Status, Breeding Ecology and Conservation of the Imperial Eagle in Austria

The Imperial Eagle (Aquila heliaca, IE) reaches the western limit of its global breeding in range in Austria. As a result of targeted conservation measures in the Pannonian region, namely Hungary and Slovakia, the species repopulated Austria in 1999, nearly 190 years after its extirpation. Since then, a strong population increase took place and was monitored closely through different species conservation programmes of BirdLife Austria. Apart from the population monitoring, the programmes comprised measures to prevent illegal persecution and the disturbance of breeding territories, the satellite tagging of young IEs and the collection of all mortality data available. By 2023, the Austrian breeding population reached 42 territorial pairs. During the past 25 years, the species, which is classified as “Endangered” on the national Red List, has pushed its continuous breeding range a further 85 km to the west. In the years 2018–2020, a pair occurred even in the floodplain forest of the Danube near Linz but vanished afterwards. The province of Lower Austria, where the species has become established as late as 2008, hosts the majority of the national population today. IEs use different habitat types as nest sites in Austria: the floodplain forests of medium and big rivers (where the recolonization started in the late 1990s) are quite densely populated; apart from that, breeding occurs in open agricultural landscapes and in big forest in dry, hilly terrain of the “Weinviertel” region. Most nests are built on only a few tree species: Populus sp., Quercus sp. and Pinus sp.; other species (Robinia pseudoacacia, Salix sp.) are used only occasionally. Regarding the number of fledged young per pair and the failure rate of nesting attempts, no clear trend could be established but weather patterns in single years seem to affect the number. Especially when combined with human disturbance (agriculture, forestry, hunting, recreation), cold, wet conditions can lead to nest failure. Interestingly, this is reflected in the proportion of abandoned nests when clustered per habitat type. Nests in open, agricultural areas, where disturbance seems to be more frequent, have a lower success rate than those in floodplain or hill forest. According to observations and the collection of prey remains below nest sites, the most important prey species by far is Lepus europaeus. Where Spermophilus citellus and Cricetus cricetus still occur in relevant densities, they are hunted frequently. Birds, such as Phasianidae, Columbidae, Corvidae, or Alaudidae, also comprise a significant proportion of the IE’s diet. Especially in winter, but also during the breeding season, the species uses carrion, particularly adult hares. The satellite tagging activities revealed that illegal persecution is the most important mortality reason for young IE born in Austria. To tackle this problem, activities to fight illegal persecution were expanded and intensified in the frame of the PannonEagle LIFE (LIFE15/NAT/HU/000902) project together with the neighbouring countries. Other relevant mortality reasons are collisions with trains and train infrastructure and collisions with wind power plants. Especially the latter must be closely monitored regarding the necessary expansion of wind farms in the future. As side notes, the monitoring produced interesting findings about the breeding biology of the species e.g., the takeover of an active nest of Buteo buteo and subsequent hatching and adoption of the chick.