We Don’t Really Know How Many Birds Die in Structural Collisions

This makes it a weak argument against windmills

Photo by Karsten Würth on Unsplash

A widely-touted argument against windmills is their alleged propensity to cause the rampant deaths of avian species. Incoming US-president Donald Trump, for example, has loudly criticized windfarms as “bird killers.” Christine Morabito, in an essay criticizing the Cape Wind project, wrote “wind farms are prolific killers of birds and bats.” These claims are oft-repeated by politicians and the media.

But all of them are repeating rhetoric, usually meant to support an agenda. While it is true that all humanmade structures are technically bird killers, we do not know definitive numbers to make apt comparisons between specific types.

The reason: it is extremely difficult to measure the actual number of birds killed by collisions into manmade objects. A study released in August does an excellent job of explaining why:

Most studies examining building collisions are conducted on recovered carcasses of collision victims that die on impact. However, though over 80% of collisions with buildings were once estimated to be immediately fatal to birds, more recent studies have not supported this pattern and suggest that far fewer than 80% of collision victims die on site. For instance, in Korner et al.’s 2022 study, only 7.2% of casualties were found dead. Therefore, estimates based on bodies found at the scene of collisions might provide a drastic underestimate of the true toll of building strikes on bird populations. [Citations omitted]

This is not to say that we should not work to figure out concrete numbers. It also is not meant to conclude that windmills are safer (or more dangerous) for birds than buildings and other manmade structures. But we should avoid listening to the proclamations of those motivated by agendas that actually have little to do with the wellbeing of our avian neighbors. Instead, given our species’ propensity to build in seemingly every available space, we should work to determine the real effect we are causing so we can devise better harm-reduction strategies.

Blackbird with cat bite injuries and thoracic puncture. Note the hunched appearance typical of this condition. Photo courtesy of Vale Wildlife Hospital via Aneesa Malik and Adina Valentine.

What is so hard about statistical conjecture?

In virtually every instance where researchers proffer large tallies, they are not counting the subjects one-by-one. For example, no one has traipsed around the United States searching for each of the approximately 40 million resident grey squirrels. Instead, scientists strategically select target areas and use a variety of observational methodologies to count. From that, they conduct statistical extrapolations based on a wide array of factors that are dependent on the species, type of living environment, migration patterns, and so on. For a detailed explanation of how this is done, see here.

As one might imagine, performing such analyses on creatures not bound to the ground are considerably complex, but estimating populations is still achievable because their patterns are predictable. When it comes to counting victims of aberrative behavior, such as collisions with myriad manmade structures, the challenges become profound. The authors of the study quoted above—Ar Kornreich, et. al.—write, “Knowing how many birds hit buildings, survive, and fly away without intervention is impossible, let alone what becomes of them later.”

Even finding those that die very near the location of their impact incident is challenging. Authors of another study discuss some of the ways researchers have thought about tackling this problem with respect to windmills. They note that simply searching designated areas around windmills for carcasses is one approach, but none have yet decided what sized radius would constitute an appropriately sized space. Another way of identifying search areas is by utilizing ballistic science, but fatally injured birds and bats can travel beyond the expected range before succumbing.

Furthermore, basing fatality numbers solely on found bodies from whatever methodology potentially suffers from human bias. Searchers tend to focus on areas near to the collision object (building, windmill, etc.), on the owned portion of the surrounding property, or on areas that are easiest for searchers to navigate. Flying creatures are, of course, not bound by these limitations.

Kornreich, et. al. point out that speculating from found carcasses likely undercounts the actual fatality figure because evidence is increasingly indicating that birds tend to survive for some time, and thus continue traveling, after a collision. As they put it:

To fully understand the crisis of building collisions, these delayed deaths, or potentially disadvantaged survivors, matter, especially if these survivors are less of an exception to the rule than previously thought.

In an attempt to overcome this problem, Kornreich’s team examined the numbers of found bodies along with data collected from rehabilitation organizations among several samples. From the latter, they calculated how many birds would not have survived in the wild with their reported injuries, including those that recovered following their treatment. In effect, birds in the wild who suffer treatable injuries, but never receive human interventions, often die directly from their wounds or from predation bolstered by their inability to flee or defend themselves.

They conclude:

These findings, along with our estimate of delayed mortality, suggest that overall collision mortality estimates based on carcass collection far exceed one billion birds in the U.S. each year. [Emphasis added]

As a reminder, they studied only accidents involving buildings, not windmills.

These researchers rightly point out that theirs is not anywhere near a perfect method. There are a number of deficiencies in the available data. Rehabilitation organizations do not have licensing requirements in many states, and do not have robust reporting requirements in many of those that do. Examining medical records is complicated by non-standardized language or assessments. Moreover, there is a bias among the birds that receive assistance. Larger breeds and those with the most visibility (i.e. brightly colored) are brought to centers more often than smaller or camouflaged ones.

What can be done?

Kornreich and her colleagues offer several solutions that would help—if not fully solve—the data problem. First, they suggest creating databases or other information repositories that allow access to rehabilitators’ data. Ideally, this would be managed at the federal level to create an interstate library rather than using the current state-by-state paradigm. Second, they advise organizations to adopt standardized language in the way they report injuries and treatments. Finally, they propose a more open reception to rehabilitators in academic journals and ornithological conferences to help spread information and ideas.

Acquiring data is only part of what needs resolution. Kornreich’s study only focused on buildings, but there is no question that windmills also cause tens of thousands of bird deaths, if not more. Because more buildings and windmills will inevitably be constructed, researchers working on collision mitigation strategies and rehab specialists will both be sorely needed moving forward.

Much work is being done on mitigation strategies. Cameras and radar that detect incoming flocks can be connected to automated systems to stop turning windmill blades at times of high aviation traffic, such as during migrations. Some have tested acoustic deterrents—the use of sound to warn approaching birds and persuade them to deviate course—though their efficacy remains in question. Painting blades certain colors allow birds to better visualize the rotation of blades and evade them accordingly. Similarly, altering the appearance of window glass and surrounding lighting reduces accidents with buildings because it allows birds to better distinguish between solid objects and open space.

Rehabilitating injured animals will always be necessary, yet it is widely ignored by the bureaucracy. Many organizations committed to assisting injured animals rely upon altruistic donors because government funding is scant and difficult to obtain. Perhaps ‘green’ funds that are often disseminated for construction projects should include rehabilitation funding as a required line item. Another solution would be to include rehabilitation funds as mandates in various subsidy packages or legislative funding for construction.

A serious situation

Bird populations are facing serious decline in many parts of the world. In the United States, according to the North American Bird Conservation Initiative (NABCI), some 70 species have lost at least 50% of their population over the last half century. Habitat loss, climate change, excessive construction of tall buildings, outdoor cats, and airplanes and motor vehicles are key factors contributing to this destruction. A researcher who published an assessment of US bird populations in 2019 called the plunging population a “sixth mass extinction” event.

Mass extinctions of bird species portends grave consequences. Ashley Dayer, an associate professor in the College of Natural Resources and Environment and a Global Change Center at Virginia Tech stated:

They are often referred to as the ‘canary in the coal mine.’ Birds are indicators of environmental health for their own species, people, and other wildlife. They also provide many important functions like eating pests and disease vectors like mosquitoes.

Managing the problem of bird strikes against structures will not solve the larger problem of species decline, but reducing the billion or more deaths per year by even a modest percentage will have a significant impact.

See you Saturday.

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