Effect of wind turbines on bird mortality often underestimated

July 6, 2020

Wind parks supply renewable energy, but they also impact bird populations. Birds can collide with wind turbine blades resulting in bird deaths. Research from Wageningen University & Research demonstrates that current wind park norms underestimate the consequences of excess mortality on bird populations.

Excess mortality due wind turbine collisions can result in decreases in bird populations. The acceptable risks of wind turbines on bird populations are examined in decision-making procedure for wind parks.

Two methods are frequently used to determine a wind park’s 'acceptable mortality limits'. In the excess mortality method (EMM) a minor increase in mortality above natural mortality is considered acceptable, as the impact is considered negligible (for example a 1% to 5% increase in mortality). Potential Biological Removal (PBR) is also examined; This is a hunting and fishing term used to calculate the proportion of a population that can be removed without significant consequences for that population. Both methods are frequently used as a feramework for issuing permits, including in the Netherlands.

Possible significant population decrease

Research associates used these two methods to examine how, according to the norms, ‘acceptable mortality’ effects starling, black-tailed godwit, marsh harrier, common spoonbill, stork, common tern and sea eagle populations. The results demonstrate that population size can be extremely sensitive to minor increases in mortality (EMM). Instead of a negligible effect, research associates found that a 1% excess mortality resulted in a 2% to 24% decrease in different bird populations after 10 years. A 5% increase of the existing mortality resulted in a 9% to 77% decrease in the different populations after 10 years, depending on the species. This is particularly the case for starlings. Species with a short life-expectancy such as the starling already have high mortality figures, making them extremely sensitive to a percentage increase in excess mortality.

Recovery factor

When the PBR method is used to determine acceptable mortality limits, the change in numbers of birds appeared to be species-dependent and was largely determined by the recovery factor: a measure of population vulnerability. When this is 1, a value typically used for robust populations that can better withstand excess mortality, this excess mortality resulted in a 50% to 55% decrease in bird numbers. With a recovery factor of 0.1, an extremely conservative value used for endangered species, the decrease in bird numbers was approximately 5%.

Serious consequences

The results demonstrate that wind turbine collisions can have significant consequences for bird populations, even if the excess mortality is considered acceptable on legal grounds. 'The consequences appear not always to be negligible as was previously assumed,' stated Ralph Buij, research associate at Wageningen Environmental Research and one of the authors. 'This certainly applies to struggling species in the Netherlands, such as the marsh harrier, common tern and black-tailed godwit. We suggest an alternative and simple method to better determine the impact of excess mortality due to wind turbines on bird populations. For the already studied bird species, this method was capable of making good predictions of the impact of mortality on population size. It is also important that the mortality estimate includes cumulative mortality in the area in which a population is present. Current mortality estimates tend to examine individual wind parks but bird populations are encountering increasing numbers of turbines across the country.'

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