Name: Roc, Thunderbird, many tribes of native americans were aware of the stories of the creature though, and each has their own name for it.
Size: Differing reports suggest animals with wingspans anywhere between ten to 20 or even 30 feet wide.
Appearance: A giant bird, with a head that can either be similar to an eagle or a vulture.
Threat: Low. While the Thunderbird is told to be extremely powerful, there are no contemporary stories of these creatures actively hunting adult humans, even though a carnivorous bird at that size would have little difficulty in such a feat.
Reported sightings remain scarce, but consistent in the southern US, with sightings as far North as Illinois. What makes the sightings hard to verify is the fact that almost all are of the animal in flight at high altitudes, giving little to make size comparisons with. The best reports are those of pilots in private aircraft who occasionally see exceptionally large birds that they can't identify, following in their tailwind.
Several photos taken in the old west, report to show slain creatures which were claimed to be the mythical Thunderbird. Most of these have since been reported as fakes, but some remain unverified. The creatures pictured however, resemble the prehistoric Pterosaurs than the giant eagles of legend. It is curious however, as the science of paleontology was not widely known during that period and so good fakes of creatures which were yet to be discovered makes one pause and wonder if the Thunderbird legend could have been inspired by surviving flying reptiles. I find that doubtful, though. The legends of the native americans across the areas where Thunderbirds were occasionally seen are remarkably consistent in what they describe, and they describe a large bird, with feathers, not something which would probably described as bat-like.
The most likely theory put forth, is that of large non-native birds getting lost somehow or searching for places to expand their territory. Sightings of Thunderbirds in Alaska, for instance, are mainly attributed to sightings of Stellar's Sea Eagles, which regularly boasts a wingspan of eight feet. There is also one reported encounter from Illinois about three boys who were playing in their backyard when a pair of large birds swooped down on them, grabbing one of them, lifting him about two feet off the ground and carrying him about thirty feet before letting go. The descriptions of the birds given by the boys and other witnesses matched up with descriptions of the Andean Condor. How the birds ended up so far from the Andes is anyone's guess.
Giant birds have appeared several times through writing and movies. More often than not, they are simply depicted as giant, flying monsters, rather than described as the mythical Thunderbird. One of the most well known is the giant bird monster from the era of old Hollywood The Giant Claw. After that are movies such as Roadkill. As stated though, these are giant birds, and not specifically the Thunderbird, which in legends was more of an ally or a warning, or even one of the Gods as opposed to an actual threat to man.
Try to keep in mind as you're traveling this summer, especially if you happen to be on a long car trip through the southern states, in addition to what could be underground, underwater, or in the woods, sometimes all it takes to see something unusual is to look up and see what's right above your head.