They are taken very seriously in the field of Cryptozoology, as they match the description of teratorns, a type of giant flying bird, now (presumably) extinct.
There have also been sightings of giant owls, but they are usually considered to be separate creatures from Thunderbirds, and other eyewitness accounts describe them to be bird-like, but with no obvious feathers like a bat or a pterosaur.
|Thunderbird in Kerala|
Here is another one:
Dr Kenneth E. Campbell is pictured here standing in front of a specimen of (now extinct) Argentavis Magnificans.
Personally, I don't think it looks really convincing, but it is on display at the Natural History Museum in Los Angeles.
Are Thunderbirds soaring through the sky as we speak?