[horsawlarway]

I think the honest answer is that we just don't have nearly as much surety around these numbers as you might expect.

I went looking for data on actual bird populations, and I get wildly different estimates.

The 3 billion number comes from the following source (as far as I can tell):

" Critical data were contributed by citizen-science participants in the Audubon Christmas Bird Count, North American Breeding Bird Survey, and other bird-monitoring initiatives. The Partners in Flight Avian Conservation Assessment Database was a critical source for the data. " - https://www.birds.cornell.edu/home/bring-birds-back/

Nat Geo quotes

" New research estimates there are between 50 billion and 430 billion birds on Earth. " - https://www.nationalgeographic.com/animals/article/how-many-...

A lot of articles seem to quote the "at least 50 billion" line, probably based on the above numbers.

Basically - I'd say we have no idea how many birds there really are. Take ALL numbers involved as wild guesses.