The habits of terns nesting in the middle Vistula valley were examined. The results are surprising

Research results of scientists from the Institute of Nature Conservation of the Polish Academy of Sciences (IOP PAN) and the Department of Water Engineering and Geotechnics of the Agricultural University of Krakow were published in Scientific Reports. The researchers began their work with a systematic penetration of the section between Tarnobrzeg and Annopol. They showed that terns were present throughout the entire studied section, but they found breeding colonies only in three places: a few nests on the spur just below Sandomierza large colony on Chwałowska Island, formed by the waters of the San River below its mouth, and the largest colony on the island near Opoka, before the watershed Vistula near Annopol. – The next step was to find out why and where the terns we see all over the river were there. We caught terns of both species and started tracking their flights during the chick feeding period. We tracked them from the boat, from the shores and driving around the entire area in a car equipped with a powerful roof antenna – said Dr. Hab. Tadeusz Zając, prof. IOP PAN, author of the study.

According to the expert, the results were clear: birds nesting on Opocka Island, which was mainly studied, do not feed further than a few kilometers from the island. It turned out that the sections where terns feed are areas of aggradation (i.e. accumulation, transfer) of sediments, which results in the occurrence of huge areas of shallow water. As ichthyological research has shown, these shallows are inhabited by fry and fish species (sometimes very rare) with small body sizes, which are hunted by terns.

The difference between tern species turned out to be striking for scientists: while the common tern (Sterna hirundo) foraged quite far away and in various habitats, the rare little tern (Sternula albifrons) fed almost exclusively in the shallows surrounding the islands. It turned out that the more complicated and diverse the island's coastline, the more often and more successfully the little tern feeds there – researchers from Krakow report. According to scientists, in the case of the “Wisła pod Zawichostem” reserve, sectional protection of the river makes sense. – On the other hand, it is known that terns feed within a radius of several kilometers from the colony, so activities in the riverbed beyond this range are possible and should not be blocked, especially if they concern human safety – noted the study authors. According to them, when planning any protective measures for terns, creating only nesting places, e.g. on barges, will not be enough, especially in the case of the little tern. There must be complementary habitat to enable effective foraging and feeding of chicks.