Onshore windfarms vs. birds

Several times recently, I've had conversations with others about environmental objections to windfarms, and specifically about their impacts on birds. I've expressed surprise that anyone could believe that large, slow-moving wind turbines could be a threat to birds. It turns out, there is research that supports the negative impacts of wind turbines on birds (see here or here), but that research doesn't actually demonstrate that wind turbines cause a decrease in bird populations. The problem, of course, is that it isn't feasible to run a randomised controlled trial with wind turbines due to cost - placing wind turbines at random across some areas and not others, and comparing the effect on bird populations in both areas. The cost of such an experiment would be enormous.

Fortunately, there are statistical methods that we can use to try and estimate the causal effects. And that is what this new article by Meng et al., published in the Journal of Development Economics (ungated earlier version here) attempts to do. Specifically, they look at the effect of onshore windfarms on bird biodiversity at the county level in China. They have two measures of biodiversity:

Bird abundance is the average number of birds of a given species per checklist observed at a county-month-year-species level. Species richness is the total number of unique species observed in a given county at the month-year level, which better reflects the diversity of the bird populations.

To establish causality, they use a difference-in-differences (two-way fixed effects) model, which essentially compares the difference in bird biodiversity before and after a windfarm is installed, between counties with and without windfarms. However, Meng et al. go a step further, using an instrumental variables approach, instrumenting for the location of windfarms by the interaction between national-level growth in windfarms interacted with county-level average windspeed at 100 metres. That instrumental variables approach should mitigate issues arising from the correlation of wind turbine location and bird biodiversity.

The novelty of this paper is not just in the methods, but in the data that Meng et al. employ. To measure bird biodiversity, they make use of data:

...from the China Birdwatching Report (CBR, similar to the eBird Reference Dataset), a citizen science dataset consisting of reports from users, including information on individual bird trips and associated characteristics, such as the specific date and time, location of a specific trip, as well as species and quality of birds encountered...

Their dataset covers the period from 2015 to 2022, and includes data collated from over 33,000 checklists. They also control for a variety of other variables:

...including average bird observed duration, average temperature, average visibility, average wind speed, total precipitation, average ozone, percentage of natural park areas in the county, average population density, and average night light value.

Using this data and the two-way fixed effects approach, Meng et al. find that:

A one standard-deviation increases in wind turbines (approximately 84 turbines)... in a given county leads to a 9.75% decrease in bird abundance per checklist from the mean value of 5.38...

...while a one standard-deviation increases in wind turbines (approximately 84 turbines) in a given county decreases the number of unique bird species by 17.67% from the mean value of 66...

Meng et al. also find evidence that the impacts are greater on migratory birds than on resident birds (important given that China is a major migration pathway for migratory birds), and that the effects are larger in forested and urban/farmland than for grassland. There is also evidence that the impact is greatest for the largest bird species.

Finally, Meng et al. show that there are effects of windfarms on neighbouring counties (as well as the counties in which the windfarms are located), and that those effects are somewhat smaller in size. That made me wonder why those analyses were not the primary results in the paper, since it seems obvious that birds may move across county borders.

So, it does appear that windfarms might cause a decrease in bird biodiversity. Meng et al. even address a bunch of concerns that jumped out at me as I was reading the paper, especially that birdwatchers, anticipating that there would be fewer birds near windfarms, do less birdwatching in those locations. On that point, Meng et al. note that:

We do not find a significant impact of wind turbine installations on birdwatcher behaviors regarding the submitted number of checklists...

And they further support that with detailed mobile phone GPS data, showing that there were not fewer trips made to the areas of windfarms, relative to areas further away. However, a couple of concerns do remain, but they are rather technical. First, I wondered why Meng et al. used the interacted variable (national growth in windfarms interacted with windspeed), rather than just windspeed alone. They describe this as a "Bartik-like variable", but we should be cautious about whether Bartik instruments are appropriate (see here). Also, two-way fixed effects models have also come in for criticism recently (see here and here). I'm not going to drag you into the technical details (read the links if you're interested). But suffice to say, this will not be the last word on whether windfarms negatively impact bird biodiversity. However, the best quality study we have so far seems to suggest they do.