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AGGRESSIVE RESPONSES OF NESTING PRAIRIE FALCONS TOTERRITORIAL INTRUDERSAuthor(s): ANTHONIE M. A. HOLTHUIJZEN and LENORA OOSTERHUISSource: The Wilson Bulletin, 116(3):257-261. 2004.Published By: The Wilson Ornithological SocietyDOI: http://dx.doi.org/10.1676/03-086URL: http://www.bioone.org/doi/full/10.1676/03-086

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257

Wilson Bulletin 116(3):257–261, 2004

AGGRESSIVE RESPONSES OF NESTING PRAIRIE FALCONS TOTERRITORIAL INTRUDERS

ANTHONIE M. A. HOLTHUIJZEN1,3 AND LENORA OOSTERHUIS2

ABSTRACT.—From 1984 through 1987, we studied aggressive responses of Prairie Falcons (Falco mexican-us) to species intruding into their nesting territories in southwestern Idaho (52 nesting attempts, 613 days, 9,085hr). Prairie Falcons responded aggressively most frequently to Common Ravens (Corvus corax; 49% of en-counters), followed by Red-tailed Hawks (Buteo jamaicensis; 24%), Golden Eagles (Aquila chrysaetos; 7%),Turkey Vultures (Cathartes aura; 4%), Northern Harriers (Circus cyaneus; 2%), American Kestrels (Falcosparverius; 1%), and bobcats (Lynx rufus; 1%). The frequency of aggressive responses toward intruders wassimilar for males and females, except in the case of American Kestrels and bobcats. Aggressive responses ofnesting Prairie Falcons to intruders may be related to predator deterrence, competition for nest sites, stage ofthe nesting cycle, food availability, and sexual size dimorphism of falcons. Received 22 August 2003, accepted16 August 2004.

Descriptions of aggressive responses ofnesting Prairie Falcons (Falco mexicanus) arelargely based on anecdotal information (e.g.,Bent 1937, Enderson 1964, Ogden and Hor-nocker 1977, Anderson and Squires 1997)rather than systematic study. Nesting PrairieFalcons respond aggressively to a wide vari-ety of avian and mammalian species that en-croach into their territories, and they displayvarying levels of tolerance to intruding spe-cies (Steenhof 1998). The lack of systemati-cally collected data has led to speculationabout both the aggressiveness and tolerance ofPrairie Falcons (e.g., Decker and Bowles1930, Kaiser 1986). Here, we describe inter-specific aggressive responses of Prairie Fal-cons to intruders in their nesting territoriesduring the breeding season. We report on (1)species with which Prairie Falcons interacted,(2) the frequency of aggressive responses, (3)the level of aggression exhibited by PrairieFalcons, and (4) differences in aggressive re-sponses between males and females.

METHODS

From 1984 through 1987, we observedPrairie Falcon pairs in 52 nesting attemptsover 613 days (9,085 hr) in the Snake RiverBirds of Prey National Conservation Area(NCA; n 5 48) and the Reynolds Creek Area(n 5 4) in southwestern Idaho. The study ar-

1 Idaho Power Company, Environmental AffairsDept., P.O. Box 70, Boise, ID 83707, USA.

2 7110 Glenridge View, Boise, ID 83709, USA.3 Corresponding author; e-mail:

tholthuijzen@idahopower.com

eas are part of the western intermountain sage-brush steppe and are characterized by coldwinters and hot, dry summers (West 1983).Climate, vegetation, and natural environmentare described in U.S. Department of the In-terior (1996). The data presented herein werecollected as part of a larger behavioral studyon Prairie Falcons (Holthuijzen 1989). Gen-eral methodology, observation techniques, andprotocol are described in Holthuijzen (1989,1990). We defined nesting territory as a lo-cality where nests were found, usually in suc-cessive years, and where no more than onepair ever nested at one time (Newton andMarquiss 1982). Between territory establish-ment and fledging of young, we observedeach pair, on average, every 6 days. Observerswere situated within 250 m of aeries (mean 5147 m 6 43 SD, n 5 52). An observation daystarted ½ hr before sunrise and terminated ½hr after sunset. Field observations were madeby two observers, each on a half-day shift.Observers were systematically rotated throughall selected falcon pairs, alternating observa-tions between mornings and afternoons. Webegan observations 1–7 weeks prior to incu-bation; observations continued until youngwere 30–35 days of age, or the nesting at-tempt failed. We used a photographic agingkey (Moritsch 1983) to determine the age ofyoung (when banded, visible at the aeries, orboth). The beginning of incubation was cal-culated by back-dating from estimated age ofyoung; we assumed that the incubation periodwas 34 days long (Burnham 1983). We pooledobservation days into 6-day intervals.

258 THE WILSON BULLETIN • Vol. 116, No. 3, September 2004

TABLE 1. Number of aggressive responses of male and female Prairie Falcons (52 nesting attempts, 613observation days, 9,085 hr) directed toward territorial intruders in southwestern Idaho, 1984–1987.

Intruding species

Female

Chase Vocal n

Male

Chase Vocal n

Pooled observations

Chase Vocal n %

Common Raven 328 224 552 364 160 524 692 384 1,076 49.5Red-tailed Hawk 99 144 243 138 149 297 237 293 530 24.4Golden Eagle 50 30 80 59 24 83 109 54 163 7.5Turkey Vulture 16 32 48 23 15 38 39 47 86 4.0Northern Harrier 21 7 28 19 3 22 40 10 50 2.3American Kestrel 3 2 5 21 0 21 24 2 26 1.2Bobcat 24 4 28 5 0 5 29 4 33 1.5Other speciesa 13 12 25 9 5 14 22 17 39 1.8Unidentified species 19 76 95 25 50 75 44 126 170 7.8All species 573 531 1,104 663 406 1,069 1,236 937 2,173

a Osprey, Bald Eagle, Sharp-shinned Hawk, Northern Goshawk, Swainson’s Hawk, Ferruginous Hawk, Rough-legged Hawk, gulls (Larus spp.), BarnOwl, Great Horned Owl, Black-billed Magpie, coyote, domestic dog, and porcupine.

The establishment of territorial boundariesearly in the nesting season (Ogden and Hor-nocker 1977, Haak 1982, Sitter 1983, Holthu-ijzen 1989) and the general openness of theterrain enabled us to record aggressive re-sponses between territorial falcon pairs andintruders in the nesting territories. We record-ed time and duration of Prairie Falcon behav-iors continuously, including aggressive re-sponses to intruders that passed through nest-ing territories (Holthuijzen 1989). We definedan aggressive response as one during whichthe focal falcon engaged in aggressive behav-ior (multiple chases, vocalizations, or both)toward an intruder. We distinguished betweentwo categories of aggressive responses: vocal(alarm call) and chase. We considered a chase,which often included vocalizing, a higher-in-tensity response than a vocal response alone.We defined a combined response as one inwhich both sexes of a nesting pair participat-ed; falcon genders were determined by thelarger size of females (Steenhof 1998), posi-tion during copulation, food solicitation by fe-males, and stylized drawings of the facial pat-terns for each nesting pair. When an interac-tion took place, gender was either known atthat time or was determined afterwards. Be-havior of falcons was reported continuouslyduring an observation day, minimizing gendermisidentification.

We analyzed the number of aggressive in-teractions for male and female falcons sepa-rately, for each pair each day. We noted spe-cies of intruder and length of aggressive en-counters. Statistical analyses were conducted

with Statistical Analysis System software(SAS Institute, Inc. 1990). Statistical testswere considered significant at P # 0.05. Dif-ferences in level of aggression between maleand female Prairie Falcons were tested withFisher’s Exact Test. We calculated the numberof interactions per hr during which pairedbirds were present on nesting territories andused one-way analysis of variance (ANOVA)to determine whether aggressive responses/pair/hr differed among nesting stages.

We did not collect information on relativeabundance of intruders during the differentphases of the Prairie Falcon nesting season.Therefore, our interpretations regarding tem-poral trends in aggressive responses to intrud-ers by Prairie Falcons should be interpretedwith caution.

RESULTS

Aggressive behavior of Prairie Falcons(both vocal and chase) was observed most of-ten with Common Ravens (Corvus corax;49% of encounters), followed by Red-tailedHawks (Buteo jamaicensis; 24%), Golden Ea-gles (Aquila chrysaetos; 7%), Turkey Vultures(Cathartes aura; 4%), Northern Harriers (Cir-cus cyaneus; 2%), American Kestrels (Falcosparverius; 1%), and bobcats (Lynx rufus; 1%)(n 5 2,173 responses; Table 1). In 8% of allinstances (n 5 170), the intruder could not beidentified. Fourteen additional avian andmammalian species intruded into falcon ter-ritories (2%, n 5 39; Table 1): Osprey (Pan-dion haliaetus), Bald Eagle (Haliaeetus leu-cocephalus), Sharp-shinned Hawk (Accipiter

259Holthuijzen and Oosterhuis • AGGRESSIVE RESPONSES OF PRAIRIE FALCONS

FIG. 1. Mean number of aggressive responses of Prairie Falcons on nesting territories (responses/falcon pair/hr; 52 nesting attempts, 613 days) directed toward Common Ravens, Red-tailed Hawks, and Golden Eagles insouthwestern Idaho, 1984–1987.

striatus), Northern Goshawk (Accipiter gen-tilis), Swainson’s Hawk (Buteo swainsoni),Ferruginous Hawk (Buteo regalis), Rough-legged Hawk (Buteo lagopus), gulls (Larusspp.), Barn Owl (Tyto alba), Great HornedOwl (Bubo virginianus), Black-billed Magpie(Pica hudsonia), coyote (Canis latrans), do-mestic dog (Canis familiaris), and porcupine(Erethizon dorsatum). Encounters averaged 1min 56 sec 6 2 min 35 sec (SD) and did notdiffer between male and female falcons (Stu-dent’s t-test: P 5 0.25).

The frequency of aggressive responses to-ward Common Ravens and Red-tailed Hawkswas greater during pre-incubation than duringincubation and brood rearing (F2,594 5 5.79, P5 0.003; F2,594 5 7.37, P , 0.001, respective-ly; Fig. 1), but did not differ between incu-bation and brood rearing for either species(Tukey’s pairwise test, experimentwise errorrates P 5 0.05). Aggressive responses towardGolden Eagles were relatively stable over thebreeding season (F2,594 5 1.70, P 5 0.18; Fig.1). Insufficient information was available todiscern temporal patterns for other species.

The number of chases as a proportion oftotal aggressive responses was highest forAmerican Kestrel (92%, n 5 26), followed bybobcat (88%, n 5 33), and Northern Harrier(80%, n 5 50) (Table 1). We recorded 148

combined responses to intruders: CommonRavens made up 32%, Red-tailed Hawks30%, Golden Eagles 16%, Northern Harriers5%, and bobcats 3%.

Generally, the frequency of combined vocaland chase responses was similar for male andfemale falcons; the exceptions were AmericanKestrel and bobcat (Table 1). Males more fre-quently chased American Kestrels than fe-males (Fisher’s Exact Test: P 5 0.030) andfemales chased bobcats more than males.Males more often chased Common Ravensthan females (69%, n 5 524 and 59%, n 5552, respectively; Table 1; Fisher’s Exact Test:P , 0.001). Males chased Turkey Vulturesmore often (60%, n 5 38), whereas femalesresponded to them more often with vocaliza-tions (67%, n 5 48; Fisher’s Exact Test: P 50.016).

DISCUSSION

Prairie Falcons responded to avian intrudersby vocalizing, chasing, or both; ground pred-ators (i.e., bobcat and coyote) were mostlychased (Table 1). Aggressive responses ofPrairie Falcons may reflect a perceived threatto nest or offspring. Prairie Falcons showed arelatively high percentage of combined re-sponses toward Golden Eagles, Northern Har-riers, bobcats, Red-tailed Hawks, American

260 THE WILSON BULLETIN • Vol. 116, No. 3, September 2004

Kestrels, and Common Ravens, but not to Tur-key Vultures. Contrary to our findings, Kaiser(1986) reported that Northern Harriers andTurkey Vultures were largely ignored by nest-ing Prairie Falcons. Ogden and Hornocker(1977) and Kaiser (1986) reported that gen-erally, Red-tailed Hawks and Golden Eaglesare attacked (chased), as we found for GoldenEagles, but not for Red-tailed Hawks (Table1). The vigorous response of Prairie Falconsto American Kestrels may be the result ofprovocation by kestrels, which tend to be veryaggressive (Brown and Amadon 1968). Coy-otes, bobcats (Ogden and Hornocker 1977,Sitter 1983, Kaiser 1986, Peterson 1988), andGolden Eagles (McFadzen and Marzluff1996) are important predators of nestling andfledgling Prairie Falcons; thus, predator deter-rence may be the predominant motivation foraggressive behavior of Prairie Falcons towardthese species, particularly during brood rear-ing.

During pre-incubation, competition for nestsites may also play a role in high levels ofaggressive responses of Prairie Falcons toCommon Ravens and Red-tailed Hawks (Fig.1). Alternatively, Red-tailed Hawks, CommonRavens, and Prairie Falcons establish nestingterritories during approximately the sametime, possibly increasing the frequency of en-counters and raising opportunities for aggres-sive responses. Several authors (e.g., Bent1937, Kaiser 1986, Anderson and Squires1997) commented on the notable tolerance ofPrairie Falcons for Common Ravens. Cade(1987) suggested a symbiotic relationship be-tween Common Ravens and Prairie Falcons,whereby Prairie Falcons use nests built byCommon Ravens and Common Ravens ben-efit by robbing prey cached by Prairie Fal-cons.

Food competition may explain some of theaggressive responses observed. Northern Har-riers (Holthuijzen et al. 1987) and Red-tailedHawks (Haak 1982) have been observed tokleptoparasitize adult falcons. Furthermore,scavengers such as Common Ravens and, per-haps, Turkey Vultures, may rob prey caches(Bent 1937, Cade 1987).

Female Prairie Falcons are about one-thirdlarger than males (Enderson 1964), whichmay result in a differential response towardpotential predators, as has been suggested for

other diurnal raptors (Newton 1979). Femalefalcons were responsible for 85% of the en-counters with bobcats, and both males and fe-males responded with high intensity to thisspecies (100% and 86% chase, respectively;Table 1). The larger size of females may be amore effective deterrent against bobcats thanthe greater agility of males. On the other hand,American Kestrels were always chased bymale Prairie Falcons; in this case, the agilityof the male may be more effective. Towardother intruders, however, male and femalePrairie Falcons showed a similarly aggressiveresponse (Table 1). Apparently, size, agility,or other factors related to the size differencebetween males and females do not provide ei-ther gender with a particular advantage in de-fense against these intruders.

ACKNOWLEDGMENTS

We are grateful to all field personnel for their assis-tance. G. Admonds and the late B. Holmes kindly al-lowed use of Idaho Power Company’s facilities atSwan Falls Dam, and D. Brakensiek and K. Gebhartallowed use of the USDA Research Service Station atReynolds Creek. L. S. Young gave organizational andadministrative support, and participated in data collec-tion. K. Steenhof guided many aspects of this study.Comments from J. C. Bednarz, L. B. Carpenter, F. B.Edelmann, A. R. Harmata, A. M. Moser, K. Steenhof,and N. S. Turley greatly improved earlier drafts. Thisresearch was supported by Idaho Power Company (anIDACORP Corporation), the U.S. Bureau of LandManagement, and the Pacific Gas and Electric Com-pany through efforts of A. R. Ansell, M. N. Kochert,and R. D. Williams.

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