptor Res.7(3/4): 105-106. Fuller, M. R., P. T. Redig, and G. E. Duke. 1974. Raptor rehabilitationand conservation in Minnesota.RaptorRes.8(1/2): 11-19.
Groinroe,I. J. 1937.Poletrap.Wis.Conserv. Bull.2(3):20-21. Hamerstrom,F. 1963. The use of Great Horned Owls in catchingMarsh Hawks. Proc. XIII Internat. Ornithol.Congr.pp. 866-869. Henderson,S. D. 1962. Effectivenessof the bal-chatritrap on raptorsother than Kestrel. EBBA News 25:205-208.
Iralet, R. H. 1937. Methodsof takingbirdsof prey for banding.Bird Banding8:156-161. Kirsher, W. K. 1958. Bal-chatritrap for Sparrow Hawks. News from the Bird Banders 33(4):41. Matray, P. F. 1974. Broad-wingedHawk nestingand ecology.Auk 91:307-324.
Mavrogordato, J. G. 1974. A hawkfor the bush:a treatiseon the trainingof the Sparrowhawk andothershort-winged hawks.London:Potter.224 pp. Meng,H. 1963. Radio controlledhawk trap. EBBA News26:185-188. Meng,H. 1971. The SwedishGoshawktrap. J. Wildl. Manage.35:832. Mueller, H. C., and D. D. Berger.1973. The daily rhythm of hawk migrationat CedarGrove, Wisconsin. Auk 90:591-596.
c
Murphy, J. R., F. J. Camenzind,D. G. Sxnith,and J. B. Weston.1969. Nestingecologyof raptorialbirdsin centralUtah. BrighamYoung Univ. Sci. Bull., Biol. Ser. 10(4): 1-36.
Nicholls,T. N. 1962. Foodhabitsof the Long-eared Owl. Passenger Pigeon24(4):130-133. Nicholls,T. N. 1973. Ecologyof BarredOwlsasdetermined by an automaticradio-tracking system.Ph.D.Dissertation. Univ. of Minn. 163 pp. Nicholls,T. N., and D. W. Warner. 1972. BarredOwl habitatuseas deterxnined by radiotelemetry.J. Wildl. Manage.26:213-224. Peeters,H. J., and E. W. Jameson,Jr. 1970. Americanhawking,a generalaccountof falconryin the New World.Davis,Calif. 150 pp. Robards,F. C. 1967. Capture,handling,and bandingof BaldEagles.U.S. Dept. Interior, Bur.SportFisheries andWildlife(Juneau,Alaska).25 pp. Southern,H. N. 1970. The naturalcontrolof a populationof TawnyOwls(Strixaluco).J. Zool, London 162:197-285.
Spring 1976
Fuller & Christenson- Capturing Techniques
19
Stewart, R. E., J. B. Cope, and C. S. Robbins.1945. Live trappinghawksand owls.J. Wildl.
Manage. 9:99-105.
l
Tordoff, H. B. 1954. An automatic live-trap for raptoffal birds. J. Wildl. Manage. 18:281-284.
Ward, F. P., and D. P. Martin. 1968. An improved cagetrap for birds of prey. Bird Banding 39:310-313.
Whitman, J. D. 1960. Somedifficultiesencounteredusingthe bal-chatrihawk trap. EBBA News. 23:104.
Sunrise
•,:::::::
.gunsot
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ß
[.................. i:•:::::::::•iiiiiiii!i!iiiiiiiiiiii:ii •'• o
L
W
SS
RB
mo
B ozoo
o•oo
•:::::::::..:•?. o•oo
oeoo
•ooo
TiME
H B
= =
Great Horned Barred Owl
Owl
L =
Long-eared Owl
S
Red-shouldered
=
OF
Hawk
•zoo
•oo
•oo
1800
2000
CAPTURE
M
=
Harrier
C =
Cooper's Hawk
G
Goshawk
R
= =
W =
Red-tailed
Hawk
Broad-winged Hawk
Figure 1. Capture Distribution of Speciesby Time of Day
2200
2400
OF TECHNIQUES
IN EAST-CENTRAL
FOR CAPTURING
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,severalspecies of raptorshad to be trapped on a 9,880-hectare study area of heterogenoushabitat types. Bal-chatri,mist net, Swedish Goshawk,and automaticbow-net traps (and combinationsof these traps) were used in severalgeneralhabitat 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-fieldedgewas also effective. Strigiformeswere most often trappedjust beforesunriseor just after sunset,while falconiformesweremostoften capturedin the late morning and late afternoon. Trappingwas least efficient from Decemberto February. A different trap type from that i•sedin the initial captureis often most effectivefor recapturing raptors.Maintenanceof healthybait animalsand frequenttrap checksare emphasized. Introduction
This paperpresentsresultsfrom a combinationof methodsusedto captureandrecapture GreatHornedOwls(Bubovirginianus), BarredOwls(Strix varia),Red-tailedHawks(Buteo }amaicensis), and Broad-winged Hawks(Buteoplatypterus)on a 9,880-hectarestudyareain east-central Minnesota. Additional information regarding the capture of Saw-whet Owls
(Aegoliusacadicus),Long-earedOwls (Asio otus), Goshawks(Accipitergentills), Redshouldered Hawks(Buteo!ineatus),anda Harrier(Circuscyaneus)areincluded. We found no data quantifyingthe resultsof trappingthat employeda combinationof techniqueson a specificstudyarea;however,there havebeen numerQuspapersdescribing varioustrapsand techniquesfor capturingbirds of prey (Bergerand Mueller 1959; Ellis 1975; Groinroe 1937; Hamerstrom 1963; Meng 1963, 1971; Nicholls 1973; Robards 1967; Stewartet al. 1945; Tordoff 1954). Severalfalconry books also providehistoricalinformation on techniquesusedfor catchingraptors(BeebeandWebster1964; Mavrogordato1974; Peetersand Jameson1970). Other papershave been concernedwith the effectivenessof particulartypesof trapsand methodsby improvingan old design(Henderson1962, Kitsher 1958, Ward and Martin 1968, Whitman 1960). Data useful in assessing the utility of a particulartrap for a particularspecies(Bergerand 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 helpfulto researchers designing studieswhich involvethe captureof birds of prey.
Raptor Research10(1):9-19, 1976
10
RAPTOR RESEARCH
Vol. 10, No. 1
The Study Area
Thestudywasconducted ontheCedar Creek NaturalHistory Area(93ø 12'E,45ø24'N) in a variety of habitatsincludingoak uplands,mixed deciduousand coniferousuplands,white
cedar (Thuja occidentalis)lowlands,tamarack(Larix laricina)lowlands,deciduousshrub
lowlands, marshes, andopenfields.Nicholls andWarner(1972)described thesehabitatsin more detail and provided a generalphenologyfor the study area. For the purposeof analyzingtrappingdata the followinghabitatswere recognized:(1) deciduousopening(a breakin the canopyof a deciduous uplandwoodsat least6 m in diameter);(2) deciduous trail (a break in the understoryof a deciduousuplandwoodsat least3 m by 3 m that may be an actual trail through the woods); (3) field-woodsedge;(4) deciduous-conifer trail (openingor trail like 442but throughamixeddeciduous-conifer upland);(5) deciduous-conifer opening(an openingin the canopylike4(1exceptin a mixeddeciduous-conifer upland);and (6) openfield. Methods
Severaltrap types and combinationof trapswere usedduringthe 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 0.7square-cm hardwarecloth cage,10 cm x 20 cm, or a 2.5-cm chicken-wirecage,25 cm x 35 cm, for mouseor pigeonbait, respectively; (3) SwedishGoshawktraps(Meng1971);and(4) a modified automaticbow-net (Tordoff 1954). Trap combinations includedputting a balchatri 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 stakinganotherin a V shape,with the openend of the V againstthe lengthwisenet. A baitedbal-chatriwasplacedunderthe triggermechanism of the automatic
bow-net.
Trap placementin 1971-1972 wasbasedon the valuableadviceof Nicholls(pers.comm.) and Hamerstromand Hamerstrom(pers. comm.). During 1972-1973 we reducedthe total numberof trap sitesand alteredlocationsbecauseof our experiencefrom the previousfield season.Generally,trapswereplacedin areaswhereraptorshad beenobserved in the habitats described.Netswere placedat the edgesof openingsor perpendicular to trails(seeNicholls 1973) and on field-woodsedges.Bal-chatris, SwedishGoshawktraps,andthe bow-netwere placedin fields or on field-woodsedges.Bal-chatriswere alsodroppedfrom vehiclesnear perchedor soaringraptorsin the mannerdescribed by BergerandMueller(1959). Pigeons(Columbalivia) were frequentlyusedasbait becauseof easeof maintenance and their heartiness(Bergerand Hamerstrom1962). We alsousedbrown and white laboratory mice (Mus musculus);hooded and white laboratoryrats (Ratms norvegicus);game-farm Ring-neckedPheasants(Phasianuscolchicus);domesticrabbits (Oryctolaguscunniculus); gerbils (Meriones unguiculatus);and Starlings(Sturnusvulgaris)and CommonGrackels (Quiscalusquiscula) obtained from pest-controlprograms.Decoy animals(Hamerstrom 1963, Andersonand Hamerstrom1967) includedGreat HornedOwls and Red-tailedHawks obtainedfrom a raptor rehabilitationprogram(Fuller et al. 1974). Tetheredpigeonsand decoy animalswere securedby leather jessesabout their legs.Bait and decoy animalswere givenfood and water at the trap site and were returnedto holdingcagesat leasttwo times eachday dependingon temperatureandprecipitationconditions.All trapswere closeddown duringperiodsof extendedor severeprecipitationor whenambienttemperatures fell below
10øF.Theseconditions werejudgedtoostressful for baitanddecoyanimals, aswellasfor raptorsif they shouldbe trapped.Trapswere checkedeveryfour hoursor morefrequently dependingon weatherconditions.
Spring 1976 Results
Fuller & Christenson- Capturing Techniques
11
and Discussion
Trappingresultsusingthe variousmethodsare presentedin table 1. The combinationof mistnets plusbal-chatriwasmostsuccessful in termsof numberscapturedandtrap daysper capture.The mist net with tetheredbait was as efficient as mist nets plusbal-chatribut resultedin fewertotal captures.The bal-chatrialonewasnext mostproductiveboth in terms of capturesand efficiency.The mist net alone rankedonly slightlybelow the bal-chatri.We capturedonly two birds in the SwedishGoshawktrap and were unsuccessful with limited useof the bow-net.Calculationof correlationcoefficients(Clarke 1969) for trap daysand capturesprovidedno significantcorrelationbetweenthe two.
Table 1
Capturesby Trap Type Days
Capturesby Species
Tot.
Trap
Used Capt.
D/C GHO BO LEO SWO RT
Mist
0279 % 20
11 12
25
5 24
Mist Teth.
#191 % 13
17 18
11
1 5
Mist B.C.
#327 % 23
30 32
11
11 52
Mist Dec.
# 91 % 6
7 8
13
3 43
B.C.
#324 % 23
14 15
23
1 14
Swed. Gos.
#182 % 13
2 2
91
1 14
Bownet
# 34 % 2
Hand*
# %
12 13
2 29
2 10
Total
1428
93
7
21
2 67
1 100 1 33
BW
Owl
4 18
5 38
1 20
9 41
3 23
2 40
2 50
2 13
1 20
1 25
1 6
1 20
1 25
3 19
3 14
3 23
1 6
1
3
22
13
BW = Broad-winged Hawk
CH = Cooper's Hawk
Hawk
1 100
5 31
SWO = Saw-whet
Owl
4 25
3 14
LEO = Long-eared Owl = Red-tailed
HA
1 5
RS = Red-shouldered GH = Goshawk
RT
CH
2 15
5
*Hand-capture data not included in total trap days or days per capture calculations. GHO = Great Horned BO = Barred Owl
GH
1 5
1 5
2 10
RS
HA:
Harrier
Hawk
4
16
1
12
RAPTOR RESEARCH
Vol. 10, No. 1
The captureof eachspecies by trap type is alsopresented in table 1. Againthe combinations of mist net pluseither bal-chatrior tetheredbait gavethe bestresults.The bal-chatri
aloneappeared to be moreefficientfor falconiformes thanfor strigiformes whencompared to other methods. Great Horned Owls were used effectively as decoy animalsin efforts to
trap otherGreatHornedOwls.Hawkstrappedby thismeanswerecaughtincidentallyto the efforts directed at Great Horned Owls. However, the attacks by hawks on Great Horned
Owlsflushedduringthe day(DunstanandHarrell1973, Murphyet al. 1969) andthe success of this owl asa decoy(Hamerstrom1963) suggest that thistechniquecanbe widely applied. BarredOwls would probably avoid a Great HornedOwl decoybecauseof apparentinter-
specificconflictswhichoccurbetweenthesetwo species (Fuller et al. 1974). No Barred Owls were availableas decoy animals.Mist nets with bal-chatrisand mist nets alone were most effective for capturingBarredOwls.Thesesametwo techniqueswere usedsuccessfully to captureRed-tailedand Broad-winged Hawks thoughthesespecies were trappedwith a variety of trap types. An important point to note here and to bear in mind throughoutthe discussion is that our trappingefforts were restrictedto a study area,and the densitiesof all specieson the area were not equal. Thesedifferencesin densityare reflectedin the "total captureby species"row of table 1. For example,BarredOwlsand Red-tailedHawksweremorenumerous on the area, and therefore more likely to be trapped, than Great Horned Owls or Broad-wingedHawks. Additionally, our efforts were concentratedon these four species. Thesedata cannot be viewed as the result of an experiment,or randomsample,designedto test the effectiveness of all trap typesin all habitat typeson all species. When trapping on a specificstudy area, trap placementis an important consideration. Table 2 presentsdata concerningcapture successin the six habitat types in which we trapped.There wasno significantcorrelationbetweenthe number of dayswe trappedin a habitat and the number of captures.The deciduousuplandsappearto be suitablehabitats for trappingnearly all species,and trapsplacedunderopeningsin the canopyof a deciduous uplandresultedin the greatestnumberof raptorscaught.Trapsalongtrailsor openingsin the understoryin deciduousupland producedthe most efficient trappingin termsof trap days per capture. Great Horned Owls, though not trapped in the uplands,were known to use these habitats and openings.These owls and two Goshawkswere trapped at field-woods edgesor in fields.The woodlots,in thesecases,were deciduousuplands.The trappingdata for three of the Great Horned Owls and three of the Goshawksreflect the logisticsof our winter trapping.It waseasierto set trapsnear accessroadsor trailswhen the snowwasdeep. Thesetrailsmost frequentlyran alongthe edgesof woodlots.In light of thisbiasit appears that all speciesusedeciduouswoodlots,except the Long-earedOwl, whichis known to use coniferousand field habitatsin the uppermidwest(Christenson and Fuller 1975, Nicholls 1962). The importanceof openingsandedgesis furtheremphasized by trappingresultsfrom the mixed deciduous-conifer openinghabitat. Raptorsapparentlyuseopeningsand trailsto their advantagein severalways. First, these breaksin the habitat provide pathwaysof unob-
structedflight-particularlyfor the BarredOwl (Nicholls1973). Secondly,edgesandopenings furnish effective hunting perchesfor the sit-and-waittype of predatorwith relatively unobstructedview and flight path towardthe prey. The raptor is alsoaffordedsomeprotection from inclement weather and predators. Though bait or decoy animalswould seem visiblein the open-fieldsituation,thesesetsdid not proveproductive.The mixed deciduousconiferoustrail habitatwasprobablynot trappedenoughto yield conclusiveresults.
Spring 1976
Fuller & Christenson- Capturing Techniques
13
Table 2
Capturesby Habitat Type
Hab.
Days Tot. Used Capt.
Decid.
#371
32
Open
% 26
40
Decid.
#140
19
Trail
% 10
23
Field/
#323
18
Woods
% 23
22
Decid. Conif.
#157 % 11
8 11
# 56 % 4
1
Field
#381 % 27
3 4
Total
#1428
81
Capturesby Species
D/C GHO BO LEO SWO RT 11 7
18
BW
RS
GH
CH
10
3
6
5
3
4
53
,100
32
38
60
36
5
5
5
1
3
26
26
38
25
27
4
2
5
2
1
2
2
80
11
26
15
20
50
18
2 11
2 11
1 8
1 20
17
HA 11
100
2 18
Open. Decid. Conif.
1 100
Trail
127
1 20 5
1 5 19
1
3
19
1 25 13
5
4
l1
1
*See Table i for key.
Long-earedOwls are uncommonon the CedarCreekNatural History Area;therefore,the singlecaptureis not unusual.Saw-whetOwlsare nestingresidents on the areaandgenerally use cedar and tamaracklowland-mixed/deciduous-coniferous upland edgesat that time (Forbes and Warner 1974, Nicholls pers. comm.). The three birds we capturedwere all trapped in deciduousuplandsin March. Theseobservations agreewith other reports on habitat useby Saw-whetOwlsduringspringmigration(Catling 1971). Goshawksarewinter visitorson the area,utilizing a variety of habitat types. Red-shouldered Hawksdid not nest
on the areaduringthisstudy,but successful Red-shouldered Hawknestlng doesoccuralong nearby dyers (Malone, Christenson,and Fuller unpublisheddata), and we have observed nestingattemptsadjacentto lakes.All Red-shoulderedHawks capturedon the CedarCreek Natural History Area were immature birds, either from the previousyear (and therefore probablynot attachedto a nestingterritory) or youngof the year. The Harrier is not an uncommonbird; two pair nestedon the area.The Harrier trapped, an incubatingfemale, was taken in deciduousopeningabout 200 m from her nest. The bird was flushedtwice from the triangularmistnet andbal-chatri(pigeonbait) set.Eachtime she "helicoptered"off the trap and up over the top of the nets. The third time we rushedthe net, and she flew into it and was captured.With the exception of a SwedishGoshawktrap set 30 m from her nest, no efforts were made to trap nestingHarriers.Observationson this femaleand anotherfemalewith youngshowedthat thesebirdswere fairly restrictedin their movements,often flying over and perchingin woodland habitats adjacent to the nesting marsh.Successful efforts to trap Harriersare describedby Hamerstrom(1963) and Berger andHamerstrom(1962).
14
RAPTOR RESEARCH
Vol. 10, No. 1
One of the objectsof the study at the CedarCreekNatural History Areawasto monitor the movementsand activity patternsof both diurnal and nocturnalraptors.Thereforeour traps were generallyset 24 hoursa day. Figure 1 presentsthe capturedistributionby time of day. Sinceour trapswere checkedasinfrequentlyasonceeachfour hours,a capturetime of 0600 may representa bird that actually enteredthe trap at 0200. As might be expected, most of the owl capturesoccurredduring the night, dawn, and dusk periods.Two owlcapturepeaksoccur:predawnandpostdusk.Hawksweretrappedduringthe day, exhibiting capturepeaksin late morningand late afternoon.Muellerand Berger(1973) foundfalconiformeswere trappedwith equal frequencythroughoutthe day duringfall migration.We have no comparativedata for hawksduringSeptember,October,and November.It is possiblethat seasonal differencesin behavioraccountfor the differenttrappingresults. The resultsof our trappingby seasonarepresentedin table3. As seenfrom the trap days per capture, our efficiency was low in January and December.Our effort was 'alsosubstantially reducedduringthesemonths,but calculationof a correlationcoefficientresultedin no significantcorrelationbetween the number of trap days per month and the number of captures. Table 3
TrappingEfforts and Success by Month
Species Jan Feb Mar Apr May Jun Jul GHO BO
1 2
1
2
1 1
1
2
3
LEO SWO
3
RT
2
4
RS
1
1
GH
1
CH
3
2
1
5
1
19
1
1
2
6
4
1
19
1
1
1
5
2 1
2
3
1 5
1
1 12
HA
3
1
4
4
2
1
1
10
17
11
7
3
4 11
1
BW Total
Aug Sep Oct Nov Dec T-C
1
2
2
12
1
13 1
1
3
81
Trap
Days 95 44 213 228 156 178 208 106 67 21 15 97 1428 TD/C
48
22
18
19
16
11
19
15
22
21
15
32
Nine raptorswere recapturedone or moretimesduringthe study(not includingcaptures by hand). Table 4 illustratesthat the time of capturevariesconsiderablyfrom captureto recapture,whereasmostbirdswere retrappedin the sameor similarhabitat associations. The type of trap involvedin recaptureswas often different from that successful in first-trapping the bird. The Great Horned Owl was initially capturedwith a pigeonin a bal-chatri.Tracks in the snowrevealedthat the sethad been struck and draggedprior to being checkedat 2200 and againat 2400. At 0100 the owl wasfound caughtin the nooses-the samebird that was
Spring 19 7 6
Fuller & Christenson - Capturing Techniques
15
recapturedthree monthslater. Radio-locationdata (Fuller and Tester 1973) showedthat this bird had had numerousopportunitiesto go after pigeonsin bal-chatrisetsduringthe three-monthinterval prior to recapture.It wasnot until a mist net with a Great Horned Owl decoywas set adjacentto the woodlot where this owl'snestwasthat he wasrecaptured. Similarly, BarredOwl g•725often perchedaround an openingin which a mist net was set. This bird hadbeencapturedin a mistnet but wasnot retrappeduntil the first nighta pigeon in a bal-chatriwasaddedto the set.Thesedatasuggest that attemptsto recaptureraptorsbe undertakenwith a different type or combinationof trap types. Table 4
Capture-RecaptureData
An # 719
717
720
725
831
813
Spa
Date
Habb
Trapc
GHO
12-10-71
0130
6
5
P
03-10-72
0330
3
4
GHO
BO
BO
BO
GH
RT
Time
B/Da
12-08-71
0615
3
5
P
05-25-72
0340
1
1
-
09-27-72
2100
1
3
P
06-28-72
1930
2
1
-
07-19-72 08-08-72
0530 2100
2 3
1 3
P
03-06-73
-
2
3
P
07-06-72
2215 0615
1
1
1 3
-
08-03-72 02-21-73 03-15-73
1630 0830
3 3
4 3
04-24-72
1745
2
5
P
07-07-72
1115
1
2
-
1800 1100
3 2
4 3
P P GHO
836
RT
04-04-73 07-10-73
808
CH
04-06-72
1730
3
5
06-29-72
2100
1
4
04-24-72
1015
2
5
P
04-26-73
-
1
3
P
812
CH
a See Table 1
GHO P P
GHO
c 1 = mist net
2 = mist net and tethered pigeon
b1 = deciduous open 2 = deciduous
trail
3 = field/woods 4 = deciduous/coniferouswoods 5 = deciduous/coniferousopen 6 = field
3 = mist net and bal-chatri
4 = mist net and decoy 5 = bal-chatri
dbait/decoy P = pigeon
16
RAPTOR RESEARCH
Vol. 10, No. 1
Our incompletedata for bait type usedduringsuccessful capturesand our biaswith the use of pigeonsmake analysisof success with differentbait animalsimpractical.All species capturedwere represented by caseswhenthe bait animalwasa pigeon.It is doubtfulthat the Saw-whetOwl was attemptingto capturethe pigeon,but it is not unreasonable to assumethat other specieswere attractedby pigeons.If oneis moreconcernedaboutcapturing specieslike Cooper'sHawks, more efficiencymight be obtainedwith smallerbait animals.
Somegeneralconsiderations regardinguseof trap typesareworth mentioninghere.There are positiveand negativeaspects of the useof any of thesetraps.The mainconsiderations in trapping for our study were ability to trap severalspecieswithin a limited area in severalhabitat types and the necessityto usetrapswhich did not requireconstantvigilance. The bal-chatriis fairly effectiveand easilytransportedand placed,but birdssometimesbreak the noosesor escapefrom them. Mist netsshouldbe set up in a situationprovidinga dark backgroundto camouflagethe net. Also, protectionfro the wind is desirablebecausewhen the "bag" of the net is blown by the wind, raptorsmay "bounce" off the net. The combination of mist net and baited bal-chatriappearsto increasethe captureefficiencyaboveeither
of thesetypesusedseparately(seetable 1). This efficiencymay be due to severalcircumstances:(1) birds may be attractedto an areaby the bait andtrappedin the net while flying by; (2) they may be trappedwhen goingin on the bait; or (3) they may be trappedin the net after an encounter with the baited bal-chatri.
The mist net plus tethered bait was as efficient as the mist net plus bal-chatri.The advantageof the bait method may be that birdsreluctantto go in on the "foreign" wire trap are attracted to the more natural-appearingtethered pigeon.A disadvantage is that if the bird is not netted, it may take the bait and escape.One might try addinga noosecarpet (Andersonand Hamerstrom1967) to this combinationfor increasedefficiency.The maintenanceof nooses,whether on a bal-chatrior noosecarpet,is time-consuming. One automatic bow-net was usedto a limited extent. This technique,usingeither tetheredbait or bait in a bal-chatri,couldbe very useful(seeMatray 1974) becauseit is easily transportedand set up, and, if camouflaged,it is inconspicuous.One would have to use a hoop largeenoughto capturethe largestbird likely to be caught.Also,the apparatuswould have to be stakeddown and a safetylatch usedso the raptor could not escapeunder the frame or hoop. The SwedishGoshawktrap is easilyset, and bait animalscan be left in it continually when food, water, and someshelterare provided.It was not very efficient for our study of residentbirds,however.Suchbirdsbecomefamiliarwith their surroundings and its contents(Southern1970, Nicholls1973). The frameworkof the SwedishGoshawktrap may dissuaderesidentbirds from attemptingto obtain the bait. Whereraptorsare attracted to unusualconcentrations of prey, suchas on gamefarms(Meng 1971), or duringperiods when winter visitantsor dispersingbirds are in an area, the SwedishGoshawktrap may be very effective.
Finally, we wishto makesomesuggestions regardingraptortrappingin general.Oncetrap typeshavebeenchosenand madereadyfor the field, onemusthavea supplyof bait and/or decoy animals and adequatefacilities for their maintenance.As Berger and Hamerstrom (1962) have emphasized,healthy bait animalsare essentialfor good trapping.The bait animalsshouldbe checkedfrequentlyin the field and replacedregularlydependingon the environmentalconditions.Similarly the trapsshouldbe checkedas frequently as possible. When trappedor bait birds are exposedto direct sunlight,wind, or precipitation,they may undergostressfulconditions.Added to this problemis the strugglecapturedraptorsor bait birdsput up in effortsto escape.
Spring 19 7 6
Fuller & Christenson - Capturing Techniques
17
Raptors may injure themselvesin mist nets. We had one Goshawk and severalBarred
Owls that upon releasewould not fly and showedsignsof wing injuries.Subsequent examinationrevealedno brokenbonesor other seriousinjuries,but the birdshad obviously strainedtheir wing muscles.Raptorsmay also pierce their bodieswith their talonswhile tryingto escape.In struggles to free themselves they may attract other raptorsto the trap.
We had severalmultiplecaptures; Bergerand Hamerstrom (1962) alsoreportmultiplecaptures.Thusthe potentialfor oneraptorto prey on anotherexists,and trappedraptorsare alsovulnerable to mammalian predators. All thispointsto the needfor frequenttrapchecks. Raptorscan be removedfrom the trap and held safelyfor sometime before processing (Fuller 1975). Whencapturingbirdsby handfrom the nestor roost,caremustbe taken not to leavea scenttrail for mammalianpredators,suchasraccoons(œrocyonlotor), to follow. Napthalene crystalscan be usedto cover one's scent. They may also be spreadaround the tetheredbait or bait animalin a bal-chatrito discourage mammalianpredation. Acknowledgments T. H. Nicholls, F. Hamerstrom, and F. H. Hamerstrom, Jr., gave helpful suggestions regardingtrap useand placement.We thank A. Peterson,the late B. Worleyand P.S. Stolen of the Cedar Creek Natural History Area for use of facilities and their cooperation. J. Malone, G. Erickson,R. E. Huempfner,and S.C. Piersonprovidedassistance with trap checks,careof bait anddecoyanimals,and raptor processing. J. R. Testerand T. H. Nicholls havemadehelpfulsuggestions on the manuscript.Our sincerethanksto L. M. Ruppertfor preparationof the manuscript.Financialsupportfor the studywasfrom NIH TrainingGrant 5 TOl
GMO1779
Literature
and ERDA
COO-1332-118
to J. R. Tester.
Cited
Anderson, R. K., and F. Hamerstrom. 1967. Hen decoys aid in trapping cock Prairie Chickenswith bow netsandnoosecarpets.J. Wildl. Manage.31:829~832. Beebe,F. L., and H. Webster.1964. North Americanfalconry and huntinghawks.Denver: WorldPress,Inc. 281pp. Berger,D. D., and F. Hamerstrom.1962. Protectinga trapping station from raptor predation. J. Wildl. Manage.26:203-206. Berger,D. D., and H. C. Mueller. 1959. The bal-chatri:a trap for the birds of prey. Bird Banding30:18-26. Berry, R. B. 1971. PeregrineFalcon populationsurvey,Assateague I•land, Maryland, Fall 1969. Raptor Res.News 5:31-43. Catling,P.M. 1971. Springmigrationof Saw-whetOwls at Toronto, Ontario. Bird Banding 42:110-114.
Christenson,G., and M. R. Fuller. 1975. Food habits of two Long-earedOwl families in
east-central Minnesota.Loon47(2) :58-61. Clark,W. S. 1971. Migrationtrappingof hawks(and owls) at CapeMay, N.J., fourth year. EBBA News 34:160-169.
Clarke, G. M. 1969. Statisticsand experimentaldesign.New York: American ElsevierPublishingCo., Inc. 161 pp. Dunstan,T. C., and B. E. Hartell. 1973. Spatio-temporalrelationshipbetweenbreeding
Red-tailedHawksand Great HornedOwlsin SouthDakota. Raptor Res.7(2):49-54.
18
RAPTOR RESEARCH
Vol. 10, No. 1
Ellis, D. H. 1975. First experimentswith capturingGoldenEaglesby helicopter.Bird Banding 46:217-219. Forbes, J. E., and D. W. Warner. 1974. Behaviorof a radio-taggedSaw-whetOwl. Auk 91:783-795.
Fisher,J. H., Jr. 1889. Goodway to trap hawks.Oologist6:192. Fuller, M. R. 1975. A technique for holding and handlingraptors. J. Wildl. Manage. 39:824-825.
Fuller, M. R., T. H. Nicholls,and J. R. Tester. 1974. Raptorconservationandmanagement
applications of bio-telemetrystudiesfroxnCedarCreekNaturalHistoryArea.Raptor Res. Rep. no. 2:33-44.
Fuller, M. R., and J. R. Tester.1973. An automaticradio-tracking systemfor biotelemetry. Raptor Res.7(3/4): 105-106. Fuller, M. R., P. T. Redig, and G. E. Duke. 1974. Raptor rehabilitationand conservation in Minnesota.RaptorRes.8(1/2): 11-19.
Groinroe,I. J. 1937.Poletrap.Wis.Conserv. Bull.2(3):20-21. Hamerstrom,F. 1963. The use of Great Horned Owls in catchingMarsh Hawks. Proc. XIII Internat. Ornithol.Congr.pp. 866-869. Henderson,S. D. 1962. Effectivenessof the bal-chatritrap on raptorsother than Kestrel. EBBA News 25:205-208.
Iralet, R. H. 1937. Methodsof takingbirdsof prey for banding.Bird Banding8:156-161. Kirsher, W. K. 1958. Bal-chatritrap for Sparrow Hawks. News from the Bird Banders 33(4):41. Matray, P. F. 1974. Broad-wingedHawk nestingand ecology.Auk 91:307-324.
Mavrogordato, J. G. 1974. A hawkfor the bush:a treatiseon the trainingof the Sparrowhawk andothershort-winged hawks.London:Potter.224 pp. Meng,H. 1963. Radio controlledhawk trap. EBBA News26:185-188. Meng,H. 1971. The SwedishGoshawktrap. J. Wildl. Manage.35:832. Mueller, H. C., and D. D. Berger.1973. The daily rhythm of hawk migrationat CedarGrove, Wisconsin. Auk 90:591-596.
c
Murphy, J. R., F. J. Camenzind,D. G. Sxnith,and J. B. Weston.1969. Nestingecologyof raptorialbirdsin centralUtah. BrighamYoung Univ. Sci. Bull., Biol. Ser. 10(4): 1-36.
Nicholls,T. N. 1962. Foodhabitsof the Long-eared Owl. Passenger Pigeon24(4):130-133. Nicholls,T. N. 1973. Ecologyof BarredOwlsasdetermined by an automaticradio-tracking system.Ph.D.Dissertation. Univ. of Minn. 163 pp. Nicholls,T. N., and D. W. Warner. 1972. BarredOwl habitatuseas deterxnined by radiotelemetry.J. Wildl. Manage.26:213-224. Peeters,H. J., and E. W. Jameson,Jr. 1970. Americanhawking,a generalaccountof falconryin the New World.Davis,Calif. 150 pp. Robards,F. C. 1967. Capture,handling,and bandingof BaldEagles.U.S. Dept. Interior, Bur.SportFisheries andWildlife(Juneau,Alaska).25 pp. Southern,H. N. 1970. The naturalcontrolof a populationof TawnyOwls(Strixaluco).J. Zool, London 162:197-285.
Spring 1976
Fuller & Christenson- Capturing Techniques
19
Stewart, R. E., J. B. Cope, and C. S. Robbins.1945. Live trappinghawksand owls.J. Wildl.
Manage. 9:99-105.
l
Tordoff, H. B. 1954. An automatic live-trap for raptoffal birds. J. Wildl. Manage. 18:281-284.
Ward, F. P., and D. P. Martin. 1968. An improved cagetrap for birds of prey. Bird Banding 39:310-313.
Whitman, J. D. 1960. Somedifficultiesencounteredusingthe bal-chatrihawk trap. EBBA News. 23:104.
Sunrise
•,:::::::
.gunsot
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ß
[.................. i:•:::::::::•iiiiiiii!i!iiiiiiiiiiii:ii •'• o
L
W
SS
RB
mo
B ozoo
o•oo
•:::::::::..:•?. o•oo
oeoo
•ooo
TiME
H B
= =
Great Horned Barred Owl
Owl
L =
Long-eared Owl
S
Red-shouldered
=
OF
Hawk
•zoo
•oo
•oo
1800
2000
CAPTURE
M
=
Harrier
C =
Cooper's Hawk
G
Goshawk
R
= =
W =
Red-tailed
Hawk
Broad-winged Hawk
Figure 1. Capture Distribution of Speciesby Time of Day
2200
2400
