[vertak]

The article and numbers are right, it’s just not clearly worded.

I had this same question a couple years back when I read this stat, so I messaged the Cornell Lab Ornithology and one of their reps explained how these numbers make sense.

“We start with a population of breeding adult birds at the beginning of the year. They breed, and multiply, and then the population is "spent" throughout the year on deaths such as window strikes, pesticide poisoning, and cat kills. If the population could keep up with these deaths, we would see a net loss of zero. Unfortunately, instead, we are seeing less and less breeding adult birds at the beginning of each year.

So when we say "we've lost 2.9 Billion birds," what we really mean is, "We've seen a net loss of 2.9 billion breeding adult birds since the 1970's." Back in the 1970's, the breeding season started with about 13 billion adult birds. This year, we had about 10 billion. Throughout the year, they multiply to about 40 billion before they die back. Of that 40 billion, cats kill 2.5 billion.”