Chlidonias niger (Linnaeus, 1758)
Sterna nigra L.: Radakoff, 1879: 178; Sterna fussipes Pall.: Radakoff, 1879: 178; Hydrochelidon nigra (L.): Reiser, 1894: 195; Klain, 1909: 172.
Conservation status: in Bulgaria: Critically endangered CR [B1a, c (iii); D; E], BDA-III; International: BeC-II, BoC-II, ECS-Spec 3, BD-I.
General distribution. A Holarctic species inhabiting Europe and Asia from France and Southern Spain to Lake Baikal and Northern America – in the 60o to 40° N latitude band.
Distribution and abundance in Bulgaria. A breeding summer visitor and passage migrant . In the past it bred along the marshes around the Danube and along the Black Sea coast [1, 2]. Now its distribution and numbers are unclear. Irregularly and in very small numbers, it breeds in the marsh near the village of Malak Preslavets and in the Srebarna Reserve [1,3]. Some fish farms in the country also have a potential for breeding . The breeding population varies between 0 and 100 pairs . According to more recent estimates, it is between 30 and 50 pairs . In the 1996-2000 period, it was 0-5 pairs . Now it is probably under 20 pairs. The species migrates until late in May, and in the middle of the month intensive migration is still observed. It is then found in many of the lakes near the Black Sea coast but this cannot be assumed as breeding. During migration it is found in the water basins throughout the country, and the flocks sometimes number hundreds of individuals .
Habitats. Freshwater stagnant water basins (marshes, lakes, fish farms), thickly overgrown with floating water vegetation. In the out-of-breeding period: basins of a different nature.
Biology. It incubates in colonies, rarely singularly. The nest is placed on the leaves of the floating vegetation, less often on tufts of reeds. Laying the eggs is at the end of May to the middle of June. The clutch usually contains 3 eggs. The young ones most frequently appear at the end of June. Nidifugous, they start flying after 18-21 days. Sexual maturity is reached not earlier than the second year. The food includes larvae of water insects, small crustaceans, etc. .
Similar species. The White-winged Black Tern (Chlidonias leucopterus), the Whiskered Tern (Chlidonias hybridus).
Negative factors. Unclarified. Probably the impermanent water regime and the quality of the waters in the basins; reasons of an inter-species nature related to a general decrease of the numbers of the species in Europe.
Conservation measures taken. Protected according to the Biological Diversity Act. Srebarna Lake, where it breeds most often in larger numbers, is a reserve with an adopted Management Plan.
Conservation measures needed. Studies of the distribution of the species in the country and clarification of its preferences to the habitat. Maintenance of a permanent water regime in the Srebarna Reserve. Restoration of wetlands along the Danube (Belene and Kalimok), which will expand the opportunities for breeding. Restoration of the functioning of the larger fish farms in the country, where the species can also potentially nest.
References. 1. Nankinov et al., 1997; 2. Red Data Book of Bulgaria, 1985; 3. Kambourova, 2005; 4. Shurulinkov et al., 2005; 5. Kostadinova, 1977; 6. Nankinov, 2004; 7. BirdLife International, 2004.
Author: Bozhidar Ivanov