Bird mortality related to collisions with ski–lift cables: do we estimate just the tip of the iceberg?

Collisions with ski–lift cables are an important cause of death for grouse species living close to alpine ski resorts. As several biases may reduce the detection probability of bird carcasses, the mortality rates related to these collisions are generally underestimated. The possibility that injured birds may continue flying for some distance after striking cables represents a major source of error, known as crippling bias. Estimating the crippling losses resulting from birds dying far from the ski–lift corridors is difficult and it is usually assessed by systematic searches of carcasses on both sides of the ski–lifts. Using molecular tracking, we were able to demonstrate that a rock ptarmigan hen flew up to 600 m after striking a ski–lift cable, a distance preventing its detection by traditional carcasses surveys. Given the difficulty in conducting systematic searches over large areas surrounding the ski–lifts, only an experiment using radio–tagged birds would allow us to estimate the real mortality rate associated with cable collision