The gorgeous and mysterious bird stones.
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Some really nice examples of bird stones at the Royal Ontario Museum.
For comparison, here is another example currently housed at the Walters Art Museum which dates to between 1500 and 1000 BC (via the Wiki Commons, 2006.15.5):
The comprehensive study done by Moorehead shows that these ambiguous prehistoric objects are found most frequently in Western Pennsylvania, New York, Tennessee, and Kentucky, and also more northward in Indiana, Michigan, Ohio, and central Canada. Loosely resembling birds, they do not usually exceed 6 inches in length, and are most commonly made of banded slate.
While still a matter of debate, it seems most likely that these bird stones were used as a counter weight for an atlatl (throwing stick), which was used throughout the Americas. One characteristic feature of the bird stone is their flat base, which is drilled at either end. These drilled holes may have been used to attach it to the atlatl. The atlatl essentially acted as an extension of the arm, and hurled a spear with great force. The earliest evidence we have of the use of the atlatl comes from the Upper Palaeolithic, dating to approximately 40,000 years ago. To visualize how these weapons were used, see this short demonstration by Dr. Elliot Abrams, professor of archaeology at Ohio University on Youtube.
It is also possible that the bird stone was worn. One interpretation along this line of thought is that they were worn on the head of women as an indication of pregnancy, appealing to the “Thunderbird” for protection.