First terns in Zagreb County area have started to come back from their wintering sites in Africa. Even a tern ringed last year in Israel came to us! In Zagreb County area of nests are found on the Siromaja Lake (Rugvica) and on the island of Rakitje (Sveta Nedelja) where the biggest colony of terns in Croatia has been nesting for years. The 25 pairs of birds are already on Siromaja lake platform, and the first eggs are hatched. There are currently about 20 pairs of birds on the Rakitja Island, and most colonies of about hundred birds are expected back in these days.
When they arrive, readings on moving follow. Before moving to Africa last year, about twenty birds had satellite transmitters that kept tracking on where they were flying. Except satellite transmitters, a large number of birds were also marked with colour rings, and some of them with small tracking devices, geolocations. Thanks to the ring, we know that some of the birds from Israel are coming to nest here!
Common terns are birds which biology is linked to life by water. They are nesting in the colonies on the river sand bars and feed with small fish and invertebrates that live in water. Terns have light-gravy to almost white feathers, they are the size of a pigeon, and their beak and legs are red. Terns are migratory birds and incredible long-distance fliers, on the way to Africa they fly up to 10,000 miles. In the spring, they usually return to the same nesting grounds where they hatch eggs on pebble islands.
As part of the Interreg Si-Hr, project ČIGRA (tern) is financed by the European Regional Development Fund, the Green Ring of Zagreb County with partners that has provided favourable conditions for terns nesting after their return from Africa. Tern nest in spring on the sand banks of large rivers like Sava. The natural islands of Sava are no longer suitable for terns due to frequent flooding and changing of terrain due to gravel extraction, construction of hydroelectric power plants (Slovenia) and building banks against flooding. Therefore, one of the project activities is to arrange islets on Rakitja and platform building. On the island on Finzula Lake near Rakitja, 400 m2 of geotextiles and 50 m3 of gravel were set up to halt (or even slow down) growth of the vegetation, which would make island suitable for terns nesting for the long time on Siromaja lake near Narta Island (Rugvica) as a replacement space for terns nesting.
The platform is made of wooden constructional elements tied to metal clutches lying on 12 plastic pontoons and is anchored at the central part of the lake. On the surface of 65m2, 8 tons of gravel is implanted to mimic the natural habitat, preferably for terns nesting. Beside gravel, platforms have a fence so that young birds do not run out and at the same time deter predators. In one corner, there is a shelter for bird watching, and small wooden shades are placed across the entire surface so that youngsters can hide from the potential predators and direct sun. The platform on Siromaja lake is the first of its kind in entire region. We hope that in the coming years it will be home to numerous young terns from the Sava River.
The idea of the project is to preserve common terns on their last remaining continental habitats along the Sava and Drava River sand banks, which are located in Croatia and Slovenia, and with cooperation of institutions from both countries. From the funds of the European Regional Development Fund under the Interreg V-A Slovenia-Croatia Cooperation Program 2014-2020, international project "Preserving Terns Population in the Sava and Drava River" was approved for 579.182,35 euros.
The project's holder is the Institute for Ornithology of the Croatian Academy of Science and Arts, and its partners are: JU Green Ring of the Zagreb County, Faculty of Science, University of Zagreb, BIOM Association, National Institute for Biology and Society for Observation and Study of Birds of Slovenia. The project started in September this year, and it is planned to be finished in February 2020.