Sterna hirundo (common tern) in literal translation is “a swallow tern”. The name probably refers to very long streamers, that is tail feathers. They are much longer than the inner streamers, hence the tail is deeply forked, similarly to a barn swallow.
Until recently the common tern was called ordinary. However, it is not more “ordinary” than the other terns. The number of this species in Poland is estimated at about 4,000-4,500 pairs, it is not a numerous bird, although in suitable habitats, that is, by rivers and lakes, it is quite common. In contrast to the little tern, it is not tied to only one type of environment. In addition to colonizing sandy banks and pits after sand or gravel mining, it can build nests in more swampy areas. It builds nests on floating plant hammocks, in areas where reeds had been cut or on low vegetation. In such cases, the nest is in the shape of a flat mound, and the eggs always lie on dry material.
It nests in a colony by the Vistula on sandy banks. In such an environment, the nest is similar to that built by the little tern, hence it is symbolic and sometimes is confined to merely a hole in the ground.
Both Vistula species of terns nest most often on the same islands. However, such coexistence is not without influence on the behaviour of the birds. The common tern usually takes its place higher above the surface of the water, because the nests located on the top of the banks are necessarily less vulnerable to flooding. While the little tern colony, often displaced in the peripheral parts of the banks, will be flooded by the floodwaters, clutches or tern chicks still have a chance of survival.
The common tern is a bird less fearful than the little tern, creating compact, several times more numerous colonies, and also taking up better and safer breeding sites. Not surprisingly, that it is several times more numerous than the little tern that is included in the Polish Red Book of Animals.
The first birds start to appear in Poland in mid-April. Mating is done by two individuals repeatedly chasing each other in flight, and often but not always, it is combined with the male giving a caught fish to the female as a gift. After preliminary “discussions” the action moves to land, that is, in the case of the Vistula to the sandbank. Both birds perform a series of gestures there: first the male with a raised neck and slightly opened wings circles the female. After initial “dancing” there is symbolic digging of the nest, that is scratching in the same place with its feet; first by the male and then by the female. After such an engagement with a fish and wedding on the sand, the relationship can then be consummated.
Both parents incubate the eggs, although more effort lies with the female. Chicks hatch out of three eggs after 20-23 days. As with all birds that inhabit the Vistula sandbanks, so in this species, losses of clutches are significant, and in some years can be 100%. The main causes are subsequent floods that flood the eggs or snatch chicks, predators or for example by being frightened by imprudent bathers.
However, terns are long-lived, and the mortality of adult birds is insignificant. Birds can afford to be without offspring for one or two seasons. However, while flooding or pressure from predators can be variable from year to year, the disappearance of nesting places as a result of human activity is usually irreversible. Regulatory work on the rivers can cause irreversible disappearance of species using the sandy banks.
Basic information about the common tern:
Photos Maciej Szymański