Charlotte McDonald

How do we count how many birds are killed by wind turbines each year?

It turns out those bird cemeteries Trump was talking about aren't far from the truth.

Sounds like a fun way to spend your weekend.

It's important to note that these studies only measure the birds killed by onshore wind farms.

It's much harder to measure birds killed by wind turbines in the ocean and the data on that isn't reliable.

Anyway, you can use the data from counting dead birds to calculate a rate of deaths per wind turbine.

Hannah found all the studies she could that did this to see if they agree.

They did not.

The problem is that dead birds are hard to count.

Sometimes small birds get missed.

Sometimes wild animals eat dead birds before they're counted.

The number and type of dead birds also varies depending on the location.

To account for the bird count problems, scientists try to model the number of birds that might die but not be counted.

Hence, the big range of answers.

Hannah thinks that because there's a risk of under-reporting in the dead bird count, the higher numbers are more likely to be in the right ballpark.

So Hannah reckons a figure of maybe one to two million birds being killed every year looks justifiable.

President Trump is roughly right.

As long as you think millions means two million and not a hundred million.

But as so often on this programme, the numbers don't tell the whole story.

And it's not just cats that are more deadly for birds than wind turbines.