Habitat alteration enables hybridisation between Lesser Spotted and Greater Spotted Eagles in north-east Poland

Abstract

SummaryWetlands in the Biebrza Valley, north-east Poland, are inhabited by two closely related Aquila species: the more numerous Lesser Spotted Eagle A. pomarina prefers human-transformed landscapes, whereas the very rare Greater Spotted Eagle A. clanga is associated with natural marshy landscapes. At least since the last decade of the 20th century, these two species have been known to hybridise in the broad zone of their sympatric occurrence in Europe. The aim of the present study was to compare habitat preferences of both spotted eagle species in order to detect which environmental factors could increase the probability of hybridisation. We analysed nesting and hunting habitats for 148 breeding territories (61 of A. pomarina, 56 of A. clanga and 31 of mixed pairs). As expected, the presence of breeding Greater Spotted Eagles was associated with non-transformed marshy landscapes, whereas Lesser Spotted Eagles clearly preferred human-transformed areas. We hypothesised that mixed pairs should occur in intermediate habitat, confirming this assumption by analysing several variables: distance to human settlements, distance to open areas, and proportion of wetlands, shrublands, grasslands, agricultural mosaic and arable land. Results of this study suggest that some landscape changes can enable two species with different habitat requirements to inhabit the same area and hybridise. This scenario has potential conservation implications for the rarer species, Greater Spotted Eagle, which has narrower habitat preferences.

Monitoring of spotted eagles in Estonia in 1994–2014: Stability of the lesser spotted eagle (Aquila pomarina) and decline of the greater spotted eagle (A. clanga)

“…The greater spotted eagle has decreased all over its European range (BirdLife International 2015) and nowadays it is one of the most threatened raptors on the continent. These reasons, accompanied with the alteration of greater spotted habitats (Maciorowski & Mirski 2014) and flexibility in habitat use by the lesser spotted eagle , have led to the extensive hybridisation of the two species (Väli et al 2010a). Mixed pairs produce fertile offspring, which, at least nowadays, form a natural part of the population in the hybrid zone (Väli 2010, Väli et al 2010a).…”

Breeding habitats and long-term population numbers of two sympatric raptors—Red Kite Milvus milvus and Black Kite M. migrans— in the mosaic-like landscape of western Poland

“…Population declines are also related to the loss of suitable wintering and breeding grounds (Moreau 2009;Maciorowski et al 2019a). The latter are heavily influenced by habitat conditions, particularly by the quality of foraging areas and the availability of suitable nesting sites (Maciorowski and Mirski 2014;Maciorowski et al 2019a). The strong tendency to cultivate tall crops (mainly maize Zea mays and rapeseed Brassica napus) as large-area monocultures, with the concomitant, dramatic shrinkage of meadows, wetlands and various types of ruderal vegetation, has markedly reduced food availability for raptors.…”

The importance of wetlands for the Greater Spotted Eagle Clanga clanga wintering in the Mediterranean Basin

“…In this study we conducted such an analysis for the Greater Spotted Eagle Clanga clanga (hereafter GSE), which is extremely rare in Europe, and in its whole range classified as ‘Vulnerable’ (BirdLife International 2017b). In its European nesting grounds, extending from eastern Poland to the Ural Mountains, GSE is closely associated with the distribution of various types of marshes, primarily semi-natural vast fens and transitional moors (Ivanovsky 1996, Lõhmus and Väli 2005, Dombrovski 2005, 2012, Mischenko and Melnikov 2013, Maciorowski and Mirski 2014, Maciorowski et al 2015a). In these areas its main prey includes various wetland bird species (ducks, waders, rails), rodents (e.g.…”

“…water vole Arvicola amphibius ) and amphibians (Graszyński et al 2002, Väli and Lõhmus 2002, Dombrovski and Ivanovski 2005, Maciorowski et al 2005, Dombrovski 2010). Drainage of wetlands has been reported as a major cause of their decline and indirectly the reduction of population size may accelerate hybridisation with the Lesser Spotted Eagle Clanga pomarina (Lõhmus and Väli 2005, Maciorowski and Mirski 2014, Maciorowski et al 2015a).…”