SEPTEMBER1997 LETTERS 287

AravaipaCreek, Arizona. Almost immediatelyat leastsix Elf Owls beganvocalizing from dispersedlocations around our campsiteand at leastfour of them began making low passesat the Great Horned Owl. Before we could relocate the horned owl to a protectedenclosure, it wasstruck once in the head by an Elf Owl. Some nocturnal behaviorsmay not be well known or understood,not becausethey are rare, but becausethey are difficult to observe.This may changewith the increasedavailability of night vision equipment (P. Henson and J A Cooper 1994, Auk 111:1013-1018). Currently, observationsof nocturnal behaviorsare likely to be sporadicand anecdotal,and therefore unreported. Such information, however,may help in understandinga speciesbiology. For example, other researchershave observedgroup mobbing by Elf Owls (F.R. Gehlbach,pers. comm.; B.A. Millsap, pers. comm.), but there are no publishedreports of the behavior.Our observations,and thoseof other researchers, suggestthat Elf Owlswill join together in mobbing and that they can be physicallyaggressive when defending their nestsagainst predators. We thank A. Duerr, T.S. Estabrookand R.L. Spauldingfor assistingwith the observations.We alsothank T. Brush, ER. Gehlbach, R. Glinski, P. Hardy, B.A. Millsap, G. Proudfoot and H.A. Snyder for sharing their observational information concerning mobbing by small owls.This manuscriptbenefitted from the constructivereviews of ER. Gehlbach,C. Marti, B.A. Millsapand an anonymousreviewer.--Clint W. Boal, Brent D. Biblesand R. William Mannan, Schoolof RenewableNatural Resources,University of Arizona, Tucson,AZ 85721 U.S•.

j. RaptorRes. 31 (3):287-288 ¸ 1997 The Raptor ResearchFoundation, Inc.

GRIFFONVULTURES (GYPS FULVUS) INGESTING BONES AT THE OSSUARIESOF BEARDEDVULTURES ( GYPAETUSBAR•ATUS)

Some African vulturesovercome the calcium deficiencyin their diets by ingestingbone fragments,and are depen- dent on the presenceof largepredators to supplythem (Mundy and Ledger 1976, S. Afr.J. Sci.72:106-110; Mundy 1982, The comparativebiology of southernAfrican vultures,Vulture StudyGroup,Johannesburg, South Africa; Rich- ardson et al. 1986, J. Zool.Lond. 210:23-43). Becauseof the lack of large mammalian carnivoresin the Iberian Peninsula,vultures apparently satisfy their calcium needsby ingestingsmall bone fragmentsfrom carcasses(K6mg 1975, Ardeola21:219-224) or smallpieces of limestone(Fern•ndez 1975, Ardeola22:29-54; Elosegi1989, ActaB,ol. Mont. 3, S•rie documentsde Travail). This note reports severalobservations of Griffon Vultures (Gypsfulvus)making use of bone splintersobtained from BeardedVulture (Gypaetusbarbatus) ossuaries, where large bonesare deliberately droppedonto rock slabs(Boudoint 1976, Alauda 44:1-21). Field work was carried out in the meridional Prepyrenees(northeast of Spain), an area of isolated calcareous massifsdescribed by Riba et al. (1976, Geografiatisica dels Paisos catalans, Ketres, Barcelona, Spain). The datawere collectedwhile we were monitoring severalBearded Vulture pairs between 1991-95 at eight ossuarieslocated in five different nestingareas (Heredia 1991,Pages 78-89 in R. Heredia and B. Heredia [EDS.],E1 quebrantahuesos Gypaetus barbatusen los Pirineos,ICONA, Madrid, Spain) selectedat random.All ossuarieshad Griffon Vulture coloniesnearby (<1 km). We made 126 visitsto the nestingareas during the nestlingperiod from February-August. Griffon and Bearded Vultures interacted at ossuariesin all five nesting areas. Occasionally,Griffon Vultures ex- plored ossuarieswhen there had been no previousoccurrence of bone drops,but more often they were observedat ossuariesafter Bearded Vultures had dropped bones. Over a 6-d-period, we observedgroups of one to sevenGriffon Vultures (• = 2.62, SD = 1.99, N = 21) visitingthe sites.During a total of 75 bone droppings,Griffon Vultures immediatelydescended to the ossuarieson 13 occasions(17.3 %) in numbersranging from one to five individuals (• = 2.30, SD = 1.63, N = 30). On five occasions,Griffon Vulturesattempted to pirate bone fragmentsfrom Bearded Vultures. Once, when an immature BeardedVulture wasdropping a bone, a Griffon Vulture flew in quicklyand ingestedsmall bone fragments next to the placewhere the impacthad occurredbefore the BeardedVulture could land. Twice,we observedgriffons trying to overtakeBearded Vultures in flight to recoverdropped bones,without success.Once, after a BeardedVulture had perched next to the bone it had dropped, three Griffon Vulturesattacked it and seizeda large bone fragment which they then proceeded to fight over and ingested.We also sawa Bearded Vulture drop a bone and, once on the ground, five Griffon Vultures attackedthe Bearded Vulture forcing it to flee with the prey. We alsoobserved three Griffon Vulturesinside a BeardedVulture nest that had been usedin the previousbreeding 288 LF.TrF. KS VOL. 31, No. 3 season.One of them spent 30 min pecking an old sheepor goat bone. Occupationof BeardedVulture nestsby Griffon Vulturesis frequentin the Pyrenees(Fernfindez and Donfizar1991, Bird Study 38:42-44; Donfizar, pers. obs.). Our observationssuggest that Griffon Vultures living near BeardedVultures benefit from this associationbecause BeardedVultures provide a sourceof calcium.The Griffon Vulture,like other speciesthat eat mainlysoft parts of carcasses(Brown 1976), is subjectto a lackof calciumbecause his diet containsonly 0.01% of thiselement (Houston 1978,J. Zool.Lond. 186:175-184). The BeardedVulture, a speciesthat in the lastcentury was spread over a largepart of the Iberian Peninsulamountains (Hiraldo et al. 1979,E1 quebrantahuesos Gypaetus barbatus (L.), Monogratias22, ICONA, Madrid, Spain),may have also facilitated the spreadof Griffon Vulturesas the distributionof both species wasextensively coincident in much of the southernPalaearctic (Elosegi 1989, ActaBiol. Mont. 3, S6riedocuments de Travail). We thank S. Mafiosafor manyhelpful suggestionsand criticismfor the manuscript'simprovement. We alsothank J.A. Donfizar,D.C. Houston and S.R. Wilbur for their reviewand commentson the manuscript.This studywas supportedby the Departamentd'Agricultura, Ramaderia i Pescade la Generalitatde Catalunya.--JoanBertran • and AntoniMargalida, GEPT (Grup d'Estudii Protecclodel Trencal6s),Ap. 43, E-25520E1 Pont de Suert(Lieida), Spain.

Presentaddress: Grases, 14-18. entlo la, 08004Barcelona, Spain.

J. RaptorRes. 31(3):288-289 ¸ 1997 The Raptor ResearchFoundation, Inc.

A GOLDEN EAGLE EATS WILD CANADA GOOSE EGGS

Golden Eagles (Aquila chrysaetos)prey primarily upon medium-sizedrodents, hares, birds and ungulates(S.IC Carnie 1954, Condor56:3-12; Boeker and Ray 1971, Condor73:463-467; M.N. Kochert 1972, M.S. thesis,Univ. of Idaho, Moscow,ID U.S.A.;P.A. Johnsgard 1990, Hawks,eagles, and falconsof North America,Smithsonian Inst. Press, Washington,DC U.S.A.). Althoughthey are known to exploit a greatvariety of prey itemsthroughout their holarctic range,eggs have not been reportedas a food item (A.C. Bent 1961,Life historiesof North Americanbirds of prey, Dover Publications, inc., New York, NY U.S.A.; Dement'ev and Gladkov 1966, Birds of the Soviet Union, Israel Programfor ScientificTranslations, Israel; Brown and Amadon1968, Eagles, hawks, and falconsof the world,County Life Books,London, UK; Beechamand Kochert1975, WilsonBull. 87:506-513; Matchett and O'Gara 1987,J. Raptor Res.21:85-94; Palmer 1988, Handbook of North American birds, Yale Univ. Press,New Haven, CT U.S.A.). We observeda Golden Eagleraid a CanadaGoose (Branta canadensis) nest and eat two eggson 4 April 1995 in Hell's CanyonNational RecreationArea in westernIdaho. At 1150 H, an adult Golden Eagle (genderunknown) flusheda CanadaGoose off a ground nest locatedon an islandin the SnakeRiver. The eaglelanded near the nest, walkedto the nest and broke open the eggsby graspingan egg in its foot and placingall of its weighton the egg until, after two to four attempts,it broke. The eagle ate the contentsof the egg (stageof embryonicdevelopment wasunknown) and then brokeand ate the secondegg. The pair of geesethat had been displacedfrom the nestand four other pairs of nearbygeese gave alarm calls during our observations,but never approachedthe eagle.Two Black-billedMagpies (Picapica) followedthe eagle to the nest and scavengedeggshell fragments while the eagle consumedthe contents.The eaglefinished eating both eggsat 1206 H and then spentthe next 5 min walkingand hopping around the island,possibly searching for more eggs.The magpiesfollowed the eagleon the ground until 1211 H when the eagle flew 50 m downstreamand perchedon a talusslope. The CanadaGoose pair returned to their depredatednest at 1430 H. AlthoughGolden Eagles have not been previouslyobserved eating eggs, we speculatethat depredationon goose eggsin Hell's Canyonmay not be uncommon.Perhaps Golden Eaglesin Hell's Canyoneat eggswhen more typical prey for this region (black-tailedjackrabbits, Lepus californicus) are rare. In contrast,Golden Eaglesnesting 128 km upstreamof Hell's Canyonin the SnakeRiver Birds of PreyNational ConservationArea, where black-tailedjackrabbits were abundantand an importantprey species(Steenhof and Kochert 1988,J. Anita.Ecol. 57:37-48), havenot been observedto prey upon gooseeggs, even though Canada Geese occasionally nest nearby (W. Bodie,pers. comm.). This studywas funded by the Idaho Power Company.Toni Holthuijzen, Mary McFadzen,Brian Herring and Mike Kochert made constructivecomments on the manuscript.Toni Holthuijzen translatedthe German papers.James