Todesfalle Strommast: Letzte Uhus in Gefahr

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Medium-voltage pylons are a major hazard for large birds such as the Great Horne
Medium-voltage pylons are a major hazard for large birds such as the Great Horned Owl. Photo: Adrian Aebischer, University of Bern.
Dangerous electricity pylons, the main threat to eagle owls: scientists sound the alarm and call for a national clean-up campaign.

We’ve known for a long time that power lines represent a serious handicap for large birds: for example, a third of our Storks die as a result of a collision with a cable or electrocution. A recent study by the University of Berne demonstrates the devastating effects of electrocution, the main source of anthropogenic mortality, on eagle owl populations. Swiss electricity companies are called upon to take measures to remediate the dangerous pylons that are common throughout the country.

The Great Horned Owl is our largest native species of nocturnal raptor. Once persecuted and decimated, this owl is slowly recovering in many parts of Europe, while stagnating or declining in Switzerland, depending on the region. Valais, one of the species’ few strongholds in Switzerland, is home to a population of around ten breeding pairs, which has managed to maintain low numbers over the past 20 years. Despite the regular discovery of owls that have died in accidents or been electrocuted, the situation of the Valais population might at first sight appear acceptable. A study, the results of which have just been published in the renowned journal Biological Conservation, shows that this is not the case: the scientists are sounding the alarm and calling for national action to clean up the dangerous electricity pylons found throughout Switzerland once and for all.

A research team from the University of Berne and the Swiss Ornithological Institute, led by RaphaŽl Arlettaz, Adrian Aebischer and Michael Schaub, wanted to find out more about the fate of eagle owls in Valais. The biologists marked young owls at the nest, using satellite tags or radio transmitters, to track them after fledging. To their great surprise, they discovered that only 10% of these youngsters survived their first year! A quarter of them perished as a result of electrocution, the main cause of anthropogenic mortality. The pylons in question are mostly old infrastructures that no longer meet modern construction standards.

The researchers then developed a demographic model incorporating several sources of information: data from the monitoring of young birds using beacons and transmitters (telemetry), surveys of nesting site occupancy and the number of young birds emancipated over the last 20 years, and an analysis of the age classes (determined on the basis of plumage) of the remains of around 100 Great Horned Owls found dead in Switzerland. This type of highly innovative model was used to estimate the main demographic parameters.

First result
The breeding success of Great Horned Owls in Valais is similar to that of other European populations. However, the mortality rate among young and adults was extremely high, at around 40% per year. Researchers were able to demonstrate that the apparently stable population was in fact only maintained by a massive influx of immigrants, particularly from Italy and France. Even a slight drop in the rate of immigration would inevitably lead to the extinction of the Valais population, which is now "on life support". The model also shows that if all dangerous pylons were removed, thus eradicating this source of mortality, the population of Valais would grow by 17% per year, theoretically tripling after just 8 years!

We’ve known for a long time how to sanitize dangerous electricity pylons to make them harmless to large birds. The measures are easy and inexpensive to implement, and they also prevent short-circuits on the lines. The Federal Office for the Environment has just published an updated catalog of hazardous pylons and recommendations for remediation online (url below), while one-off remediation operations have already taken place locally, notably in the Seeland (Storks) and in the Martigny region (Great horned owls). There are, however, thousands of pylons of a type that are dangerous for large birds, and they are present all over Switzerland.

This study provides an exemplary demonstration of the potential benefits for endangered native fauna of systematic measures to clean up dangerous pylons. "Electricity companies, always keen to promote their green electricity, have a heavy responsibility when it comes to the survival of large birds whose populations are decimated by their distribution networks. When it comes to eco-labeling energy, we need a new, global approach that considers not only the nature and origin of the current, but also the electricity distribution network. Companies should clean up their infrastructures without delay, if not to meet the requirements of current legislation, then at least to truly earn the green label they boast," asserts Professor RaphaŽl Arlettaz. "Why not set up a vast clean-up campaign, as certain regions of France and Germany have already done?". The survival of the Great Horned Owl and other large bird species in Switzerland depends on it.

Documentation: Michael Schaub, Adrian Aebischer, Olivier Gimenez, Silvia Berger, RaphaŽl Arlettaz: Massive immigration balances high anthropogenic mortality in a stable eagle owl population: Lessons for conservation. Biological Conservation, 2010, doi:10.1016/j.biocon.2010.04.047