The following is from the facebook page of cryptozoologist Ken Gerhard:
For years now, I've been compiling an extensive database of most of the North American Thunderbird accounts dating back to the 1850s. Overall, an intriguingly consistent model has emerged -
1. They are reported from pretty much all over the U.S. and Canada (which corroborates a plethora of widespread Native legends).
2. The average estimated size is about 5' tall with (on average) a 16' wingspan. Implausible wingspans of up to 36' have been described.
3. Their bodies/feathers are almost always described as being solid black, dark, chocolate brown or gray. Occasionally the head or wing tips are described as being lighter in color, even white.
4. The wings are often described as being unusually slender and squarish.
5. The beak is almost always described as curving downward, like a raptor's.
6. The head is often described as being 'bald' or leathery. Many witnesses have remarked that these animals have a 'prehistoric' or 'nasty' look about them.
7. They are typically observed soaring through the sky, occasionally landing for a brief period of time. Some have remarked that they appear awkward or clumsy when they take off.
8. There have been observed predating fish... in one case a fawn... and of course there is the famous Lawndale Incident, as well as other accounts of them carrying away people (mostly children).
9. Reports are almost always of a solitary bird, though there are a handful of accounts that describe pairs, or rarely small 'flocks.'
10. Two reports describe noticeably 'large eyes' and one that was allegedly shot was said to have a bad odor.
Check out his upcoming new book on giant birds here.