The response of common terns Sterna hirundo to power lines: An assessment of risk in relation to breeding commitment, age and wind speed Original Research Article

Common terns breed within the industrial complex at Shotton Steel Works, North Wales. Each journey between the breeding colony and the main feeding areas on the Dee estuary requires the terns to negotiate two spans of power lines. Here we investigated the risk of collision with power lines in relation to the time demands on adults, the age of birds and wind speed. Systematic observations were made of terns journeying to and from the estuary during four phases of the breeding season (courtship, incubation, nestling and juvenile). Casualty surveys of the ground beneath the power lines were also completed, and in order to quantify the potential removal rate of bird casualties by predators, bird corpses were placed underneath the power lines and recounted after two, five and 14 days. There was a greater than three-fold increase in the frequency of combined journeys made by terns from courtship to the nestling phase and this increase coincided with an increase in the proportion of adult terns which passed under or between the wires of the power lines. Terns also flew closer on average to the top earth wire during the nestling and fledging phases than during the courtship or incubation phases. Juvenile terns flew consistently closer to wires than adults with most juvenile crossings being < 1 m above the top wire. Only 7% of adults flew this close. Only two common tern casualties were found beneath wires (during the nestling phase) representing only 0·4% of the colony population. Since the removal rate of corpses by predators was very slow (around 6% in 14 days), the mortality rate of terns due to collisions with wires was also considered low. Common terns are agile birds and suffered only low rates of direct mortality through collisions with power lines. However, the demands placed on parent birds during the breeding season caused a significant increase in their vulnerability to collision. This is a factor which has not previously been studied and has implications for a wider range of species.