FOOD OF THE BOOTED EAGLE (HIERAAETUS PENNATUS) IN CENTRAL SPAIN
JoskP. VEIGA
ABSTRACT.m The identificationof 202 prey remainsof the BootedEagle (Hieraaetus pennatus) shows that mammals (41.6%of preyitems identified), birds (36.6%) and reptiles(21.8%) are importantprey in CentralSpain. Most mammals capturedwere youngrabbits, and the majorityof the bird prey were fledglingsor juveniles.Lizards were adult or subadultindividuals. Over 90% of the preycaptured weighed between 27 and 243 g.
Little is known about the biologyof the Booted but no eagleswere seen in 1981.Visits were made approximately Eagle(Hieraaetus pennatus), as it occursin countries every 15 d from shortlybefore incubation (mid-late April) until after the youngleft the nest(mid-late August). During the feeding with little ornithologicalactivity. Most published periodnests were occasionallyvisited every 7 d. Pelletsand prey accountsof food habitsare singleenumerations of remainswere sought in andaround nests and below perches which prey remainsrecorded mainly during sporadicvis- were usuallywithin a 200 m radiusof the nests. its to nests(Valverde 1967; Arafijo 1973; Garz0n A total of 110 pellets,containing 130 identifiableprey items, 1973;Iribarren ! 975). This procedureprovides an and72 preyremains were collected. Each species found in anyone pelletwas counted as 1 individualunless it waspossible to show inaccuratepicture of diet, sinceprey that are large that more than 1 wasrepresented. Therefore, it wasnecessary to and leave persistentremains are over-represented countpieces of remainssuch as nails, beaks, teeth, etc. Weight and in samples(e.g., Valverde 1967;Delibes 1975). In approximateage of the prey were estimatedby comparingre- spiteof this, severalrecent papers dealing with the mainswith materialfrom zoologicalcollections and with speci- menscollected in the studyareas. In order to establisha frequency trophicrelationships between members of various distributionfor prey,weight classes were established whose limits raptor communitieshave made use of such data followeda geometricprogression (Fig. 1). This insuredthat the (Jaksi•and Soriguer 1981;Jaksi• 1983;Jaksi• and resultingdistribution would be more or lessnormal (Schoener Braker 1983).In my opinionthis has led to errone- 1969;Hespenheide 1971). Only some prey identified in thepellets ousconclusions regarding the ecologicalposition of could be assignedto one of the establishedweight categories, particularlyin thecase of species,like rabbits and ocellated lizards, the BootedEagle in Mediterraneanenvironments. whoseweights vary a great deal. The present paper presentsmore accurateinfor- mation about the diet of this raptor, obtainedusing RESULTS AND DISCUSSION a more systematicdata collectionprocedure. I also take into considerationsome attributes of prey, Mammals,birds, and reptiles,in decreasingor- suchas sizeand age, that have been overlooked. der of capturefrequency, comprised the diet of the Booted Eagle in the studyarea. Percentagediffer- STUDY AREA AND METHODS ences of these taxa in the diet increased considera- bly when biomasswas taken into account.(Table 1). Thisstudy was carried out in 3 areas, each about 35 km 2 insize, Among mammal prey, rabbitswere the most im- located on the northern slope of the Sierra de Guadarrama mountains (40ø 35'-40 ø 60' N, 0ø 5'-0 ø 60' W). Area 1 is about 60% portantprey species.Birds captured were primarily pastureinterspersed with thick scrub.The only arborealforma- speciesthat forage on the ground. The Ocellated tionspresent are 3 smallpine groves of between1 and 5 ha. Area 2 Lizard (Lacertalepida) was the only reptile prey, is I0 km awayand about40% coveredwith maturenatural pine although other lizards are common in the study trees(Pinus silvestris) over 15m tall.The restof area2 ismade up of area. a sparseevergreen oak grove(Quercus rotundifolia) with extensive clearingsin whichlow scrubmixes with pastureland. Area 3, 15 The weightof prey itemsvaried between 10 and km from area 2 and 30 km from area 1, is similar to area 1 in that it 800 g. However,most were in the 27 to 243 g range hasonly 2 arborealformations, one of 2 ha and the otherof 25 ha. (Fig. 1). A major part of the diet consistedof prey in Area 1 wasvisited from 1978to 1981.One pair of BootedEagles the 81 to 243 g weight-class(Fig. 1). Prey-sizedis- used the samenest year after year. Area 2 wasalso visited from 1979to 1981.In 19792 pairsof nestingeagles were present, but in tributionsdo not appear to be the samefor the 3 1980 to 1981 no nests were found. Area 3 was also visited from taxa present in the diet: most mammal and lizard 1979to 1981.In both 1979and 1980 1 pair of eagleswas located, prey weighedbetween 81 and 243 g. Avian prey was
120 RAPTORRESEARCH VOL. 20 (3/4): 120-123 FALL/WINTER 1986 BOOTED EAGLE DIET 121
methodologicalbiases. However, using the same methodology,these small prey havebeen found in the diet of other similar sized raptors in the same 5O areasin whichthe BootedEagle was studied, Fur- thermore, by samplingprey remainsregularly and at relativelyshort intervals the potentialbias possi- bly causedby the greater detectabilityof certain prey when collectedat longer intervalswould be 25 diminished. The fact that the material to be analyzedwas collectedfrom the nestsas well as from the perchesof the adultsreduces the possibil- ity of obtaininga distortedimage of diet if it is I assumedthat food taken to the nestlingsis different from that of the adults. I have not been able to demonstratethis in the Booted Eagle. 0 9 27 81 243 729 2217 Earlier studiesof Booted Eagle feeding habits carried out in the Palearctic and in South Africa prey-weight classes g describe them as a hunter of small birds and, to a Figure 1. Diet of the BootedEagle. Thick line histogram: lesserdegree, lizards (Valverde 1967; Arafjo 1973; percentof the total biomasssupplied by the Garzhn 1973; Iribarren 1975; Steynand Grobler prey-items;thin line histogram:percent of the 1981). It is worth noting that althoughthe scarcity totalnumber of prey-items.Sample size = 165. of mammalsin the South African Booted Eagles' predominantlybetween 27 and 81 g (Fig. 2). The diet could be due to a lack of appropriate sized majority of birds in this classwere the Spotless individuals in the field, the low representationof Starling,(Sturnus unicolor) weighing 70 g. Nearlyall this taxon in reports from Spain where rabbits rabbitscaptured were very young individuals.Of aboundin a varietyof sizesis surprising.My results 27 bird prey itemsof knownage, the number of suggestthat the BootedEagle behaves, in my study fledgling and juveniles was greater than the numberof adults(22 youngvs. 5 adults).All Ocel- lated Lizards identified were adults or sub-adults. Prey-sizedistribution could merely reflect the sizedistribution of availableprey, assumingBooted lOO[ ß Eagleson the study area selectedprey randomly with respectto size.Nevertheless, the lackof insects, % o amphibians,and smallreptiles in the diet of some other raptors of similar sizesuch as the Common 50 Buzzard (Buteobuteo), Black Kite (Milvus migrans) and Red Kite (M. milvus)in the same study area (Veiga 1982) suggeststhat prey below a certain weight were avoided.Prey might alsobe selected 25 accordingto ageand experience.This maybe par- ticularlytrue for arian prey, sincethe poor flying abilitiesof young birds make this age classmore ß I I I I vulnerableto predationby BootedEagles. 0 9 2•7 81 243 729 2217 g It has been reported that the analysisof pellets and prey remainsfor Order Falconiformestends to prey-weight classes underestimatethe amount of some prey while overestimatingothers (Valverde 1967; Delibes Figure 2. Distributionof the prey remainsin the prey- weightclasses in eachtaxonomic group., Black 1975; Collopy 1983). The absenceof small prey circles = mammals; open circles = birds; suchas insects,amphibians or smallreptiles in the squares= reptiles.Sample sizes: mammals = Booted Eagles' diet could be due to these 61; birds = 64; reptiles= 40. 122 Josi•P. VEIGA VOL.20, NO. 3/4
Table 1. Prey of the BootedEagle in centralSpain.
NUMBER OCCURRENCE BIOMASS SPECIES OF ITEMS PERCENT PERCENT
REPTILES OcellatedLizard (Lacertalepida) 44 21.8 14.3 Total 44 21.8 14.3
BIRDS
Common Kestrel (Palcotinnunculus) 2 0.99 1.0 Quail (C oturnix c oturnix) 2 0.99 0.46 Unidentified Phasianidae 1 0.49 0.46 Little Bustard (Otistetrax) 1 0.49 1.76 Stone Curlew (Burhinusoedicnemus) 1 0.49 1.05 Wood Pigeon(Columbia palumbus) 1 0.49 1.08 Unidentified Columbidae 3 1.48 2.54 Swift (Apusapus) 1 0.49 0.09 Hoopoe (Upupaepops) 7 3.46 1.03 Green Woodpecker(Picus viridis) 1 0.49 0.39 Unidentified Alaudidae 1 0.49 0.08 Mistle Thrush (Turdusviscivorus) 1 0.49 0.27 SpotlessStarling (Sturnus unicolor) 28 13.86 5.13 Magpie (Picapica) 9 4.45 4.33 Jackdaw(Corvus monedula) 4 1.98 2.1 Carrion Crow (Corvuscorone) 1 0.49 1.18 Unidentified 10 4.95 1.83
Total 74 36.6 24.8
MAMMALS Common White-toothed Shrew(Crocidura russula) 1 0.49 0.03 Blind Mole (Talpacaeca) 2 0.99 0.19 Rabbit ( Oryctolaguscuniculus) 65 32.18 48.71 Hare (Lepusgranatemis) 2 0.99 5.0 Unidentified Lagornorpha 1 0.49 o.75 Water Vole (Arvicolasapidus) 8 3.96 5.0 Weasel (Mustelanivalis) 3 1.48 0.94 Unidentified 2 0.99 0.19
Total 84 41.6 60.8
TOTAL ITEMS 202 F•,i•i•/W•'rr, a 1986 Boo'r•r• E•,c;I• Dm'r 123 area, like a taxa-generalistthat concentrateson ter- ranean habitats of Chile and Spain: a comparative restrial prey weighingbetween 70 and 240 g. It is analysis.J. Anim.Ecol. 50: 269-281. probablethat the generaldecrease of the rabbit in SC•OE•ER,TW. 1969. Modelsof optimalsize for solitary Iberian ecosystemsin the last decades,resulting predators.Am. Nat. 103:277-313. from the effect of mixomatosis, has influenced the STEYr,P. A•I) GROBLZR,J.H. 1981. Breeding biologyof the BootedEagle in SouthAfrica. Ostrich52:108-118. compositionof the Booted Eagle'sdiet. However, VALwRI)Z,J.A. 1967. Estructura de una comunidad there are no detailed studiesof the population mediterrfinea de vertebrados terrestres. C.S.I.C. Mad- dynamicsof the rabbit and other prey species, rid. whichwould be necessarybefore this could be seri- Vzxo^, J.P. 1982. Ecologia de las rapaces de un ouslydiscussed. ecosistemaMediterrfineo de montaha.Aproximaci6n a su estructuracomunitaria. Ed. Univ. Compl., Mad- ACKNOWLEDGMENTS rid.
I amindebted toG. Bortolotti, C.Griffin and B. Millsap for their MuseoNacional de Ciencias Naturales C.S.I.C. Josi• Gutii•rrez criticalcomments of an earlierdraft. Abascal,2. Madrid-28006.SPAIN.
LITERATURE CITED Received30 March 1985; Accepted8 April 1986.
ARAUJO,J. 1973. Falconiformesdel Guadarrama sur- occidental. Ardeola 19: 257-278. CoI•I•OV¾,M.W. 1983. A comparisonof direct observa- tions and collectionsof prey remainsin determining the dietof GoldenEagles.J. Wildl. Mange. 47: 360-368. DEI•mES,M. 1975. Alimentaci6ndel Milao Negro (Milvus migrans)en Do-nona(Huelva, Espana). Ardeola 21: 183- 2O7. GARZ0N,J. 1973. Contribuci6n al estudio del status, alimentaci6ny protecci0nde las falconiformesen Es- paha central.Ardeola 19: 279-330.• HEsPrNVirxr)r,H.A. 1971. Food preferenceand the ex- tent of overlapin someinsectivorous birds, with special referenceto the Tyrannidae.Ibis 113: 59-72. IRmARRrN,J.J. 1975. Biolo•ia del figuila calzada (Hieraaetuspennatus) durante el periodo de nidifica- ci6n en Navarra.Ardeola 25 (Vol. Esp.):305-320. JA•cS•6,F.M. 1983. The trophic structureof sympatric assemblagesof diurnal and nocturnalbirds of prey. Amer. Mid. Natur. 109: 152-162. JAKSX6,F.M. ANDBRAKER, H.E. 1983. Food niche re- lationshipsand guild structure of diurnal birds of prey: competitionversus opportunism. Can. J. Zool. 61:2230-2241. JAKSI•,F.M. ANDSORIGUER, R.C. 1981. Predationupon the Europeanrabbit (Oryctolagus cuniculu,) in Mediter-