Chlidonias hybridus Pallas, 1811
Chlidonias hybrida Pallas 1811: Ivanov, 1985: 118; Simeonov, Michev, 1991: 148; Nankinov et al., 1997: 306.
Conservation status: in Bulgaria: Vulnerable VU [A 3 + B 2a, 2c], BDA-II, III; International: BeC-II, ECS-Spec 3, BD-I.
General distribution. A species of the Old World breeding in 5 separate regions: European-Central Asian, Eastern Asian, Indian, African, Australian.
Distribution and abundance in Bulgaria. A breeding summer visitor and passage migrant. In the past it bred in singular habitats: the area of the town of Svishtov , Belene islands  Srebarna Lake, [3, 4] possibly in the Burgas lakes. . At present, it ismainly found in the large Danubian water basins. The quantity of multiplying birds varies greatly during the years. In the Kalimok-Brashlyan Protected Locality, 20-500 pairs multiply , in the Mechka fish farms 5 – 210 , in Srebarna Lake up to 250 , in the fish farms Hadzhidimitrovo, Persinski marshes, Malak Preslavets Swamp – 40 – 120 pairs . The total numbers in the country for the recent years are 220 – 800 breeding pairs . During migrations it has been observed in many internal water basins and microreservoirs, as well as along the Black Sea coast and the Black Sea lakes .
Habitats. Marshes, swamps, bogs, permanent freshwater lakes, fish farms, thickly overgrown with vegetation over and around the water. During migration it is also found in salt water basins.
Biology. Colonial breeding. The nest consists of floating plants. They lay 3 eggs that are incubated by both parents. Incubation continues for 18 to 20 days. The newly hatched young ones can run and swim, but they stay in the nest for up to 2 weeks. At the age of 20 to 25 days, the young ones can fly. It feeds on small fish, frogs and a large number of insects that it catches mainly over the water and in flight.
Similar species. The Black Tern (Chlidonias niger), the White-winged Black Tern (Chlidonias leucopterus).
Negative factors. Intensification of freshwater fish production; accidental lethality in fishing with nets; natural disasters.
Conservation measures taken. A protected species according to the Biological Diversity Act. Included in the Red Data Book of Bulgaria (1985). Some of the places of distribution are declared Protected Territories: Srebarna Lake, Malak Preslavets, Kalimok-Brashlen.
Conservation measures needed. Declaring the fish farms Hadzhidimitrovo and Mechka Protected Territories.
References. 1. Ivanov, 1985; 2. Nankinov et al., 1997. 3. Kambourova, 2005. 4. Shurulinkov et al, 2005. 5. Kostadinova, Gramatikov, 2007.
Authors: Victor Vasilev, Bozhidar Ivanov