Sterna hirundo
The wagtail is a bird of consummate elegance, with an agile and swift flight. It is known for its spectacular attacks by diving vertically into the water to catch small fish. In the Danube Delta, its colonies are noisy and very active.
A slender silhouette with long narrow wings and a deeply forked tail — hence the popular nickname "sea swallow". In breeding plumage it has a complete black cap, grey back and white-grey breast. The bill is red with a black tip (a key distinction from similar terns), and the legs are red.
Breeds on freshwater lakes but also hunts frequently in lagoons or at sea. In the Danube Delta it is a widespread migratory breeder on the majority of large lakes, with colonies on sand islands, floating vegetation mats or specially built artificial platforms.
Hunts by hovering above the water before plunging vertically with the full body into the water. Highly colonial and extremely aggressive in nest defence against predators — it will not hesitate to attack gulls or even humans that approach too closely. Its call is a prolonged, strident "kierrr" — unmistakable in the Delta soundscape.
Arrives in Romania in April. The female lays 2–3 eggs directly on the ground or on platforms, incubated by both parents for approximately 22–26 days. Chicks are semi-nidifugous — they can walk but are fed by the parents until fledging. Colonies are active and noisy throughout the summer.
Globally Least Concern (LC). Colonies are vulnerable to flooding, loss of nesting sites through vegetation overgrowth, and disturbance from uncontrolled tourism. Maintaining islands and nesting platforms is essential.