Eagles and power lines: the applied value of long-term data

In this post Associate Editor Jeroen Minderman discusses a paper he recently handled by Clément Chevallier and colleagues ‘Retrofitting of power lines effectively reduces mortality by electrocution in large birds: an example with the endangered Bonelli’s eagle

I have spent much of the past few years studying the effects of wind turbines and wind farms on bird and bat populations, and as a result when I think of the impact of energy generation on wildlife, I tend to think of turbines. However, what I, and I think many others, often forget is that the rapid development of renewable energy technologies comes hand in hand with the need to expand and upgrade the electricity grid, and in many countries this involves the construction of new above-ground power lines and pylons. Recently this has got extremely close to home for me personally as a big gouge was cut out of the forested hillside I could see from my old office window, to accommodate a section of the new Beauly to Denny power line stretching the length of Scotland.

Quite apart from visual impacts, as pointed out by Chevallier and colleagues in their recently published paper in Journal of Applied Ecology, power lines and associated pylons can have significant negative effects on bird and bat populations. Larger birds, such as raptors, are particularly at risk from electrocution as they frequently use the wires and pylons as perches, roosting or even nesting sites. Unfortunately, mortality caused by anthropogenic sources such as power lines may be especially problematic for longer-lived species such as raptors, particularly where populations are already small or isolated.

With the rapid growth in renewable energy generation, further expansion of power lines seems inevitable. Thus, to avoid widespread adverse effects on wildlife, we need to know what the most effective mitigation strategies are. For example, can we either improve the technology of new power lines or pylons, or change something about existing installations, so that bird electrocution risk is minimised? In turn, to do this effectively, we need to know exactly how, when and why electrocution mortality operates.

Bonelli’s eagles in flight. Picture credits: VivekMalleshappa (Wikimedia commons) and Birdwatching Barcelona (Flickr).
Bonelli’s eagles in flight. Picture credits: VivekMalleshappa (Wikimedia commons) and Birdwatching Barcelona (Flickr).

It is precisely these two issues that are addressed by Chevallier et al. They use 20 years’ worth of capture–recapture data on a long-lived raptor species in France, the Bonelli’s Eagle Aquila fasciata, to estimate key demographic rates both before and after a period of power line retrofitting. This retrofitting involved targeted insulation of power lines in or near known breeding pair home ranges and dispersal areas. Using a combination of live colour ring resightings and dead recoveries, they estimated survival probability of different age classes both before and after power line retrofitting, allowing an assessment of the effectiveness of this mitigation option. In doing so they were able to separate out mortality caused either by electrocution or other factors, and they show that survival of juvenile and immature Bonelli’s Eagles is significantly higher after power line insulation compared to the period before insulation. Although this improved survival of young birds is likely to improve population growth rates, the authors show that this still would not lead to a sustainable population in the long run, due to limited adult survival (which was not impacted greatly by electrocution). Irrespective of the need for wider conservation measures to improve the prospects of French Bonelli’s Eagles, this paper does neatly demonstrate that retrofitting existing power lines to improve insulation can be an effective mitigation option to limit bird electrocution risk, one that is likely to be much more cost effective than many alternatives.

What this paper also shows, however, is the huge conservation value of long-term, large-scale monitoring and capture–mark–recapture studies. Development of effective mitigation or conservation strategies depends on the ability to measure how and where adverse effects, for example on survival, operates. Estimation of demographic parameters such as survival, particularly in longer-lived animals, requires long-term data. And in many cases, it is particularly longer-lived animals that are more at risk from anthropogenic disturbance. Unfortunately, however, the current scientific funding climate is becoming so squeezed, that unless a particular project can produce results in a very short time frame, it is unlikely to attract many resources – making the collection of long term capture–mark–recapture data very difficult. We can only hope that studies such as the one by Chevallier at al. will highlight and stress the vital importance of investment in long-term applied research projects, and not just scientific short-termism.

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