Study system
The study sites are located in the Hortobágy National Park, East Hungary (Figure 1). Hortobágy is one of the largest continuous open landscape in Europe where alkaline grasslands cover approximately 80 000 hectares. The climate is temperate continental, with a mean annual temperature of 9.5 ºC and mean annual precipitation of 550 mm (Fick & Hijmans 2017). The soil reference group of the study area is classified as Vertic Solonetz, characterised by high salt content (IUSS Working Group 2015). Hortobágy is a lowland plain area, the elevation ranges between 87 and 110 m above sea level. The landscape is characterised by a high diversity of dry grassland, wet meadow and marsh habitats as well as a small amount of croplands. Alkaline grasslands, meadows and marshes are included in the Annex I of the Habitats Directive (92/43/EEC) as “Pannonic salt steppes and salt marshes (1530)”, a priority habitat type of the European Union. The most widespread dry grassland type isAchilleo setaceae – Festucetum pseudovinae , which are short, dry grasslands that occur on moderately alkaline soil and are managed by extensive grazing. The dominant grass species covering 50-80% of the vegetation is Festuca pseudovina , characteristic forb species include Achillea collina , A. setacea , Plantago lanceolata , Podospermum canum and Trifolium spp. (Deák et al. 2014).
The Eurasian crane (Grus grus ) is a large-bodied bird species with a Palaearctic distribution. Its global population is currently strongly increasing and estimated as approximately 500 000 individuals (Wetlands International 2015). It is a protected bird species in Hungary and across the European Union, being a bird of community interest and listed in the Annex I of the Birds Directive (2009/147/EC). Hortobágy is the largest stopover area during Baltic-Hungarian Flyway, connecting the eastern part of Scandinavia, western Russia, the Baltic states and Tunisia, crossing Poland, Hungary and the Balkan Peninsula. During autumn migration, the highest crane numbers can be observed between mid-September and late November. Since 2000, the total number of cranes found simultaneously roosting at the Hortobágy National Park has exceeded 150,000 individuals several times and averaged over 130,000 birds (database of the Hortobágy National Park Directorate). The peak number of cranes recorded at the Hortobágy National Park were 158 740 in the autumn 2019 and 95 400 in 2020 (www.hnp.hu).
While staying in the Hortobágy, the cranes spend most part of daytime in foraging on croplands, especially in maize stubbles, whereas they occasionally also utilize grasslands as feeding habitats (Végvári 2002). They spend the nights on fishponds where they are more protected from predators. In grasslands, cranes display a special feeding habit that is called as ‘crane-ploughing’. This means that the birds search for invertebrates in the grasslands (Anteau et al., 2011) and during this activity they heavily disturb the soil surface using their bills. The disturbed soil surface resembles a ploughed area (Figure 1). The area of ploughs typically ranges between 10-100 m2, but occasionally, larger ploughings also occur.