The Big Bird Killers: Cats or Wind Turbines

Getting our claws into the argument of whether renewable wind energy or free-roaming cats pose a bigger threat to birds.

by Luke Kenny

Cat, Birds, Windmill

Between a cat and a fast blade, are birds caught in the middle, or has kitty got a lot more to answer to?1

A recent argument in support of wind turbines has drawn on the fact that they kill orders of magnitude fewer birds than cats do. The US Fish and Wildlife Service reports that as of 2017, US land-based wind turbines kill between 140,438 and 327,586 birds annually, while cats kill between 1,4 billion and 3,7 billion.

Ascertaining exact mortality figures like those above in any country is extremely difficult. Turbine-killed birds may be hidden by vegetation or carried off by scavengers (or current in the ocean), while the true number of cats in any one country is typically unknown, and therefore their exact predation effect remains a mystery.

The argument is really one of deflection, and is missing the sad truth: that birds, and wildlife in general, is under serious assault from a variety of anthropogenically created sources e.g. climate change, habitat loss, and the assisted spread of invasive species. Of the latter, domestic cats rank as the third most destructive invasive alien species after both the chytrid fungus wiping out amphibians and the cumulative effect of all rat species.

The issue hidden by the smaller incidental mortality of turbines is often the species it effects. Raptors e.g. eagles, owls, vultures etc., especially those employing soaring, updraft and thermal-riding flight techniques can be most at risk of collision with the high speed turbine blades (the tips can travel at speeds of up to 300 kmh). Some of these raptors are known to have very low reproductive success, so any increased mortality is best avoided.

The Altamont Pass wind-farm in California highlights the need to carefully site these important renewable energy facilities, where the turbines intrude on the highest nesting density habitat of golden eagles in the world. Estimates of up to 67 golden eagles were killed every year, as well as approximately 1000 other raptors.

Under pressure from conservation organisations including the Golden Gate Audubon Society, the largest turbine operator in Altamont, NextEra Inc., began to replace older turbines with larger, more bird-friendly models, as well as siting the new turbines in locations determined by biologists and engineers as having the lowest possible risk to those birds. The results of the work (after 1/3 of the turbines were replaced) has been positive, with a reduction of mortalities in four key raptor species by up to 50%.

Other areas to be avoided are migration corridors, where again the increased traffic of birds is likely to cause significant mortality. More research is required however, as a study at Utgrunden (an offshore wind-farm in Sweden) recorded 500,000 eider flights through the farm without any observed collisions.

Bird Mortality Chart

Median/average estimated bird mortality in the USA, 2017. Source: US Fish and Wildlife Service.

Cats are on an entirely different level. As the numbers opening this blog suggest, free-roaming domestic cats, which includes both owned and feral animals, are by instinct killing machines. Even saying that much, one risks prodding cat owners to get their claws out in defence.

Like many emotive issues, it is a complex one and both sides often make up for the lack of sufficient scientific evidence with a strong and unshakable belief that their side is right.

On the subject of cats killing birds in the UK, the RSPB state that “…there is no clear scientific evidence that such mortality is causing bird populations to decline”. With numerous species in decline for other reasons, mostly habitat loss and climate change, the RSPB however does note that reducing predation by cats would be a prudent move. This is especially important as pet cats may bring home as few as one in five kills, indicating that the true picture may be far worse than it would initially appear.

Keeping cats indoors, or within enclosures, would certainly reduce the avian death toll. A Dutch report noted that the mere presence of a cat (even a taxidermy specimen) increased stress levels in birds, and “…reduced subsequent feeding of the young by one-third, and notably augmented the risk of subsequent nest raiding by corvids or other predators.”

Feral cats quite likely have more effect on vulnerable bird species, and pose much more of a challenge to deal with. Directed culling potentially leaves empty territories for adjacent cats to expand into, so often trap-neuter-release programmes are suggested as the most successful means to their control.

With the development of wind farms come many lessons, and it is our duty to learn from them so as to successfully tackle any arising problems. In addition to better placement of turbines, pioneering studies offer hope of further reducing associated bird mortalities.

Radars can detect not only approaching birds but also inclement weather that may increase the risk of collisions, and as a consequence trigger turbine shutdown. In a more low-tech study that shows promise, a reduction in mortality can seemingly be brought about by painting a single blade of each turbine black, which increases the visible effect to nearby birds.

While the issue of cats and birds continues with much heated debate, wind farms, with correct mitigation can certainly take their rightful place in the renewable energy portfolio, helping to fulfil a desperately-needed shift towards a low carbon future.

1 Photos (from L to R) by Henda Watani, Maria Orlova and Expect Best respectively, from Pexels.

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