Ibis 142:139-142

Predators a s prey at a Golden Eagle Aquila chrysaetos eyrie

in Mongolia DAVID H. ELLIS1*, P. TSENGEG2, PETER WHITLOCK3 &

MERLIN H. ELLIS" 'uses Patuxent Wildlife Researcfi Center, NCR 1 Box 4420,

Oracle, AZ 85623 USA ^307 Sweet Avenue No. 7, fvloscow, ID 83843 USA

3PO Box 325, Eastfiam, fAA 02642, USA "Institute for Raptor Studies, HCR 1 Box 4420, Oracle,

AZ 85623, USA

Although Golden Eagles Aquila chrysaetos have for decades been known occasionally to take large or danger­ous quarry, the capturing of such was generally believed to be rare and/or the act of starved birds, principally in winter, when prey as large as adult Pronghorn Antelope Antilocapra americana are taken (Bruns 1970, Deblinger & Alldredge 1996]. Trained eagles are known regularly to capture foxes Vulpes sp. [Yrsaliev 1962) and even occasionally Wolves Canis lupus [Masefield 1914], but we know of no sizeable study wherein w i l d eagles preyed to any significant extent on adult foxes or other medium-sized mammalian carnivores (see review in Watson 1997]. The same is true for medium-sized avian predators: Golden Eagles occasionally capture them [Sherrod 1978], but to no great extent. In a review of Golden Eagle prey in North America, Olendorff [1976] tabulated 7094 items. O f this number, 54 [<1%] were mammalian predators of Domestic Cat Felis domesticus size or larger. Avian predators were even less frequently represented: ten medium to large sized hawks [< 1%). Further, some of the predators on Olendorff's prey list could have been young and therefore more easily taken. Collopy [1983] reported two Coyotes Cants latrans among 463 prey items from Idaho, but both were pups.

This report provides details of an exceptional diet at a Golden Eagle eyrie in eastern Mongolia w i t h unquantified notes on the occurrence of foxes at other eyries in Mongolia. The eyrie in question lies 4 5 - 4 6 ° N and 112-113°E at about 1020 m elevation in a dry steppe cut by numerous canyons, each about 30 m deep. The eyrie was one of two stick nests and was about 12 m up on a near-vertical, west-facing cliff about 25 m tall.

The nest had one young [age about 49 da.ys] and an assortment of unusual prey [Fig. 1, Tables 1 & 2] when we found i t on 1 July 1995. We revisited the site on 16-17 June 1997 and found that the nest used in 1995 had

"Corresponding author. david_h_ellis@usgs. gov

recently fallen. Although the other nest on the cliff had been recently bui l t upon and a new foundation had been laid at the 1995 site, we found no evidence that eggs had been laid in 1997. Many moulted eagle feathers were present in both years, but we saw only one adult in 1995 and none in 1997. The collapse of the 1995 nest gave us an unexpected opportunity to sort through the entire stick matrix, a rubble heap consisting of strata represent­ing 4-6 breeding years.

The identification of avian skeletal remains was by Storrs Olson, US National Museum [ U S N M ] . Robert Fisher [ U S N M ] assisted w i t h identification of mammalian remains. Prey were identified in 1995 by gross morph­ology [Fig. 1]. Two birds [i.e. the Litt le O w l Athene noctua and Pallas' Sandgrouse Syrrhaptes paradoxus) were identi­fied from feathers only. Except for the Daurian Hedgehog

Figure 1. Golden Eaglet, 1 July 1995, remains of unusual prey, (a) Photograph; (b) drawing with prey Identified. A, Upland Buzzard wing; B, gazelle leg; 0, Eagle Owl legs; D, gazelle pelvis and vertebrae; E, Raven head; F, fox skulls, G, crane leg; and H, Upland Buzzard leg.

© 2000 British Ornithologists' Union

140 Short communications

Table 1. Avian prey from a Golden Eagle eyrie in eastern Mongolia.

Prey On nest rim, 1995 From disassembled nest, 1997 Totals (minima)

Tadorna ferruginea (Ruddy Shelduck) 3 3 Tadorna tadorna (Common Stielduck) 1 1 Sufeo hemiiasius (Upland Buzzard) 1 1 -t-Anthropoides virgo (Demoiselle Crane) 1 " 3 3 Chiamydotis unduiata (Houbara Bustard) 1 1 Larus argentatus (Herring Guli) 1 _ 1 1 Syrrhaptes paradoxus (Paiias' Sandgrouse) i 1 Bubo bubo (Eagle Owi) - g - 2 Athene noctua (Little Owl) 1 1 Cofvus corax (Common Raven) 1 i xgv 1 ,

Total birds 7 12 15

Erinaceus dauricus spines and the dehydrated Mongolian Gazelle Procapra gutturosa legs, all of the 1997 identifica­tions were from bones compared either w i t h specimens found dead in Mongolia or on deposit at the U S N M . For some prey, identification to species was by comparing known distribution using Reading et al. (1994).

The prey listed in Tables 1 and 2 provide some surprises. O n our single visit i n 1995, most of the prey we recorded were unusual. Ravens Corvus sp. are seldom recorded in Golden Eagle eyries. They appeared only twice in OlendorfPs (1976] list. Although Golden Eagles regularly hunt Demoiselle Cranes Anthropoides virgo on migration over the Himalayas (Thiollay 1979] and they frequently attacked migrating experimental cranes in the Rocky Mountains and Arizona (Ellis et al. in press], we know of no N o r t h American study and only t w o European studies (see Table 9 in Watson 1997] where cranes of any species numerically constitute more than 5% of the prey during the nesting season. I t was more

surprising to see such an accumulation of predators and large prey on the nest top at one time (Fig. 1]. This included one Upland Buzzard Buteo hemiiasius, three owls, three foxes, and an adult and calf of Mongolian Gazelle. The number of unusual prey recovered from the nest matrix in 1997 was even more remarkable.

Although no study w i t h a large sample size has shown fox-size predators important in Golden Eagle diet (Watson 1997], young ungulates are sometimes impor­tant, so our observation of gazelle calves was no great surprise. McGahan (1967] reported 24 (3%] Mule Deer Odocoileus hemionus fawns among 702 prey items for breeding Golden Eagles in Montana. Our value for gazelle calves (10 m i n i m u m count, 1997], however, represents 13% of 78 prey items and at least one adult was also present.

I t must be stated that the high proportion of large or predatory prey reported herein (Tables 1 & 2] is yet more surprising owing to the propensity of Golden Eagles to

Table 2. Mammalian prey from a Golden Eagle eyrie in eastern Mongolia.

From disassembled On nest rim, 1995 nest, 1997 Totals (minima)

Prey Adult Juv. Neonatal Adult Juv. Neonatal Adult Juv. Neonatal

Erinaceus dauricus (Daurian Hedgehog) V 1 Lepus toiai (Tola! Hare) 4 14 1 14 1 Marmota sibirica (Siberian Marmot) 2 3 1 3 1 Vuipes vulpes (Red Fox) 2 9 2 9 2 Vuipes corsac (Corsac Fox) 1 10 6 10 6 Musteia eversmanni (Siberian Polecat) 1 1 2 = Fells manui (Pallas' Gat) 2 1 2 1 Procapra gutturosa (Mongolian Gazelle) 1- 1 1 103 1 10

Total mammals 11 1 41 11 10 42 11 10

'Several clusters of spines present, probably representing several individuals. Two counted because one skull was removed In 1995. 3These calves were represented by 12 legs Identified as gazelle and 10 brain case lids photographed, then lost during preparation but judged (by comparison of photos with crania at US Natural History Museum) to be Procapra gutturosa rather than similar-sized Domestic Sheep Ovis aries lambs.

© 2000 British Ornithologists' Union, Ibis, 142, 139

Short communications 141

remove the uneaten remains of larger prey items that are more likely to draw sarcophagic insects. If this pair of eagles was at all normal in nest sanitation, then the number of foxes, polecats, cats and gazelle that we report is a small sample of those actually taken. Furthermore, our method of combing completely through the stick matrix should have allowed us ample opportunity to recover the skeletal remains of small prey (i.e. those which are less likely to be removed for nest sanitation purposes) had they been present. However, the Sandgrouse and the Little O w l , both detected by feathers in 1995, did not appear as skeletal remains in the fallen nest. Clearly, smaller prey, for which legs and bones are largely consumed, were under-counted in 1997. Finally, although our method of sifting through the remains of the nest matrix is acceptable for fallen nests, we very much discourage the removal of any nest for this purpose.

Some numerical comparisons are of interest. Our total of only 15 Tolai Hares Lepus tolai and four Siberian Marmots Marmota sibirica compared w i t h 27 foxes probably suggests not so much a preference for foxes, but rather that populations of more normal prey were depressed at this site. Indeed the whole region surround­ing the eyrie was heavily overgrazed and marmot burrows were noticeably lacking. Finally, at about 3 kg average adult mass (Silva & Downing 1995), the Corsac Fox Vulpes corsac is little above the size range normally taken by Golden Eagles. However, Red Foxes (approximate average mass 4.5-9 kg; Silva & Downing 1995] occurred nearly as frequently as the Corsac and was better evidence that the adults at this eyrie were either desperate for prey or had a propensity for larger prey. Interestingly, the majority of both species of foxes (70%) were adults (by dentition], and none was a pup. A t about 20 other eagle nests in Mongolia, we found that, as expected, the normal diet was marmots and hares. However, we did observe foxes in two other nests (five foxes, all Corsac] and, inter­estingly, both of these were in the same region as the eyrie detailed here. Viewed another way, we have observed several times more foxes as prey in this one eyrie than in all other Golden Eagle eyries in Mongolia combined. O f course, the numbers would change i f the other eyries were similarly disassembled.

That this pair was forced to exploit an untraditional feeding niche is further supported by the presence of the other unusual prey. Besides the foxes, an additional five predatory mammals brings this total to 4 1 % . The unusual birds, including raptors (four), raven (one) and cranes (three), comprise 10% of the diet. When we add the 11 gazelle (14%), the total for unusual prey was 51 (65%) at this eyrie. Most of the prey we identified were larger than the 0.5-2.0 kg range proposed by Watson (1997) as comprising the opt imum foraging size for this eagle.

A n adaptability to local prey conditions characterizes the species. In one environment. Golden Eagles subsist largely on Hedgehogs Erinaceus europaeus (Zastrov 1946;

Hogstrom & Wiss 1992], in another on tortoises [Testudinidae) (Grubac 1987] and in another on hawks and herons (Spofford 1969, 1971). It is not perhaps surprising that individuals of the largest race of Golden Eagle are able to survive where their normal diet is preda­tors as large as themselves and ungulates even larger. These observations lend credence to the unquantified observation by Andrews (1932) that Golden Eagles frequently take young and adult gazelle in south-central Mongolia.

Data were gathered peripheral to other ecological studies financed by the USGS Patuxent Wildlife Research Center, NASA-Goddard Space Flight Center, Mr Howell Wynne, and an anonymous philanthropist, with supplemental support from the National Avian Research Center (United Arab Emirates] and the Institute for Raptor Studies.

REFERENCES

Andrews, R.C. 1932. Central Asiatic Expeditions. Vol. 1. The New Conquest of Central Asia: a Narrative of the Explorations of the Central Asiatic Expeditions In fVlongolia and China, 1921-1930: 124, 170-171. New York: American Museum of Natural History.

Bruns, E.H. 1970. Winter predation of Golden Eagles and coyotes on pronghorn antelopes. Can. Field-Nat. 84: 301-304.

Collopy, M.W. 1983. A comparison of direct observations and col­lections of prey remains in determining the diet of Golden Eagles. J. Wlldl. h^anage. 47: 360-368.

Deblinger, R.D. & Alldredge, A.W. 1996. Golden Eagle predation on pronghorns In Wyoming's Great Divide Basin. J. Raptor Res. 30: 157-159.

Ellis, D.H., Clegg, K.R., Lewis, J.C. & Spaulding, E. In press. Golden Eagle predation on experimental Sandhill and Whooping Cranes. Condor.

Grubaq R. 1987. L'Algle Royal en Macedone. In Michel, 8. (ed.) LAIgle Royal en Europe: 37-39. Arvleux: Actes de Premier Col­logue International.

Hogstrom, S. & Wiss, L.-E. 1992. Diet of the Golden Eagle Aquila chrysaetos (L.) In Gotland, Sweden during the breeding season. Ornis Fenn. 69: 39-44.

Masefield, J. 1914. The Travels of fVlarco Polo the Venetian. New York: E.R Dutton.

McGahan, J. 1967. Quantified estimates of predation by a Golden Eagle population. J. Wlldl. Manage. 31 : 496-501.

Olendorff, R.R. 1976. The food habits of North American Golden Eagles. Am. Midland Nat. 95: 231-236.

Reading, R.P., Sumya, D., Samya, R., & Batsalhan, N., eds 1994. Dictionary of the Vertebrate Species of Mongolia. Ulaanbaatar: Mongolia Biodiversity Project.

Sherrod, S.K. 1978. Diets of North American Falconlformes. Raptor Res. 12: 49-121.

Silva, M, & Downing, J.A. 1995. CRC Handbook of Mammalian Body Masses. London: CRC Press.

Spofford, W.R. 1969. Golden Eagles In the United States. In Mickey, J.J. (ed.) Peregrine Falcon Populations: Their Biology and Decline: 528. Madison: University of Wisconsin Press.

Spofford, W.R. 1971. The breeding status of the Golden Eagle In the Appalachians. Am. Birds 25: 3-7.

Thiollay, J.-M. 1979. La migration des grues a travers I'himalaya et la predation par les algles royaux. Alauda 47: 83-92.

© 2000 British Ornithologists' Union, Ibis. 142, 139

142 Short communications

Watson, J. 1997. Tiie Golden Eagle. London: T. & A.D. Poyser. Yrsaliev, D. 1962. Materialy po biologii berkuta. [Data on the biolo­

gy of the Golden Eagle.] Izv. Akad. Nauk KIrg. SSR Ser. Blot. NaukA: 83-87.

Zastrov, M. 1946. Om Kungsornens Aquila chrysaetos ut bredning och biologi I Estland. VSr Figelvarld 5: 64-80.

Received 21 April 1998; revision accepted 30 November 1998