Species, Imperial Eagle

The imperial eagle is among the rarest birds in Bulgaria and worldwide. It had been among the most widespread raptors in Bulgaria but today its population numbers only 23 pairs. Most of these have remained in South-eastern Bulgaria and occur in highest numbers in Sakar.

The Imperial eagle has for centuries been considered a sacred bird, because our ancestors had thought that it drives away storm clouds and saves the crops. Even today it is believed in some parts of Bulgaria that killing or harming an Imperial eagle will bring serious disaster upon people. Its popular name is the 'cross eagle' because of the epaulet-like white spots on its wings which are visible in flight.    

There are many reasons for the drastic reduction of the species, but the mass-scale killing of raptors in the 50-70s period of the past century, habitat change and intensive agriculture are among the main causes leading to the reduction and disappearance of the wild animals upon which the Imperial eagle feeds.  Improper use of pesticides in agriculture, illegal poison baiting and poaching still cause the death of many eagles.  A new threat to the Imperial eagle has come about recently, a threat of which no evidence for it had been present.  Satellite tracking of juvenile Imperial eagles allowed the BSPB to prove that a substantial number of the young birds die from electrocution, when attempting to perch on power grid poles without safeguards. 78% of the satellite tagged eagles whose destiny is known, have died of electrocution in Bulgaria and Turkey.

DESCRIPTION OF THE SPECIES

The Imperial eagle (Aquila heliaca) is among Bulgaria's largest eagles, slightly smaller than the Golden eagle (Aquila chrysaetos). Its body size varies between 72 and 84 cm, and its wingspan is 180 to 215 cm. The adult birds are dark brown, almost black, with a very characteristic golden colour at the rear part of the head and neck. Usually, there are two white spots on the shoulders, of varying size, which may, in some individuals, be fully absent. The tail feathers are mainly yellowish-gray in colour.

The juvenile birds have brown feathers, ochre in colour, which gives them their specific light brown appearance. The young birds' flight feathers are uniformly dark. Adult feather pattern is reached only after the 6th year.

Concerning the intra-species systematic arrangement of the Imperial eagle (Aquila heliaca Savigny,1809), opinions vary. One opinion is that the species represents two sub-species - the Eastern Imperial eagle (Aquila heliaca heliaca Savigny, 1809) and the Western (Spanish or Pyrenean) Imperial eagle (Aquila heliaca adalberti Brehm, 1861). Other authors say that A. heliaca adalberti should be considered a separate species, Aquila adalberti.

THE BIOLOGY

The Imperial eagle prefers rolling terrains and plains where forested areas or groups of trees alternate with open spaces such as pastures, agricultural lands and fallow lands. The Imperial eagle nests on single or in groups of tall trees growing alongside rivers, frequently in the immediate proximity to settlements, roads and arable lands. In the past, the species had nested in orchards as well. Its preferred altitude varies between 25 and 1,230 m.

The Imperial eagle is a monogamous species. The pair is strongly attached to its nest and some nesting areas have been occupied by Imperial eagles for many years. The minimal distance (established using GPS) between the nests of various pairs is 4,700 m. The mating displays begin in February and are most intensive in March. They are very beautiful - the birds rise in the air and dive steeply with folded wings.

The nest is 7 to 26 m above the ground. Both birds participate in its construction, using dry branches. The home of the eagle reaches 1.2 to 2.2 meters. It is 30 to 90 cm thick and can weigh as much as 200 kilos. Often, the pairs have more than one nest, using one and keeping the others as spare. The main trees used for nesting are hybrid poplars (Populus sp.) followed by various oak species (Quercus frainetto, Quercus pubescens, Quercus cerris, Quercus petraea). Less often, the nests are built on Scots pine (Pinus silvestris), European beech (Fagus sylvatica) and gumarabic acacia (Robinia pseudoacacia).

Usually, the female lays 2 eggs, or, less often, 1 or 3 eggs, in the second half of March or in early April. Nests with 4 eggs have been known, although they are exceptional. Mainly the female sits on the eggs, replaced by the male to feed, or having food brought to the nest by the male. The incubation period is around 43 days long. The young are fed mainly by the female, while the male is responsible for delivering the food. The young leave the nest after the second half of July and early August. For some time they return to the nest to spend the night or stay near-by, still fed and trained to hunt by their parents.

The birds remain in their nest region until the second half of September - the end of October. After that, the young birds migrate, usually reaching as far as Turkey, Israel, or Syria. One of the juvenile birds with a satellite transmitter was followed during the project as far as the Sudan. The adult birds stay and winter here permanently, because they are experienced hunters, unlike the juveniles, and are capable of feeding even during the cold months.

The migration of the East European Imperial eagle was studied for the first time during the LIFE+ Project "Save the Raptors". Using satellite tracking of 22 juvenile Imperial eagles, we know now far more about the birds' wintering places, about their temporary quarters and about their lives.

The Imperial eagle hunts in open areas such as pastures, meadows, and bare hills. Most often, the bird surveys, for hours in some cases, its hunting territory from a suitable observation point. It swoops on any prey flying precipitously low. In areas with abundance of sousliks, the eagles fly low, several centimeters above the area most densely occupied by these rodents. In some cases the Imperial eagles may be observed perching next to the entrances toward the shelters of their prey.

The main food of the Imperial eagle in Bulgaria is the hedgehog (Erinaceus roumanicus). It is followed by the souslik (Spermophilus citellus), the Hare (Lepus europaeus), the shrews (Microtus sp.), the White stork (Ciconia ciconia) etc. Not being a specialized predator, this bird has a varied menu including more than 150 animal species. In the winter, the share of crows and carrion increases. Sometimes the Imperial eagle takes the prey of other raptors. Such behavior is particularly typical of the young Imperial eagles who are not sufficiently experienced as hunters.