The irony is, in areas where these round ups are repeated year after year, the rattlesnakes seem to be evolving NOT to rattle anymore to give their presence away. This obviously makes them more dangerous to people and children now. Quite a few herpetologists are studying this phenomena, and the snakes within these areas are very reluctant to rattle, even when touched and moved. It seems Darwinism is in favour of the rattlers there. Kinda tells you something about the humans doesn't it
That's actually not a new trait. Rattlers have been known to not always rattle for as far back as our recorded history. It's only in the movies and popular culture that rattling was a given.