Nest sites and reproductive success

Five nests were located in cliffs, three in rock columns, two in sandy precipices and one in a rock ledge. These nest sites were similar to Eagle Owl nests in similar habitat in Kazakhstan (Karyakin et al. 2007, Pazhenkov & Korzhev 2006) but differed from those in forest and taiga habitats northeastern Kazakhstan (Karyakin et al. 2005a) where nests were on the ground at the base of pine trees, and human disturbance was a major cause of breeding failure.

Nest sites consisted of natural large cavities, holes, or crevices that had 2–4 m tall bushes (Amygdalus pedinculata) at the entrance and were in the proximity of a lake or other water source. Nest sites were located among small broken rocks or on rock ledges on mountain slopes and rocky mountain outcrops, or in sand cliffs on riverbanks (Fig. 2). Mean height (above ground) of nest site structure (e.g. cliff faces) was 7.6 (±5.9 m, 0.5–20) and height of nest entrance was 3.1 (±2.5 m, 0–7). Nest cavities were well sheltered from the elements and close to nests of other steppe birds including the Upland Buzzard Buteo hemilasius, Redbilled Chough Pyrrhocorax pyrrhocorax, and Northern Raven Corvus corax. Three breeding pairs had 2–5 potential nest sites 5–20 m from each other within their breeding territory. Many breeding pairs occupied nests for more than one year, a habit reported for this species in Kazakhstan (Karyakin & Barabashin 2006).

Eggs were laid in a scrape on sandy and gravel substrate. Average clutch size was 1.7 (±0.9, 1–3) and brood size was 1.5 (±0.5, 1–2), smaller than those reported in Kazakhstan (Pazhenkov & Korzhev 2006, Karyakin et al. 2007). Nest structure or entrance height was not correlated with the number of eggs laid (r = 0.1, r = 0.4) or young fledged (r = 0.001, r = 0.1). We suspect that variation in the reproductive success of the Eagle Owl was related to prey availability. A more detailed study is required of prey availability vs. prey taken to understand this relationship.

Prey use

We identified 276 prey remains from pellets and at 11 Eagle Owl nests (Appendix 1). Air-dried pellets averaged 63.5 ± 14.2 by 22.2 ± 4.1 mm and weighed 10.61 ± 4.6 g. Of the 276 prey identified, 72.1% were mammals, 26.1% birds, 0.7% amphibian, and 1.1% beetles. These results are consistent with Eagle Owl diet studies in Russia (Dementiev & Gladkov 1954) where it rarely eats fish, amphibians or invertebrates.

The most frequent mammalian prey identified were the Mongolian Five-toed Jerboa Allactaga sibirica, Campbell's Hamster Phodopus campbelli, and the Daurian Hedgehog Mesechinus dauricus (Appendix 1). We reason that the relatively high percentage of Mongolian Five-toed Jerboa and Campbell's Hamster in the diet was because they are active at night when Eagle Owls are hunting.

The predominant avian prey identified were Common Sparrowhawk Accipiter nisus, Saker Falcon Falco cherrug, Little Owl Athena noctua and Chough (Appendix 1). Most (65%) of the avian prey species were observed to breed in the nesting territories of Eagle Owl (unpubl. data), whereas 35% were passage migrants, and included Common Sparrowhawk, White Thrush Zoothera dauma, and Eye-browed Thrush Turdus obscurus that were observed to perch (and roost) on bushes near Eagle Owl nest sites — probably the reason of high numbers in the owl diet.

Figure 2.

Nesting habitat of the Eagle Owl in Mongolia (photo by S. Gombobaatar).

Prey biomass and size

The average weight of bird prey was 411.9 g (range 15–1500, n = 18) and 371.8 g (range 17–2000, n = 13) for mammals (Appendix 1). The heaviest (Goshawk) and lightest (Flycatcher) avian prey species were captured while migrating through Eagle Owl territories. The Mongolian Marmot Marmota sibirica and Tolai Hare Lepus tolai were the heaviest mammal prey identified. Weight of the most frequent prey species in the diet varied from 15 to 300 grams for birds and from 17 to 200 g for mammals (Appendix 1). Average length of prey species was 336.1 ± 176 mm (140–700) for birds and 252.8 ± 161 mm (102– 630) for mammals, whereas the most frequent length of prey species was 101–400 mm. There was no difference between the body mass (t22 = 2.07, P = 0.1) and between the body length (t27 = 2.1, P = 0.2) of bird and mammal prey identified.

We conclude that prey identified from pellets over-represented mammalian prey and under-represented avian prey whereas prey remains over-represented large prey and under-represented small prey (Appendix 1). Combining pellet and prey remains data did not eliminate these biases. We suggest that pellets are still useful for estimating prey diversity but direct observations are necessary to help quantify the aforementioned diet estimate bias. Eagle Owls in our Mongolia study capture comparatively lightweight birds and mammals that are easily accessible to breeding pairs. This concurs with Karyakin et al. (2005b) and Kondratenko & Tovpinets (2001) who state that in the Altay Kray, Russia, the predominant prey species were small and medium-sized rodents, including the Water Vole Arvicola terrestris; the most frequent large mammal prey was the Tolai Hare. The variety of raptors included in the diet of Eagle Owl was surprisingly high (Appendix 1). Other Eagle Owl diet studies have recorded more diverse diets, e.g. tree bats, colubrid snakes, batrachians in Ramanujam (2001), and domestic chicken Gallus domesticus in Serrano (2000). This variation emphasizes the opportunistic and generalist predatory nature of the Eagle Owl across its range.