©Andras Kovacs/RaptorImages

The sacred bird

The imperial eagle was considered a sacred bird in Bulgaria because people believed that it chased storm clouds away, saving the harvest. Even today in some parts of the country people believe that if you kill or harm an imperial eagle, evil will come upon people. Its traditional name is ‘cross eagle’ – in flight the white patches on its wings together with the light-coloured head resemble a cross.

The imperial eagle is one of the rarest birds in Bulgaria and in the world. In the past, it was among the most widespread raptors in the country. Today its population numbers only 35 pairs. Most of them are found in Southeastern Bulgaria, with the highest concentration on Sakar Mountain.

© Sv. Spasov/Imperial eagle
© Sv. Spasov

Close to people

Imperial eagles prefer hills and plains with a mosaic structure of forests, groups of trees and open spaces – pastures, agricultural land and abandoned land. They nest on tall trees – individual or in groups, – along rivers, often in the immediate vicinity of human settlements, roads or agricultural lands.

© Sv. Spasov

Strongly attached to home

Imperial eagles are monogamous. The pair is strongly bound to their nest and some nesting territories have been occupied by imperial eagles for many years. The minimum distance between nests is 4700 m. The courtship displays of the species are very impressive. The two birds fly up in the air, then fold their wings and dive abruptly.

The nest is between 1.2 and 2.2 m in diameter and weighs up to 200 kilos. It is built by both partners from dry branches. Often a pair has more than one nest, using one as a primary nest, and the other(s) – as secondary. The nests are built mostly on hybrid poplar trees, different species of oak, and rarely on European red pine, beech and acacia trees.

In the second half of March and the beginning of April the female lays 2 eggs, and more rarely – 1 or 3. Although recorded, clutches of 4 eggs are an exception. The female does most of the incubation, and the male replaces her only when she needs to feed, or he brings food to the nest. The female feeds the chicks too, and the male is responsible for providing the food. The young leave the nest in the second half of July – beginning of August. For a while they return to the nest to roost or live in the area while their parents keep feeding them and teach them how to hunt.

The juvenile Imperial eagles remain in their natal territory until the second half of September – end of October. Then the young birds start migrating to reach Turkey, Israel and Syria. The adults are resident and spend the winter in Bulgaria.

© Andras Kovacs/RaptorImages/Imperial eagles

Eagle’s Menu

The main food source for the imperial eagle in Bulgaria is the hedgehog, followed by sousliks, hare, voles, etc. As an unspecialized raptor it has a diverse diet, including more than 150 species of animals. In winter, corvids and carrion form a larger portion of its diet. It would steal prey from other raptors too. This behaviour is particularly typical for young imperial eagles that do not have enough hunting experience.

© Bogdan Boev/Hedgehog
© Sv. Spasov/European ground squirrel

Restoration and sustainable management of pastures

In cooperation with farmers we work for the revival of traditional pasture livestock breeding through the creation of herds of livestock, the restoration of tilled pastures and the partial removal of scrub in order to restore the mosaic habitats.

All these actions are aimed at combatting the most serious threat for wildlife and for raptors in particular – the destruction of natural habitats. Many habitats are being reduced in area and fragmented at the same time.

Recently land owners and users have started to till former pastures that served as hunting grounds for the imperial eagle in order to turn them into intensive agriculture plots, vineyards and orchards. This leads to the destruction of dense, abundant souslik colonies, and to the removal of the natural grasses and scrub that provide shelter for many wild animals used as prey by the imperial eagle. The food sources for some pairs are destroyed completely and this may lead to them abandoning their territory.
© Sv. Spasov

Nearly homeless

A survey on land use in 24 nesting territories of the imperial eagle in Bulgaria reveals that the key grassy habitats for the species have dwindled by half. This is an alarming trend – in 2006 grassy habitats constituted 21.02 per cent of imperial eagle nesting territories, whereas in 2019 they constitute a mere 9.14 per cent of the area.

This drastic change is the result of agricultural policies and farming subsidies. Thus, due to omissions and discrepancies in the legislation, vast grassy areas that were used as extensive pastures for more than 20 years, are tilled and turned into arable land, even when the soil and climate are not suited for intensive agriculture.

Safe electricity pylons and poles

There are different ways to make power lines safe for the birds:
By mounting special insulation on potentially dangerous electricity pylons we can prevent the birds from being electrocuted when they touch the line and the pylon simultaneously.
The replacement of bare lines with fully insulated ones (the so-called PAS System) protects the birds from electrocution along the entire length of the overhead line.
Installing perches on transmission towers – they provide a safe perch for the birds away from the conductors.
Underground power lines to replace the overhead lines that are dangerous for the birds. This approach completely eliminates the risk of electrocution or collision for the birds and guarantees a reliable power supply for people.
Taking steps with regards to dangerous electricity poles and pylons is an exceptionally urgent measure for the conservation of the imperial eagle and of many other species of birds, as uninsulated power lines are the most common cause for the death of young birds. This was revealed by a satellite-tagging study on young imperial eagles – 67 per cent of the tagged birds were electrocuted after perching on dangerous electricity pylons and poles and touching the power line and the pylon simultaneously.

In Bulgaria, a great percentage of the power distribution lines pose a risk for birds, for large raptors and white storks in particular. Some of them are true death traps that kill dozens of birds every year. Imperial eagles often perch on electricity pylons and poles, particularly in open spaces without any trees. In some countries they even build their nests on electricity pylons and poles. Unfortunately, the birds cannot tell the difference between safe and unsafe pylons.

© BSPB/Installation of insulation on electric poles in the area of  Atanasovsko Lake
© Andras Kovacs/RaptorImages/Imperial eagle

Fight against wildlife crimes – fight against poisons

The only team with a specialized dog trained to find poisoned dead animals and baits in Bulgaria is the BSPB’s own anti-poison team. The team started operating in 2016 chiefly in the area of the Eastern Rhodope Mountains and Sakar Mountain but it responds to situations across the country. The task of Bars and his handler Nikolay is to search for carcasses of poisoned animals and for poisoned baits and to alert the respective authorities. After finding a carcass the handler determines what steps to take, as the animal could have died from natural causes. In case it has been poisoned, it must be removed from the wild as soon as possible; samples are sent to a laboratory to establish the cause of death.

Poisoned baits

The mass use of poisoned baits against predators in the 1950s was one of the reasons for the decline in the numbers of the imperial eagle. Another threat are pigeon keepers that use poisoned baits to eliminate the goshawk and the peregrine falcon – the nemesis of their pigeons. However, the poison can affect any raptor, and all raptors are protected by law. In 2011 one of our satellite-tagged imperial eagles was poisoned in the area of the town of Perushtitsa. The use of poisoned baits is strictly prohibited as it poses a risk not only to domestic animals and wildlife but to humans too. Unfortunately, the use of poisoned baits is still practiced.

© Marton Horvath / ММЕ

Nest guarding

The job position ‘nest guard for the imperial eagle’ appeared when we first started our work for the conservation of the species in Bulgaria. Nest guards are necessary because human disturbance often may result in failure of the breeding. Many of the trees with nests in the country are located in open spaces, close to arable lands. The eagles are used to the usual agricultural activities, such as the pasture of domestic herds and the tractor activities in the fields.

On Sakar Mountain there have been observations of a shepherd passing under the nest with their herd without a response from the incubating eagle on the tree. But the appearance of unfamiliar machinery, or of people with unfamiliar behaviour causes the eagles to abandon the nest, for example, a truck with workers, tourists, logging activities in the vicinity of the nest. When left unattended in the nest, the eggs may cool down or warm up too much, or they can be eaten by crows that are used to human presence.

Installing man-made nests

This is one of the most effective measures for the conservation of the imperial eagle, as the construction of a new nest is hard and requires a lot of energy, particularly for young pairs that breed for the first time. Before the installation a suitable tree and territory must be selected. The spot should be far from other pairs’ territories, dangerous power lines and sources of disturbance such as roads; it should provide enough food resources. Ideally, there would have been observations of young wandering birds. The tree needs to be tall and strong, so the pair can safely observe their surroundings and fly off with ease.

The actual installation of a nest takes about a day. The base is constructed in advance – a round shape made of branches that is hoisted up the tree using climbing gear. Once it is on the top of the tree, more branches and twigs are put on the nest so that it becomes sturdy and stable. A pair can use a nest for decades.

Installing man-made nests

Satellite tagging

So far the BSPB has tagged 28 young birds through the programme for satellite tagging of the imperial eagle. Our aim was to learn more about the biology of the species and to establish the causes for its high mortality rates. Thanks to the satellite telemetry now we know a lot more about the places the eagles live at, about the wandering and migration of the young birds, as well as for the threats and dangers they face along the way.

The transmitters are fitted on the young eagles just before they leave the nest, when they have already reached the size and strength of adult birds. The satellite transmitter is a small device with GPS and an antenna that is fitted on the back of the bird via specially designed straps. It weighs only 70 g so the bird gets used to it and can carry it all its life. The device has a solar panel to charge its battery so it has a long life span.

You can also follow the movement of the eagles on the map of the interactive module for satellite tracking.

© Dimitar Gradinarov
© Sv. Spasov/Placing rings on an Imperial eagle