Ecology and Evolution study suggests death rate of birds due to wind turbines is underestimated

A June 2020 study published in Ecology and Evolution suggests that bird mortality rates caused by collisions with wind turbines may have been underestimated. 

The study examined the mortality rates of the common starling, black-tailed godwit, marsh harrier, Eurasian spoonbill, white stork, common tern and white-tailed eagle. 

Study documents stated that mortality in up to 5% of the bird populations would have negligible effects long-term. Researchers, however, found that the bird species actually can be quite sensitive to small increases in mortality, and as little as a 1% increase in death rates in the bird populations resulted in an up to 4% population decrease over 10 years. 

"Allowing a 5% mortality increase to existing mortality resulted in a 9%–77% reduction in the populations after 10 years," the study said. "Wind turbine collisions have proven to affect bird populations, with potentially important negative, cumulative effects from additional mortality caused by multiple wind farms."

The European Union prohibits the deliberate capture, disturbance or killing of wild bird species by wind development contractors. However, the capture, keeping or other judicious use of certain birds or small numbers is permitted "under strictly supervised conditions."

Researchers in the study said that although the mortality thresholds frequently used to meter the anticipated effect of wind turbines on native bird populations offer science-based and clear-cut methods, questions have been risen recently regarding the methods' validity. 

Scientists said the questions are whether the methods are generally applicable, and if they are too uncertain in predicting the cumulative impacts of wind turbine collisions on bird populations. Study documents stated that the processes may be "highly uncertain, generating misleading conclusions."

The study concluded that the mortality threshold to which the bird species are unexpectedly sensitive, can "severely underestimate" the effect of the wind turbines on bird populations. 

"Our results indicate that methods to estimate allowable mortality resulting from wind turbine collisions currently used in several northwest European countries may severely underestimate population losses in vulnerable bird populations," the study said.