HUNTING BEHAVIOR OF AND SPACE USE BY ... - Semantic Scholar

j. RaptorRes.36(3):194-199 ¸ 2002 The Raptor ResearchFoundation, Inc.

HUNTING BEHAVIOR OF AND SPACE USE SCREECH-OWLS DURING THE BREEDING

BY EASTERN SEASON

JENNIFER E. BUHAY 1 ANDGARYRITCHISON 2 Department of Biological Sciences, EasternKentuckyUniversity, Richmond, KY 40475 U.S.A. ABSTRACT.--Thehunting behavior of and spaceuse by radio-taggedEastern Screech-Owls(Otus asio; three pairs, one unmated male, and four fledglings) were observedin central Kentuckyfrom MarchJune 1999. Screech-owlsperched at a mean height of 2.2 m. The mean giving up time was 340.2 sec, while the mean time until initiating an attackwas361.4 sec.Owlsattackedprey located a mean distance of 2.27 m from perches,and the mean distancebetweensuccessive percheswas10.33 m. Male screechowlsperched higher in taller trees during the fledgling period, and thesehigher percheswere apparently usedwhen hunting insects.Male screech-owls alsohunted in different areasof their rangesduring different breeding periods, possiblyto take advantageof temporarily abundant prey and avoid prey depletion. During the fledgling period, the hunting ranges of paired male and female screech-owls overlapped,but males and femalesdid not hunt in areasof overlapat the sametime.

K•¾Wombs: EasternScreech-Owl; Otus asio;huntingbehavior, perchtime;, rangeuse;,breeding season.

Comportamientode cazay uso de espaciode Otusasiodurante la estaci6nreproductiva RESUMEN.--E1 comportamientode cazay uso del espaciode Otusasiodotadoscon radio transmisores (tres parejas,un macho solitarioy cuatrovolantones)rue observadoel centro de Kentuckydesdemarzojunio 1999. Los bfihos se posaronen perchasa un altura de 2.2 m. E1 tiempo de esperafue de 340.2 sec,mientras que el tiempo medio hastael inicio de un ataque fue de 361.4 secLos bfihos atacarona presaslocalizadasa una distanciamedia de 2.27 m de las perchasy a un distanciamedia entre perchas sucesivas de 10.33 m. Los machosse ubicaron en las perchasm•tsaltas de •trbolesgrandes,aparentemente estasfueron utilizadaspara la cazade insectos.Los machostambi•n cazaronen areasdistintasa sus rangos durante las diferentes etapas de su reproducci6n, posiblemente para aprovechar la abundanciatemporalde presasy evitaragotarlas.Durante el perlodo del crecimientodel plumaje,losrangos de caza de las parejasse traslaparonpero los machosy hembras no cazaron en •treasde traslapeal mismo tiempo. [Traducci6n de CasarMarquez]

Eastern Screech-Owls (Otus asio) are found

throughout easternNorth America and have been the subject of many studies (e.g., Belthoff et al.

1993,Sparkset al. 1994,Duguayet al. 1997). However,as with other nocturnal predators,direct observations are difficult

and, as a result, little is

known about the hunting behaviorof screech-owls. In one study,Abbruzzeseand Ritchison(1997) observed the hunting behavior of Eastern ScreechOwlsin central Kentuckyand found no diffbrences betweenmales and femalesduring the nonbreeding season. • Presentaddress: Departmentof BiologicalSciences, 410 ScientificCollections,Universityof Alabama,Tuscaloosa, AL 35487

U.S.A.

• Corresponding author'se-mailaddress: gary.ritchison@ eku edu

194

As the breeding seasonapproaches,males and females in many owl speciesexhibit a division of labor (Reynoldsand Linkhart 1987). The reproductive successof male raptors may depend on their ability to supplyfood for matesand offspring throughout the breeding season,beginning with courtshipand continuing through the post-fiedging period. Once young are able to thermoregulate and manipulateprey,femalesresumehunting and assistthe male in provisioningnestlingsand fledglings (Hovis et al. 1985, Gehlbach 1994). The changing roles of male and female screech-owls during the breeding seasonmay influence their hunting behavior.Our objectivewith thisstudywas to examine how changingfood demandsand the changing roles of male and female Eastern Screech-Owlsmight influence hunting behavior and spaceuse during the breeding season.

SEPTEMBER 2002 METHODS

SCREECH-OWL HUNTIN(;

AND MATERIALS

The hunting behavior of Eastern Screech-Owls(three breeding pairs, an unpaired male, and four fledglings) was studied from 5 March-8 June 1999 at the Central

Kentucky Wildlife ManagementArea, located 17 km southeast of Richmond, Madison County, Kentucky. Adult screech-owls were capturedby checkingnestboxes or by luring them into mist nets using the playback of bounce songs(Ritchisonet al. 1988). Captured owlswere fitted with 5-6 gm (about 3-4% of screech-owlbody mass) radio-transmitterswith activity switches(Wildlife Materials, Inc., Carbondale, IL) attached backpackstyle (Smith and Gilbert 1981). Yellow reflective tape (about 5 X 1 cm) was attached to the antennas of the transmitters to make it easier to locate hunting owls. Owls were allowed

1 wk to become

accustomed

to the transmitters

before observationsbegan. Several daysprior to anticipated fiedging dates, nestlings(N = 13 in three nests) were banded with a numbered aluminum leg band, and four (two from one nest and one each from the other two nests) were fitted with radio-transmitters.

One or two radio-tagged owls were observed each night and general locationswere determined usinga receiver (TR-2, Telonics Inc., Mesa, AZ) and a hand-held, two-elementyagi antenna. Specificlocationsof owlswere determined using a red-tinted flashlight. Owls were assumed to be hunting when their attention appeared to be focused on the ground (probably searching for ground-basedprey) or on nearby vegetation (probably searchingfor birds or insects) or flying insects.When hunting, owlsalso exhibited t?equenthead movements. In addition, hunting fledglings,in contrastto non-hunting fledglings,were alwaysperched severalmeters away t?om siblings. Observationswere made at least four times per week. Observationperiods startedshortlyafter sunsetand were usually 3-4 hr in duration. We typically watched oMs

BEHAVIOR

195

we determined the distanceusing ArcView 3.1 (Environmental SystemsResearchInstitute, Redlands,CA). The location of each perch was recorded in Universal TransverseMercator (UTM) coordinates using Global Positioning System (GPS) receivers (Garmin XLS). To determine the size of the areas used by owls, we produced 100%-minimum-convexpolygons (MCP) using ArcView 3.1 with the SpatialToolsand Animal Movement extensions.

We

also

measured

the

distance

from

each

perch to the nest in each territory. Repeated measuresanalysisof variance wasused to examine the possible effects of breeding period on the hunting behavior of male screech-owls. Small samples•zes precluded comparison of male hunting behavior to that of females and fledglings. All analyseswere conducted using the StatisticalAnalysisSystem(SASInstitute 1989). All valuesare presentedasmean + standarderror. Individual owlsare referred to by the last three digitsof their (or their mate's or parents') radio-transmitter frequency. RESULTS

Eastern Screech-Owls (N = 4 males, 3 females,

and 4 fledglings of unknown sex) were observed for a total of 182 hr on 55 nights from 5 March-8 June 1999. Overall, these owlsperched at a mean height of 2.20 - 0.08 m (N = 304 perches) in trees witt• a mean height of 5.63 -+ 0.24 m (N = 301 perches) and mean DBH of 0.12 -+ 0.01 m (N = 301 perches;Table 1). Owls (N = 11) perched a mean distanceof 0.51 -+ 0.05 m (N = 302 perches) from the main trunk of perch trees or snags.Forty different plant specieswere used as perches by screech-owls, with snags (21.6%), American ash from a distance of 8-12 m. Adult females were difficult (11.1%, Fraxinusamericana),and American sycato observeduring the nestlingperiod becausethey often more (7.8%, Platanus occidentalis) used most frerespondedaggressively. Once femalesbegan hunting fur- quently. ther away from nest sites (about 1 wk after young The mean giving-uptime (N = 231 perchesof 9 fledged), our presence seemed to have little effect on their behavior.As a result, only female hunting perches owls) was 340.2 -- 18.7 sec, while the mean time located during the fledgling period were usedin analyses until initiating an attack (N = 18 perchesof 6 owls) of hunting behavior.The breeding statusof eachpair was was361.4 -+ 32.0 sec.Owls (N = 6) attackedprey categorized as: pre-nesting, egg-laying/incubation,nestling, or fledgling. During focal owl observations,we tape-recordedall information and perchesand attacksiteswere markedwith flagging (placed severalmeters away) for later analysis. Perch timeswere classifiedas either giving-up time (the owl flew to another perch without initiating an attack) or time until attack. These times were only determined when we either observedowls landing on a perch or could estimatelanding time basedon a changein a transmitter's pulse rate (all transmittershad activityswitches). Within 2 wk, we returned to perchesand measured perch height, substrate(tree, shrub, or vine) height, the distancefrom the tree trunk to the perch, and the diameter at breastheight (DBH) of the perch tree. We also noted tree speciesand measured the distanceto the next perch. If successive percheswere more than 50 m apart,

located

a mean

distance of 2.27

-

0.28 m from

perches (N = 21). The mean distancebetweensuccessivepercheswas 10.33 _+1.42 m (N = 65 perch changesby 9 owls). We observed21 attacksby screech~owls, with 11 successfuland 10 unsuccessful.Eight of 18 attacks by males were successful,resulting in the capture of three birds, one small mammal, two crayfish (Cambarusspp.), and two insects.Attacks by females (N = 2) and fledglings (N-- 1) on insects were

all successful.

Although adult females and their young were only observedhunting during the fiedging period, two males were observed during the pre-nesting,

196

BUHAYAND RITCHISON

VOL. 36, NO. 3

Table1. Perchcharacteristics andhuntingbehaviorof male,female,andfledgling(unknownsex)EasternScreechOwls.Valuesare presentedasmean -+ one standarderror.Samplesizesare in parentheses.

Perchheight(m) Tree height (m) Distancefrom tree trunk (m) Tree DBH (m) Distanceto nextperch(m) Attackdistance(m) Giving-uptime (sec) Attacktime (sec)

MAI •ES

FEMALES

FLEDGLINGS

(N= 4)

(N= 3)

(N= 4)

1.93 + 0.07(258) 4.70 + 0.22 (255) 0.42 + 0.05 (256) 0.10 _+0.01 (255) 11.07-+ 1.72(53) 2.32 + 0.30 (18) 355.5 + 20.4 (201) 381.3 + 32.3 (16)

3.50 + 0.36 (22) 9.79 + 0.92 (22) 0.97 + 0.27 (22) 0.19 + 0.03 (22) 2.25 -+ 0.48 (2) 0.75 -+ 0.25 (2) 183.2+ 62.4 (13) 150 (1)

3.99 _+0.34 (24) 11.67_+0.79 (24) 0.97 -+ 0.23 (24) 0.23 _+0.05 (24) 8.75 _+1.53(10) 2.5 (1) 279.9 -+ 52.0 (17) 255 (1)

egg-laying/incubation, nestling,and fledglingpe- es among periods in either attack time or attack riodsand a third male wasobservedduring all but distance. For male screech-owls, the mean distance of the pre-nestingperiod.Analysisof the huntingbehavior of these males revealed that mean perch huntingperchesfrom nestsitesdid not differ (F3,• height (F.•,s= 14.85, P = 0.006) and tree height = 0.1, P = 0.95) amongbreedingperiods.Similar(b•,•= 7.99,P = 0.024)variedamongperiods(Fig. ly,the sizeof malehuntingrangesdid not vary(F,.3,s 1), with malesperching higher in taller treesdur- = 4.13, P = 0.056) amongbreedingperiods.Howing the fledgling period (Tukey's test, P < 0.05). ever, specifichunting areasused by male screechWe found no differencesamongbreedingperiods owlsdid vary amongbreedingperiods.For examm other variables,including DBH of perch trees ple, male 957 huntedprimarilyin the northeastern (F•,• = 2.43, P = 0.18), distanceof perchesfrom portion of his territory during the pre-nestingpethe main trunk (F3, • = 3.5, P = 0.11), giving-up riod, but in the southwestern sectionduring the time (F3,5= 0.71, P = 0.59), and distancebetween egg-laying/incubation period.During the nestling successive perches(F•,3= 2.07, P = 0.27). Sample period, male 957 hunted primarilyin the souths•zeswere too small to examine possibledifferenc- eastern section of his territory and, during the fledgling period, again used the southwesternsection (Fig. 2). Based on a limited number of observations, the

Perch Height I•Tree Height

N

Nest sile

Pre,-nesting Egg-laying/ Incubation

Pre-nesting Egg-laying/

Nestling

Fledgling

Incubation

Period

4()0 meters

Nestling Fledgling

Figure 1. Variation among breeding periodsin mean Figure 2. Variation in areasused by screech-owlmale perch heightsof and treesusedby male EasternScreech- 957 during differentbreedingperiodsin centralKenOwls. tucky.

SEPTEMBER 2002

SCREECH-OWL HUNTING

sizesof hunting ranges of the three adult female screech-owls during the fledgling period were 7.0 ha (two observationperiods, five perches), 9.4 ha (two observationperiods, 12 perches),and 12.2 ha (five observationperiods, 10 perches),respectively. During the fledgling period, the hunting range of male 957 completely encompassedthat of his mate, while the rangesof males 997 and 037 included87% and 59% of the hunting rangesof females997 and 037, respectively. Althoughranges overlapped,paired malesand femaleswere not observedhunting in the samelocationson the same night. Fledglings (N = 4 from three different broods) were observed during 10 observation periods (froln the day of fledging through 29 d post-fledging). Youngowlswere first observedhunting 26 d after fledging. The first (and only observed) successfulattackby a fledglingwas27 d after fledging. DISCUSSION

Eastern Screech-Owlsin our studyperched at a mean height of 2.2 in. Similarly, Gehlbach (1994) found

that

Eastern

Screech-Owls

in

suburban

Waco, Texas, hunted from perches at a mean height of 2.6 m. However, at the same location where we conducted our study, Abbruzzese and

Ritchison(1997) reporteda mean perch height of 1.66 in for screech-owlsduring the non-breeding season.The higher mean perch height in our study may be due to seasonaldifferencesin the typesof prey being hunted and seasonaldifferencesin vegetation height. During the non-breedingseason, screech-owlsin central Kentucky appeared to be hunting primarily crayfish and slnall mammals (Abbruzzese 1996). In contrast, screech-owlsin our

BEHAVIOR

197

Other nocturnal owls hunt froln perches at heightssimilar to thoseusedby screech-owls in our study.For example,Boreal Owls (Aegoliusfunereus) have been reported to hunt froln perchesat mean

heightsof 1.7 in (Norberg 1987) and 3.1 in (Bye et al. 1992). Lower perches may provide a better view of ground-dwellingprey in areas where lowgrowing cover and shrubs obscure much of the ground (Norberg 1987). In addition, low perches may allow owlsto better locate prey usingacoustic cues (Norberg 1987,Abbruzzese1996). The mean giving-uptime for screech-owls in our studywas340.2 sec (or 5.7 min). Abbruzzesseand

Ritchison(1997) reported a mean giving-uptime of 278.9 sec (4.6 min) for Eastern Screech-Owls during the non-breeding season,while Gehlbach (1994) found that screech-owls hunting primarily insectshad a mean giving-uptime of just 72 sec

(1.2 min). Gehlbach (1994) suggestedthat prey abundance influences giving-up times, with owls able to assessareas of dense prey more quickly. Differencesin the type of prey being hunted may alsoinfluence giving-uptimes.For example,Bye et al. (1992) suggestedthat owls need more time to searchfor slnall mammals than other prey because their

movements

are difficult

to detect.

The mean time before initiating an attack by screech-owls in our studywas361.4 sec (or 6 min), similar in duration to the mean giving-up time (340.2 sec). During the non-breeding season,Abbruzzese and Ritchison (1997) also reported no difference between giving up and attack times for screech-owls,and suggestedthat predators detecting invertebrateprey do not wait long before initiating an attack becausesuch prey are relatively

easyto capture. In addition, we observed screechowls capturing or attempting to capture flying insectson severaloccasions.Becauseflying prey can crease the visual search area and to scan areas quickly move out of range, owls likely attack such above vegetation for flying insects.For example, prey shortlyafter detection. In contrast,Bye et al. Village (1990) noted that Eurasian Kestrels (Falco (1992) suggestedthat Boreal Owls had longer attinnunculus)chose higher percheswhen hunting tack, or detection, times becausethey waited loninvertebratesbecausesuch perchesprovide larger ger before initiating attacks on slnall mammals. scanning areas than low perches. Owls attackingslnall mammalsmay wait longer beDuring the breeding season,vegetationis taller cause such prey are difficult to capture (Toland and thicker than during the non-breedingseason 1987, Bye et al. 1992, Atkinson and Cade 1993). and, in response,screech-owlsmay perch higher. Hayward and Hayward (1993) noted that, after deSimilarly,Morrison (1980) found that Loggerhead tecting prey, Boreal Owls sometimeswaited 10 min Shrikes (Lanius ludovicianus) selected higher or more if a prey item was not in a vulnerable poperchesduring the summerthan during the winter sition. becausesuch perchesprovide a wider field of view Paired malesin our studyperched higher in tallin taller, more dense summer vegetation. er trees during the fledgling period than during

studyoften appeared to be hunting for insects,and owlsmay perch higher when hunting insectsto in-

198

BUHAYAND RITCHISON

the pre-nestingand egg-laying/incubationperiods. This increasein perch height may have been due to a change in the type of prey being hunted. Male screech-owlsappeared to hunt primarily small mammals and crayfishduring the pre-nestingand egg-laying/incubationperiods, but were only observedhunting insectsduring the fledgling period. Although this switchmay have been due in part to changesin availability,other factorsmay have also contributed. Gehlbach (1994) suggestedthat male screech-owlsprovision females with large prey items during the pre-nesting period because females require substantialamounts of energy to produce a clutch of eggs. However, during the nesding and fledgling periods, male screech-owls may provide their youngwith smaller,more easilyhandled prey like insects (Gehlbach 1994). McClain (1997) found that adult screech-owlsdelivered primarily small prey items (including beetles [Coleoptera] and moths [Lepidoptera]) to nestlings. Male screech-owlsin our study were observed hunting in different parts of their ranges during different breeding periods. One possible reason for such shifts may be to take advantage of temporarily abundant prey. For example, Gehlbach (1994) observed that screech-owlssometimes made

repeated trips to and from concentrations of invertebrates such as emerging cicadas (Cicadidae) and earthworms (Lumbricidae). Village (1990) noted that raptors sometimes return to hunting

siteswhere prey havebeen captured.Similarly,we sometimes

observed

male

screech-owls

make

re-

peated visitsto the edgesof temporary pools and •ntermittent streams,possiblysearching for crayfish that were

sometimes

abundant

at such loca-

tions (pets. observ.).Abbruzzese (1996) also reported that screech-owls in central Kentucky frequently hunted for crayfish.Another reasonwhy male screech-owlsmight hunt in different areas at different times is to avoid depletion of prey resources.For example, Village (1990) found that the hunting ranges of Eurasian Kestrelswere not used uniformly and that kestrelswere familiar with their territoriesand the prey resources,as shown by the use of the same hunting locationson successivedays. The hunting rangesof paired male and female EasternScreech-Owls in our studyoverlapped,particularly around nest sites.However, our observations also suggestthat members of a breeding pair did not use the same hunting locations on the

VOL. 36, No. 3

same nights. Other investigatorshave also found that paired male and female raptorsforage in different areas probably to avoid competition and prey depletion (Newton 1986, Craig et al. 1988, Village 1990, Gehlbach 1994). Gehlbach (1994) observed

that

male

and

female

screech-owls

leav-

ing nest siteson hunting foraysusuallyflew in different directions. Similarly, male and female EurasianSparrowhawks(Accipiter nisus)typicallyhunt in different directions from the nest (Newton 1986, Selas and Rafoss 1999).

We first observedhunting by fledgling screechowls 26 d after fiedging. However, we only observedthe four radio-taggedfledglingson four occasions during the period from 16-26 d post-fiedging (on days 16, 19, 22, and 26). Gehlbach (1994) reported that young screech-owls attempt to capture insectpreyjust 9-14 d after fiedging and are able to feed themselvesregularly3-4 wk after fiedging. ACKNOWLEDGMENTS

We thank Meg Bommarito, Kevin DeFosset,Matt Ricketts, and Sue Anestisfor help with field work, and Jim Belthoff,Jeff Duguay,and Fred Gehlbachfor their many helpful comments on the manuscript. The Frank M. Chapman Fund of the American Museum of Natural History and the University ResearchCommittee at Eastern KentuckyUniversityprovidedfinancialsupport. LITERATURE

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SCREECH-OWL HUNTING

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Received13 November 2001; accepted16 April 2002 Former

Associate

Editor:

Cole

Crocker-Bedford