Thunderbird

A Comparison of Certain North American Birds to the Thunderbird
An article by Willie Shughart that compares "five of the largest birds to occur in the contiguous United States and southern Canada" to the thunderbird.
http://cryptosquared.tripod.com/insights/tbirdcomp.html

Cryptozoology.com - Thunderbird
An article by Craig Heinselman discussing the cryptid, sightings, and the elusive "Thunderbird Photograph".
http://www.cryptozoology.com/cryptids/thunderbird.php

Evidence of the Giant Birds
Evidence that the Native Americans legend of the thunderbird is based on fact.
http://sped2work.tripod.com/evidence.html

Thunderbird
A discussion of the cryptid and what it might be.
http://www.geocities.com/capedrevenger/thunderbird.html

The Thunderbird Petroglyphs of the Upper Midwest
Pictures and article by Kevin L. Callahan discussing thunderbird petroglyphs.
http://www.geocities.com/Athens/Oracle/2596/thunderbird.html