I don't know much about snakes, but....
This is exactly the sort of question people regularly ask in the Research and Craft area of the Compuserve Writers Forum, and usually get pretty good answers.
If you want to try asking there as well, the URL is:
[
go.CompuServe.com]
You can go to the Research and Craft section, and post your question there and likely get lots of answers (I know several folks there (including Diana Gabaldon who is one of the moderators) do live in the American West, and may have personal experience with rattlers). They're an entertaining group too, so you may want to browse other threads while you're there.
Just title your post something meaningful, so the folks likely to know the answer will read it.
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"I've often said that the difference between British and American SF TV series is that the British ones have three-dimensional characters and cardboard spaceships, while the Americans do it the other way around."
--Ross Smith