In 2018 and 2019, the breeding success of two Common Tern colonies on artificial lakes near the River Sava in Zagreb, Croatia, was studied. The colonies were visited weekly from May to July and we collected data on phenology, number of breeding pairs, clutch size as well as egg and chick survival. We also conducted a comparison between early and late breeders. Hatching and fledging success was within previously observed ranges, apart from a low hatching success on Siromaja in 2019. The smaller colony on Siromaja had a higher productivity in both years than the colony on Rakitje, although in 2018 an avian pox virus killed much of the late chicks on Rakitje. Early breeders seem to have had higher hatching success and average clutch size. Furthermore, a greater proportion of them managed to hatch all their eggs compared to late breeders, but the differences were not statistically significant. Our study provided baseline data for future monitoring of phenology and breeding success with regard to the management of breeding colonies.
Between 2002 and 2019, monitoring of Common Tern Sterna hirundo and Little Tern Sternula albifrons along the Sava River near Zagreb, Croatia was conducted. Natura 2000 site “Sava kod Hrušćice” was designated to protect colony at river islands, with estimated population sizes of 100–150 pairs of Common and 20 pairs of the Little Tern. Flooding of the colony caused breeding failure in several years. Common Terns moved to breed on islands in gravel pits with a total population around 150 pairs, while Little Tern did not breed after 2010. In the last few years, terns have not bred at Hrušćica and the only colony inside the Natura 2000 site is situated on a breeding platform at Siromaja gravel pit. Channelling of rivers and hydropower plants are the main threats, changing natural dynamics of water level and causing reduction of gravel sediment in rivers.
Background and Purpose: The species Pistia stratiotes L. (Araceae) is South American (or pan-tropical) free-floating, aquatic macrophyte, present on all continents, except Antarctica, today. In Europe it occurs occasionally and casually in many countries, but it is established only in thermally abnormal waters in Slovenia, France and Germany. Although it was found in different European countries, even in Slovenia, it has never been recorded in Croatia. Materials and Methods: Floristic research in the Sava-Strmec Special reserve (Zagreb County) was carried out during autumn (September and November) of 2017, using GPS Receiver to determine investigated localities. Results: The species Pistia stratiotes was firstly recorded on 9th September 2017 on one locality in the Sava-Strmec Special reserve (Zagreb County). On 24th and 27th November 2017 several other localities in the vicinity were found. On each locality a small colony of plants was found. Although exact introduction pathway is not known, several possible pathways were taken into consideration and discussed. Conclusion: A first record of the macrophyte Pistia stratiotes for Croatia is reported here. For now, it should be treated as a casual alien species.
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