Its likely pewter. Some attribute these to military purposes, some say hunting whistles. I'm not sure if it could be proven without marks? But either way they can date back over 150 years ago, and have a fair bit of value. But the same style is probably made today. And I see some on eBay which seem to be from the early 1900's, again similar, with relatively lower value. I just don't know how to tell them apart. Here's a link to a similar find;
http://www.treasurenet.com/forums/t...litary-whistle-found-colonial-house-site.html