Artificial Floating Island for the Common Tern (Sterna hirundo)

LIVING MOHELNICE – Species Conservation Concepts in Gravel Mining Environments

As part of the "Živá Mohelnice" project, a new floating nesting platform for Common Terns (Sterna hirundo) was installed in a gravel pit near Mohelnice. This endangered waterbird species has been regularly observed in the area during flyovers, although breeding has not yet been confirmed at the site.

The Common Tern typically breeds on naturally formed islands, gravel bars and sandbanks found in river deltas, oxbow lakes, or shallow lakes. Historically, it relied on dynamic river systems with fluctuating water levels that regularly created open, predator-free surfaces with minimal vegetation. Due to river regulation, construction of dams, and vegetation overgrowth, these habitats have largely disappeared. As a result, terns are increasingly dependent on artificial nesting structures, such as gravel islands, flat rooftops or floating pontoons.

To create the floating nesting platform, the following measures were implemented:

  • Construction of a square island measuring 2.5 × 2.5 m.
  • The floating structure consists of a wooden frame reinforced with cross beams.
  • The top surface is made of OSB 3 boards.
  • Buoyancy is provided by 8 sealed plastic barrels (120 L each), evenly distributed.
  • A 5–7 cm layer of gravel was applied to the surface, combining 4–8 mm and 8–16 mm fractions to mimic natural nesting substrate.
  • To reduce gravel displacement, small wooden partitions and corner shelters were integrated into the design.
  • The island is anchored on all four sides using steel cables connected to 60–90 kg weights, submerged at a depth of 10–12 m.
  • As a protective measure against predators, a vertical sheet metal skirt was installed around the perimeter, and anti-perching spikes (commonly used to deter pigeons) were added to prevent colonisation by undesirable bird species.

The Common Tern (Sterna hirundo) is a long-winged piscivorous bird that requires predator-free nesting sites with low vegetation. In the Czech Republic, it is classified as an endangered species, with an estimated 600–800 breeding pairs. The breeding season typically lasts from May to July. Terns lay 2–3 eggs in a simple scrape and chicks usually hatch after 3–4 weeks, leaving the nest shortly thereafter.

Efforts to support the Common Tern also benefit other waterbird species and contribute to the overall ecological value of post-extraction sites. The island will be monitored and maintained regularly in the coming years and adapted based on practical experience and monitoring results.