The most likely zoological candidates for Thunderbirds could be the suposedly recently extinct Teratorns.
"Thunderbird" is the name given to a large, condor-like bird reported throughout North America since the beginning of historic records. They were mentioned in Native American mythology as well. They have also been seen in Texas. From their descriptions as large dark-colored, gliding birds with wingspans over fifteen feet wide, Thunderbirds seem to be associated with the largest known soaring bird accepted by ornithologists - the condors. The Andean condor is the world's largest classified flying bird, with a wingspan of up to twelve feet. The largest classified flying bird of North America is the Californian condor, with a nine-foot wingspan. Descriptions of the Thunderbird from eyewitnesses match the description of the Andean condor more