I can’t seem to find The National Chicken Council’s 2024 Wing Report to view how they counted a wing. The wing of a chicken consists of 2 muscle groups which are usually divided and sold as ‘wings.’ Each chicken would (usually) have 4 such wings; meaning it’s only 362.5 million chickens. Even if they’re counting whole wings it’s only 725 million chickens. 4 billion does seem to be an exaggeration.
There are already non-EU members that are members and participants in Schengen.