J. Raptor Res. 25(1):9-17 ¸ 1991 The Raptor ResearchFoundation, Inc.

DEVELOPMENT OF FORAGING BEHAVIOR IN THE AMERICAN

DANIEL E. VARLAND AND F•RWIN E. KLAAS U.S. Fish and WildlifeService, Iowa CooperativeFish and WildlifeResearch Unit, Iowa StateUniversity, Ames, IA 50011

THOMAS M. LOUGHIN AgriculturalExperiment Station, Department of Statistics,Iowa State University,Ames, IA 5001

ABSTRACT.--Weobserved the developmentof foragingbehavior after nestdeparture in 12 siblinggroups of American (Falcosparverius). Perch resting decreasedwhereas perch hunting, eating self- capturedprey, and flying increasedover the 5-wk periodthat youngwere observed.Kestrels used perch hunting more than other typesof hunting and fed exclusivelyon invertebrates,primarily . Perch hunting success(captures/pounces) increased significantly 3 wk after fledging.After this period there was no significantchange. Significant increasesin capture rate (captures/hour) occurred4 and 5 wk after fledging due to increasedpounce rates. We observedsocial hunting among siblings,families, and alsoamong unrelated kestrels. Social hunting occurred during both perchhunting and groundhunting. Socialforaging in thesekestrels was imitative rather than cooperative.

Desarrollo de los h•tbitos de caceria en los Halcones Cernlcalos EXTe,•CTO.--Hemosobservado el desarrollode losh•tbitos de bfisquedade alimentoen HalconesCernlcalos (Falcosparverius) con 12 gruposde hermanos.Despu•s de un perlodo de 5 semanasde observaci6n, notamosque estosj6venes halcones disminuyeron la frecuenciadel posarsepara descansar,mientras que aumentaronla frecuenciaen el posarsepara cazar, el comersu presacapturada, y el volar. La cacerla desdeuna perchaocurri6 m•ts que otrostipos de caceria.Se alimentaronexclusivamente de invertebrados, principalmentede saltamonteso longostas.E1 •xito de la cacerladesde una posici6nperchada (captura/ embestida)creci6 significativamente despu•s de 3 semanasde haber dejadoel nido. Desputsde estono hubo cambiosignificativo. Un notableincremento en la proporci6nde capturas(captura/hora) ocurri6 entre 4 y 5 semanasdespu•s de dejar el nido, y se debi6 al incrementoen la proporci6nde embestidas. Se observ6cacerias en gruposentre hermanos,familias, y tambi•n entre individuossin parentesco.Las caceriasen grupo se realizaron o bien desdeel sueIv o desdeuna percha. La provisi6nde comidaen grupoen estoscernicalos rue m•tsbien imitativa que cooperativa. [Traducci6n de Eudoxio Paredes-Ruiz]

The post-fledgingperiod, here definedas the pe- In 1988 we begana study of American Kestrels riod of parental dependencyfor foodin youngbirds nestingin nest boxesattached to the backsof high- after leaving the nest (see van Tyne and Berger way signsalong InterstateHighway 35 (I-35) in 1966), hasreceived relatively little attentionin avian Central Iowa. In this paper we describethe devel- research.This is partly becauseof the difficultiesin opmentof foragingbehavior in youngkestrels during observingthe behaviorof youngonce they leavethe the post-fledgingphase and during the period of nest (e.g., Brown and Amadon 1968, Newton 1979, recent independencefrom parents.

Alonso et al. 1987). STUDY AREA AND METHODS The post-fledgingperiod and the subsequentpe- Severalyears prior to the initiation of this study,kestrel riod of recent independencefrom parents are im- nest boxeswere attachedto the backs of highway signs portantlife historystages, when youngdevelop for- along1-35 at approximately2-km intervals,from northern aging skills essentialto survival (Weathers and Polk County to northern Worth County in northcentral Sullivan 1989). High mortality ratesof recentlyin- Iowa. The studyarea was a corridorapproximately 2 km wide on either side of 1-35 from 18 km south to 99 km dependentjuveniles and othersduring their first year north of Ames. Land bordering1-35 was farmed inten- of life reflectsthe critical nature of this time (e.g., sively with row crops. Lack 1954, Henny 1972, Sullivan 1989). We banded 97 fledglingsobserved in 1988 and 1989 10 VARLAND •.T AL. VOL. 25, NO. 1

with U.S. Fish and Wildlife Serviceleg bands,and indi- appearedfrom view. We did not use data if the left vidually marked them with coloredvinyl legjesses before in < 5 min. We analyzeddata for 93 observationsessions they fledged.We captured76 percent(35/46) of the adult (mean length = 57.5 min, SD -- 32.0). kestrels in the nest box or with bal-chatri noose traps A metronometiming device(Wiens et al. 1970), set at (Berger and Mueller 1959). We bandedand individually 20 sec intervals, cued spot observationsof behavior and marked adults with coloredvinyl leg jesses. social activity. At each sound of the tone, we recorded To locatefledged young for behavioralstudies we used behavior and social activities of the focal kestrel. We re- the signalsfrom back-mountedradio transmitters (Holohil cordedfour main classesof activity:general behavior, so- Systems,Ltd., Woodlawn,Ontario, ). We attached cial behavior, hunting behavior, and allopreening and transmittersto severaldays before fiedging. In 1988 beaking.We recognizednine subclassesof generalbehav- we attached radio transmittersto 12 nestlingsin 10 nest ior and five of social behavior. boxes.Survival of radio-markedkestrels was high (11 of General Behavior. "Perch resting"describes a kestrel 12 survivedthe post-fledgingperiod) and siblingsgenerally perchedand not engagedin any other observedbehavior. maintained closecontact for 4 to 5 wk after fiedging.This Rudolf (1982) and Toland (1987) distinguished"perch confirmed the technique'susefulness and feasibility for hunting" from other perching activity by alert posture, monitoring family group activity. We made observations erectbody or bodyleaning slightly forward, frequent star- in 1988 to gain insightinto AmericanKestrel post-fiedging ing at ground (Fig. 1), and head bobs. Becauseyoung behaviorand to developan efficientdata recordingsystem. kestrelsthat have never huntedmay exhibit someof these These data are not part of the presentanalysis. behaviorswithout attemptingprey captures,behavior was We testedthe transmittersused in 1989 alongthe high- not recordedas perch hunting until at least one pounce way right-of-way at a heightof I m. Signalrange averaged was observed.Flights to/and from the ground and flights 2.3 km (N = 13, SD = 0.60, range = 1.1-3.5 km). In betweenperches during perchhunting bouts were included 1989, we radio-taggedone randomly selected nestling from in perchhunting behavior.We defined"ground hunting" eachof 13 nests.Young observedin 1989 (50 individuals as a bird on the ground searchingfor prey for >20 sec. from 13 nests)fledged between 27 and 31 d after hatching Searchesof shorterduration involvingflight from a perch (mean = 29.2, SD -- 1.4), from 13 June to 3 July. were consideredperch hunting. "Flight" was any non- One radio-taggednestling died 7 d after fledgingbefore huntingflight. We usethe term "eating" only for kestrels we could collect behavioral data. We lost signals from 3 eating self-capturedprey. "Maintenance activity" includ- of the remaining 12 transmitterswithin 5 d after the tagged ed preening, rousals (shaking), and stretching. birds fledged. For two of these sibling groups,we were "Lying on belly" describesa postureyoung kestrels often unable to determine whether the transmitters failed or if assumedon fenceposts,utility poles,and largetree branch- the individuals left the area. For the third, transmitter es. "Begging"was solicitationof food from parents."Out failure became evident when we observed the radio-marked of sight" refersto a focal kestrelconcealed by vegetation kestrel with another siblinggroup in the study 37 d after or other objects.A sessionwas discontinuedwhen a bird fiedging.Despite the early lossof signalsfrom thesethree was out of sight >5 min. "Other" was usedto categorize transmitters, we were able to collect data on behavior of behaviorsobserved relatively infrequently,and included individuals in these broods. walking,hover hunting, aggressive interactions among sib- We observedfledglings between 0600 and 1300 H at a lings, parent-to-youngprey transfers,and eating prey distance of 70-100 m with a 20x or 20-60x spotting caught by parents. During observationsessions, one or scope.We did not use a blind becausebirds under obser- both adults frequently vocalizedaggressively at us. We vation frequently changedlocations. We monitoredfledg- therefore suspectthat the interactionswith parents oc- ling groupson a rotational basisat 1-3 d intervalsuntil curredless frequently than they would havein the absence we lost contact with the brood. When we could not find of observers. a brood,we searchedby vehiclean area of approximately Social Behavior. Lett and Bird (1987) defined social 6 km 2 around their last known location. behaviorfor American Kestrel fledglingsas any behavior We adoptedWyllie's (1985) definitionof dispersal,which which occurredwithin 2 m of one or more siblings.We is movementof a fledgedbird farther than I km from its adoptedthis operationaldefinition with two modifications. nestwithout return. We determinedtime of dispersalonly We extendedthe distanceto 3 m and includednon-sibling for kestrelswith transmittersknown to be functioning1 kestrelsin socialinteractions (adults late in the post-fledg- wk after fledging(N = 9). ing periodwhich no longerfeed their youngand kestrels At the beginningof each observationsession, we ran- from outsidethe parent/sibling family unit). "Associa- domly selectedone fledgling,which was not necessarily tion" was any ac,tivityof the focalkestrel except social the one with the transmitter, as the focal bird (Altmann hunting,which occurred< 3 m from oneor more kestrels. 1974). Two people observedbehavior; typically one in- "Social hunting" was hunting activity by the focalkestrel dividual collecteddata on a siblinggroup while the other which occurred < 3 m from one or more kestrels that also observedanother group elsewhereon the study area. In were hunting (Fig. 1). "Nonsocial"refers to activity of 39 casestwo peoplecollected data simultaneouslyon two the focalkestrel occurring > 3 m from oneor more kestrels. birds in the same sibling group, or one personmade con- When we could not see whether other kestrels were < 3 secutiveobservations on differentbirds in the samesibling m from the focal kestrel becauseof densevegetation we group. For analysis,we combinedthese simultaneous or recorded its social status as "Undetermined." consecutive observations into one observation session. Foraging Behavior. We recordedpounces, captures, Sessions lasted 5 to 60 fmin or until the focal bird dis- and prey type. Foraging successwas the percentageof SPRING 1991 FORAGING IN KESTRELS 11

Figure 1. American Kestrelshunting sociallyafter fledging. pounceswith known outcomesthat were successful.Out- 1989:116). Thus, the level of significancefor thesetests is comeswere unknownin 5% of the observedpounces (18/ 0.05 divided by the total number testsbeing made on a 345). We convertedpounces and capturesto hourly rates set of non-independentbehaviors. basedon sessionlength. To comparedifferences in meansfor foragingactivity Allopreeningand Beaking.We recordedthe frequen- between weeks after fledging, we used least significant ciesand the individualsinvolved in allopreeningand beak- difference (LSD) t-tests (SAS 1985). We selected0.05 as ing, forms of direct socialcontact. Allopreening is the the level of significancefor t-tests. preeningof a conspecificindividual's plumage. Our ob- servationsof beakingparalleled those of Sherrod(1983: 182), who adoptedthe term beakingto describebehavior RE$ULTS in youngPeregrine (Falco peregrinus) in which "one falconnibbles at the beakand lore area of its sibling." Kestrelsspent progressively less time inactiveand StatisticalAnalysis. We groupedbehavioral data ac- more time foragingas they grew older (Table 1). A cordingto 7-d intervalsstarting with fledging.The ex- significantdecrease occurred in perch resting be- perimentalunit (n) wasthe siblinggroup, and observations havior (P • 0.001) with weekspost-fledging, where- of the number of groupsobserved ranged from 12 during the first wk after fledgingto 4 during the fifth. We com- as significantincreases occurred in perchhunting (P puted statisticsfor behavior,social, and hunting activity • 0.001), eating self-capturedprey (P • 0.001), for each sibling group in each 7-d, post-fledginginterval and flying (P • 0.002). We did not observeyoung for which data were available. eating prey capturedby their parentsafter the third We usedthe generallinear model procedure(PROC week post-fledging.Mean time of dispersalfor ra- GLM, SAS 1985) to obtainan analysisof variance(ANO- VA) and testedfor linear trendsin specificbehaviors dur- dio-markedkestrels (N = 9) was 23.6 d after fledg- ing 5 wk post-fledging.Because not all siblinggroups were ing. representedin all weeksand data were missingfrom some Perch hunting constituteda greater percentageof cells, we usedType III sum of squaresto calculateP foragingtime than groundhunting in all 5 wk post- values. We selected0.05 as the level of significancefor linear trends in behavior. Because behaviors were not in- fledging (Table 1). Significant increasesoccurred dependent,we adjustedthe significancelevel of P values with time in perch hunting pounces(P • 0.001), using Bonferroni'sinequalities (Snedecor and Cochran captures(P • 0.001), and success(P • 0.05; Fig. 12 VARLANDET AL. VOL. 25, No. 1

Table 1. Time (mean% _ SE) spentengaged in 10 behaviorcategories by post-fledgingAmerican Kestrels in Iowa.

WEEKS POST-FLEDOINO

1 2 3 4 5 1-5 BEHAVIOR ME•,½ _ SE MEAN _ SE MEAN _ SE MEAN _ SE M•t 4- SE P-VALUF.,Sa

Perch resting 75.3 _+4.0 53.8 _+5.2 41.4 + 3.3 19.5 _+7.2 23.8 + 2.1 <0.0010 Perch hunting 0.2 + 0.2 6.0 + 2.0 18.3 + 2.7 43.4 + 8.8 48.6 + 2.8 <0.0010 Ground hunting 0.0 0.9 + 0.7 3.6 + 1.6 10.0 + 5.4 1.8 + 1.1 0.0580 Flying 0.4 + 0.1 2.1 + 0.4 3.9 + 0.9 5.8 + 3.3 7.5 + 2.6 0.0018 Eating self-capturedprey 0.0 <0.1 + 0.1 1.5 + 0.8 6.6 + 2.6 7.9 + 0.8 <0.0010 Maintenance 14.5 + 2.0 19.1 + 4.2 17.4 + 3.4 9.3 + 3.8 8.7 + 0.8 0.3215 Lying on belly 4.1 + 3.2 7.3 + 4.1 2.9 + 1.6 0.0 0.0 0.1750 Begging 1.7 + 1.1 2.5 + 0.6 2.6 + 0.9 0.0 0.0 0.1394 Out of sight 2.3 + 0.9 5.4 + 1.2 7.9 + 2.4 5.3 + 1.6 1.6 + 0.6 0.1794 Other 1.4 + 0.4 2.8 + 1.4 0.4 + 0.2 0.1 + 0.1 0.1 + 0.1 0.0547 aP-values are based on ANOVA F-testsfor lineartrends across 5 wk post-fledging(dr = 1, 27). All testsfor lackof fit werenot significant (P > 0.05).

2). Ground hunting successalso increasedsignifi- five flycatchingattempts (see Suring and Alt 1981) cantly (P < 0.01). amongbirds 23-25 d post-fiedgingduring three ses- We identified nearly all prey items caught by sions, four were successful. young kestrelsas grasshoppers(order Orthoptera). When perchresting, fledged kestrels became pro- We saw one kestrel feeding on a (order gressivelyless social with time. The significantde- Odonata),and someitems were too small to identify. creasein association(P < 0.001) and the significant During four sessionswe observedseven brief bouts increasein nonsocialbehavior (P < 0.001; Table 2) of hover hunting in birds 12-37 d post-fledging. reflect this trend. None of theseattempts were successful.We observed Allopreening and beaking exchangesoccurred

Table 2. Time (mean% ___SE) spentengaged in socialand nonsocialactivity by post-fledgingAmerican Kestrels in Iowa.

BEHAVIORBY WEEKSPOST-FLEDGING 1-5 SOCIAL ACTIVITY 1 2 3 4 5 P-VALUES a Perchresting (N) b (12) (10) (10) (6) (4) Association 57.7 + 10.8 48.9 + 8.2 38.3 + 6.0 25.8 + 16.3 13.5 + 8.2 <0.0010 Social hunting ...... Nonsocial 28.2 + 7.5 48.9 + 8.1 56.1 + 5.8 74.2 + 16.3 86.5 + 8.2 <0.0010 Undetermined 14.1 + 8.6 2.2 + 1.0 5.6 + 3.3 0.0 0.0 0.2722 Perch hunting (N) (1) (6) (10) (6) (4) Association 0.0 17.3 + 11.7 19.4 + 6.8 6.6 + 6.0 3.3 + 1.7 0.1658 Social hunting 0.0 11.6 + 5.3 21.3 ___7.1 30.4 ___16.2 14.5 + 8.4 0.0772 Nonsocial 100.0 69.8 ___12.5 53.0 + 6.1 63.0 + 18.3 82.2 + 8.5 0.4673 Undetermined 0.0 1.3 + 0.8 6.3 + 4.9 0.0 0.0 0.7646 Ground hunting (N) (0) (5) (7) (5) (3) Association 0.0 15.0 + 15.0 21.7 ___10.3 1.2 + 1.2 0.0 0.0955 Social hunting 0.0 20.4 + 13.6 33.6 + 13.9 45.9 + 22.7 0.0 0.9385 Nonsocial 0.0 44.6 + 17.5 44.6 + 8.4 52.9 + 22.1 100.0 0.4887 Undetermined 0.0 20.0 + 20.0 0.0 0.0 0.0 -- P-valuesare basedon ANOVA F-testsfor lineartrends across 5 wk post-fledging.Perch resting df = 1, 26; perchhunting df = 1, 12; h•untingon grounddf = 1, 7. All testsfor lackof fit were not significant(P > 0.05). Numberof siblinggroups observed. (a) Perch Hunting Hunting on Ground

24 Pouuces/hr (P < 0.001) Pouuces/hr (u.s.) +/- +/- SE 2O

18

18

14

12

10

4

2

0 o I g 3 4* 5 0 I g 3 4* 5 Weeks Post-fledging Weeks Post-fledging

(b) Perch Hunting Hunting on Ground

100 r • ,oo[%Success (P < 0.05) /% Success (P < 0.01) t / - SE 80 50

•of+/_70 I SE 70

60 50

50 50

40 40

30 30

20 Z0

tO 10

0 0 o I g 3* 4 0 I g 3* 4 5* Weeks Post-fledging Weeks Post-fledging

(c) Perch Hunting Hunting on Ground

Captures/hr (P < 0.001) •' [Captures/hr(u.s.) +/- t SE •2[+/- I SE lO

4

2

o o I 2 3 4* 5* 0 I 2 3 4* 5 Weeks Post-fledging Weeks Post-fledging Figure 2. Mean foraging pounce(a), percentsuccess (b), and capture rates (c) for post-fledgingAmerican Kestrels at weekly intervals. P values are basedon ANOVA F-tests across5 wk post-fledging(perch and ground hunting pounceand capture rate df = 1, 27; perch and ground hunting successdf = 1, 10). Weekly meanswith * differ significantly(P < 0.05, least significantdifference t-test) from the precedingweek. 14 VARLAND ET AL. VOL. 25, NO. 1 during 15% (14/93) of the sessions.We observed Komen and Meyer (1989) observedallopreening in the behaviorsin 9 of 12 family groupsamong young fledglingCommon Kestrels (Falco tinnunculus). Other ranging from 3-23 d post-fledging. researchershave reported closeassociations among Kestrelswere socialwhile perchand groundhunt- fledged American Kestrels (Sherman 1913, Cade ing and spenta substantialamount of time in these 1955, Roest1957, Smithet al. 1972, Balgooyen1976, activities(Table 2). We observedsocial hunting dur- Wheeler 1979, Lett and Bird 1987), but we have ing 41% (20/49) of the sessionsin which hunting not found any referencein the literature to allo- occurred.We saw socialhunting in 10 of 12 sibling preening or beaking. groupsand quantifiedit in 8. In three of theseeight Development of Foraging Behavior. Bird and groups,social hunting involvedsiblings and nonsib- Palmer (1988) describedvarious foraging methods lings. We saw extra-familial socialhunting in 20% usedby AmericanKestrels. Toland (1987) grouped (4/20) of the sessionswith socialhunting. In one of American Kestrel foraging methodsinto three cat- thesegroups we observedsocial hunting involving egories:perch hunting (which he observed70-97% siblings,a parent, and a non-siblingfemale of un- of the time); hoverhunting (2-20%); and horizontal known age.The femaleparent did not feedthe young flight ((5%). The American Kestrel is a generalist but calledand flew aggressivelyat a Red-tailedHawk predatorof invertebratesand small vertebrates,and (Buteojamaicensis)perched within 20 m of the group, its diet varies with season and geographic area causingthe hawk to leave the area. Socialhunting (Heintzelman 1964, Bent 1938). among non-siblinggroups occurred just before or In this study,young kestrels progressed from rel- after dispersalfrom the natal area. We observed ativeinactivity to activeforaging within 3 to 4 weeks socialhunting on oneor two occasionsand then lost of leavingthe nest(Fig. 2, Table 1). The two hunting contactdue to signalloss from the radio-taggedkes- methodsobserved most frequently, perch hunting trel. We are uncertain whether these groups re- and groundhunting, were probablyleast dependent mained together. on flying ability. Early reliance on hunting tech- niquesrequiring relatively simple flight patternshas DISCUSSION been reported for post-fledgingCommon Kestrels Association.Association among fledgling kestrels (Shrubb 1982), PeregrineFalcons (Sherrod 1983), occurredmostly during the first 2 wk after fledging, Red-tailed Hawks (Johnson 1986), Northern whenyoung are mostdependent on parents.Moreno Wheatears(Moreno 1984), and SpottedFlycatchers (1984) found that fledgling Northern Wheatears (Muscicapastriata; Davies 1976). (Oenantheoenanthe) fed by oneparent perched closer Fledged American Kestrelsfed on easily-caught to each other than fledglingsfed by both parents, invertebrateprey. Dunstan(1970), Johnson(1986) and that a tendencyfor fledglingsto associate(perch and Shrubb(1982) reportedinvertebrates as the ear- ( 1 m apart)diminished as they became increasingly liestprey of Great Horned Owls (Bubovirginianus), moreindependent. Distance between sibling Spanish Red-tailed Hawks, and kestrels.Toland (1987) found Imperial Eagles(Aquila heliaca; Alonso et al. 1987) an 82% successrate amongAmerican Kestrels(both and Black Kites (Milvus rnigrans;Bustamante and sexes,all ages) hunting invertebrates,with lower Hiraldo 1990) increasedwith age, and there was a rates for rodents(66%) and birds (33%). Collopy positivecorrelation between increased sibling dis- (1973) reportedthat kestrelswintering in California tance and flying proficiency. had 64% hunting successfor invertebratesand 25% Wittenberger(1981) suggestedthat the allopreen- for vertebrates. Smallwood (1987) found kestrels ing in breedingbirds is important in maintaining wintering in Florida fed only on arthropods,with pair bonds.Our observationsof allopreeningand comparablesuccess rates for males (76%) and fe- beakingprovide evidence that AmericanKestrels are males (73%). socialafter fledging.Thus, fledglingkestrels do not In this study, mean perch hunting successin- perchclose together merely to improvetheir chances creasedsignificantly from 3.3% in the secondweek of being fed or becausethey lack flying skills. We after fledging to 49.7% in the third, but did not suggestallopreening and beakingmay maintain so- changesignificantly thereafter (Fig. 2). These suc- cial bondsbetween siblings during the post-fledging cessrates for invertebratesare substantiallylower period.Both behaviorsoccur in the socialrepertoire than rates cited above and indicate that kestrels fur- of fledglingPeregrine Falcons (Sherrod 1983), and ther developtheir foragingskills after dispersal.We SPRING 1991 FORAGING IN KESTRELS 15

observedsignificant increases in mean capturerates (Cade 1955, Wheeler 1979, Wilmers 1982). We also by perchhunting kestrels at 4 and 5 wk post-fledging observedpost-breeding adults and juveniles hunting due to increasedpounce rates (Fig. 2). The observed in groups, but social hunting was observedmost increasesin perchhunting success and pouncerates frequently amongsiblings prior to or just after dis- may be at least partially due to increasesin grass- persal.Young kestrelshunted socially from 12-46% hopper density during the post-fledgingperiod. of the time (Table 2). We saw nothingto indicate Grasshopperswere abundantin centralIowa in July that individualsin groupswere coordinatingtheir and August 1989 (Rice 1989). efforts or using signals to coordinate movements. Reportsof increasingnumbers of kills by matur- Thus, socialforaging in thesekestrels was imitative ing PeregrineFalcons released from hacksites (Sher- rather than cooperative. rod 1983) and increasinghunting successwith age We consideredthe possibilitythat differencesmight in fledgedRed-tailed Hawks (Johnson1986) were existbetween the huntingefficiency of kestrelshunt- supportedby few quantitativedata. Increasedhunt- ing sociallyand thosehunting nonsocially,but the ing successover time was quantifiedfor fledgling studydesign was not adequateto testthis idea. Fur- Ospreys (Pandion haliaetus;Edwards 1989a) and ther research is needed to document whether social passerines,including Northern Wheatears(Moreno hunting influencesforaging efficiencyin the Amer- 1984), SpottedFlycatchers (Davies 1976), and Yel- ican Kestrel. low-eyedJuncos (Junco phaeonotus; Sullivan 1988). SocialHunting. Wilson (1975:51)described two ACKNOWLEDGMENTS typesof socialforaging, imitative and cooperative. This paper is a contributionof the Iowa Cooperative The net effect of such socialhunting probably is Fish & Wildlife Research Unit, U.S. Fish & Wildlife greater foragingeffidency. Service;Iowa State University; Iowa Department of Nat- ural Resources;and the Wildlife Management Institute. During imitative foraging, individuals observe It is Journal Paper No. J-14074 of the Iowa Agriculture othersin the groupand may initiate,copy, increase, and Home EconomicsExperiment Station, Ames, Iowa, or learn foragingbehavior. All of thesemay occur Project No. 2845. Support was receivedfrom the Iowa during social hunting but are difficult to differen- Department of Natural ResourcesNongame Program, Iowa Ornithologists' Union, North American Bluebird tiate. Communicationamong imitative foragers Society,Iowa ScienceFoundation, Big BluestemAudubon probablyis indirect,and group membersdo not co- Society of Ames, Iowa, Max McGraw Wildlife Foun- ordinate their efforts during the hunt. Several in- dation, Iowa Agriculture Experimental Station, and the vestigatorsreported feeding benefits associated with Iowa State University Department of Ecology. imitative foraging (e.g., Krebs 1973, Rubensteinet For assistancein the field we are grateful to ISU students T. Holmes, K. Moeller, P. Emge, V. Windsor, and D. al. 1977, Sullivan 1984). Edwards (1989a, 1989b) Mallinger and to DNR NongameTechnicians B. Ehres- comparedthe foragingbehavior of sibling pairs of man and P. Schlarbaum.We thank Carroll Rudy (W3866 Ospreysand singleyoung and foundthat pairs and Highway H, Chilton, WI 53014) for drawingthe socially singlesboth reached the same level of successbut huntingkestrels. Thanks alsoto the followingindividuals who read drafts of the paper and providedhelpful com- that siblingsdeveloped their skills sooner.Sibling ments:G. McLaughlin, J. Bednarz,L. Best,J. Dinsmore, pairs alsoused similar foragingtechniques and had P. Delphy, J. Gionfriddo,K. Shaw, R. Bowman,J. Small- similar diets. Edwards suggestedthese differences wood and J. Bustamante. were a result of observationallearning betweensib- lings. LITERATURE CITED Hector (1986) listed six characteristics distin- ALONSO,J.C., L.M. GONZALEZ,B. HEREDIAAND J.L. guishingcooperative foraging from imitative forag- GONZALEZ. 1987. Parental care and the transition to ing, includingdivision of labor and useof signalsto independenceof SpanishImperial Eagles Aquila heliaca coordinatemovements. He reportedthat mated pairs in Dofiana National Park, southwestSpain. Ibis 129: 212-224. of AplomadoFalcons (Falco femoralis) cooperatively hunting for birds had greater success(45%) than ALTMANN,J. 1974. Observationalstudy of behavior: sampling methods.Behaviour 49:227-267. when alone(21%). Group sizein cooperativelyfor- BALGOOYEN,R.G. 1976. Behavior and ecologyof the aging Harris' Hawks (Parabuteounicinctus) was American Kestrel (Falcosparverius) in the Sierra Ne- positivelycorrelated with capture (Bednarz 1988). vada of California. Univ. Calif. Publ. Zool. 103:1-87. After the breedingseason American Kestrels may BEDNARZ,J.C. 1988. Cooperativehunting in Harris' hunt in socialgroups of 10-20 juvenilesand adults Hawks (Parabuteounicinctus ). Science239:1525-1527. 16 VARLANDET AL. VOL. 25, No. 1

BENT, A.C. 1938. Life historiesof North American birds MORENO,J. 1984. Parental care of fledgedyoung, di- of prey. Part II. Bull. U.S. Nat. Mus. No. 167. visionof labor and the developmentof foragingtech- BERGER,D.D. AND H. MUELLER. 1959. The bal-chatri: niquesin the Northern Wheatear(Oenanthe oenanthe). a trap for birds of prey. Bird-Band.30:19-27. Auk 101:741-752. BIRD, D.M. AND R.S. PALMER. 1988. American Kestrel. NEWTON,I. 1979. Populationecology of raptors.Buteo Pages 253-290, in R.S. Palmer [Ed.], Handbook of Books, Vermillion, SD. North American birds: diurnal raptors. Yale Univer- RICE, M.E. 1989. Crops, soils, and pests newsletter. sity Press,New Haven, CT. Iowa State University Ext. Entomol. NewsletterIC- BROWN,L.H. AND D. AMADON. 1968. Eagles, hawks 458(17), Ames, IA. and falconsof the world. McGraw-Hill Book Go., New ROEST,A.I. 1957. Noteson the American Sparrow Hawk. York. Auk 74:1-19. BUSTAMANTE,J. ANDF. HIRALDO. 1990. Factorsinflu- RUBENSTEIN,D.I., R.J. BARNETT,R.S. RIDGELYAND encingfamily rupture and parent-offspringconflict in P.H. KLOPFER. 1977. Adaptive advantagesof mixed- the Black Kite Milvus migrans.Ibis 132:58-67. speciesfeeding flocks among seed eating finches in Costa CADE, T.J. 1955. Experimentson winter territoriality Rica. Ibis 119:10-21. of the American Kestrel, Falcosparverius. Wilson Bull. RUDOLF,S.G. 1982. Foraging strategiesof American 67:5-17. Kestrelsduring breeding.Ecology 63:1268-1276. COLLOPY,M.W. 1973. Predatoryefficiency of the Amer- SAS INSTITUTE. 1985. SAS/STAT Guide for Personal ican Kestrel wintering in northwestern California. Computers.Version 6. SAS Institute, Gary NC. Raptor Res. 7:25-31. SHERMAN,A.R. 1913. The nestlife of the SparrowHawk. DAVIES, N.B. 1976. Parental care and the transition to Auk 30:406-418. independentfeeding in the young SpottedFlycatcher SHERROD,S. 1983. Behaviorof fledglingperegrines. The (Muscicapastriata). Behaviour39:280-295. PeregrineFund, Inc., Ithaca, NY. DUNSTAN,T.C. 1970. Post-fledgingactivities of juvenile SHRUBB,M. 1982. The hunting behaviorof somefarm- Great Horned Owls as determinedby radio telemetry. land kestrels.Bird Study 29:121-128. Ph.D. thesis.University of SouthDakota, Vermillion, SMALLWOOD,J.A. 1987. Sexual segregationby habitat SD. in American Kestrels wintering in southcentralFlor- EDWARDS,T.C. 1989a. Similarity in the development ida: vegetativestructure and responsesto differential of foraging mechanicsamong sibling Ospreys.Condor prey availability. Condor89:842-849. 91:30-36. SMITH, D.G., C.R. WILSON AND H.H. FROST. 1972. 1989b. The ontogenyof diet selectionin fledg- The biologyof the American Kestrel in Central Utah. ling Ospreys.Ecology 70:881-896. Southwest Nat. 17:73-83. HECTOR,D.P 1986. Cooperativehunting and its rela- SNEDECOR,G.W. ANDW.G. COCHRAN. 1989. Statistical tionshipto foragingsuccess in an avian predator.Ethol- methods.Iowa State University Press,Ames, IA. ogy 73:247-257. SULLIVAN,K.A. 1984. The advantagesof socialforaging HEINTZELMAN,D.S. 1964. Springand summerSparrow in Downy .Anita. Behar. 32:16-21. Hawk food habits. Wilson Bull. 76:323-330. 1988. Ontogenyof time budgetsin Yellow-eyed HENNY, C.J. 1972. An analysisof the populationdy- Juncos: adaptation to ecologicalconstraints. Ecology namicsof selectedavian species.U.S. Fish and Wildl. 69:118-124. Serv., Wildl. Res.Report 1:1-99, Washington,D.C. 1989. Predation and starvation: age-specific JOHNSON,S.J. 1986. Developmentof hunting and self- mortality in juvenilejuncos (Junco phaeonotus ). J. Anita. sufficiencyin juvenile Red-tailed Hawks. RaptorRes. Ecol. 58:275-286. 20:29-34. SURING,L.H. ANDC.J. ALT. 1981. Flycatchingbehavior KOMEN,J. ANDE. MEYER. 1989. Observationson post- by American Kestrels. SouthwestNat. 26:76. fledgingdependence of kestrels(Falco tinnunculus rupi- TOLAND, B.R. 1987. The effect of vegetativecover on colus)in an urban environment.J. Raptor Res. 23:94- foragingstrategies, hunting successand nestingdistri- 98. bution of American Kestrels in central Missouri. J. KREBS,J.R. 1973. Social learning and the significance Raptor Res. 21:14-20. of mixed-speciesflocks of chickadees(Parus spp.) Can. vAN TYNE, J. ANDA.J. BERGER. 1966. Fundamentals J. Zool. 51:1275-1288. of ornithology.John Wiley and Sons,. LACK,D. 1954. The natural regulation of animal num- WEATHERS,W.W. AND K.A. SULLIVAN. 1989. Juvenile bers. Clarendon Press, Oxford, U.K. foragingproficiency, parental effort, and avian repro- LE•r, D.W. ANDD. M. BIRD. 1987. Post-fledgingbe- ductive success.Ecol. Monogr. 59:223-246. havior of American Kestrelsin southwesternQuebec. WHEELER,S. 1979. New Hampshire kestrels.New Wilson Bull. 99:77-82. HampshireAudubon Annual. Spring:25-27. SPRING 1991 FORAGING IN KESTRELS 17

WIENS, J.A., S.G. MARTIN, W.R. HOLTHAUSAND F.A. WITTENBERI3ER,J.F. 1981. Animal social behavior. IWEN. 1970. Metronome timing in behavioralecology Duxbury Press, Boston, MA. studies.Ecology 51:350-352. W¾•J.IE,I. 1985. Post-fledgingperiod and dispersalof WILMERS,T.J. 1982. Kestrel use of nest boxeson re- young SparrowhawksAccipiter nisus. Bird Study 32: claimed surfacemines in West Virginia. M.S. thesis. 196-198. West Virginia University, Morgantown, WV. WILSON,E.O. 1975. Sociobiology:the new synthesis. Received 13 July 1990; accepted13 December 1990 Belknap Press,Boston, MA.