What many believe to be the only written account of the original gathering and feast mentions nothing of Turkey specifically, but "fowl". In the letter by Edward Winslow dated December 12, 1621 was first published in 1622, and is chapter 6 of Mourt's Relation: A Journal of the Pilgrims at Plymouth.
Our harvest being gotten in, our governor sent four men on fowling, that so we might after a special manner rejoice together after we had gathered the fruit of our labors. They four in one day killed as much fowl as, with a little help beside, served the company almost a week. At which time, amongst other recreations, we exercised our arms, many of the Indians coming amongst us, and among the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five deer, which they brought to the plantation and bestowed on our governor, and upon the captain and others. And although it be not always so plentiful as it was at this time with us, yet by the goodness of God, we are so far from want that we often wish you partakers of our plenty.
Other accounts of the time surrounding the first harvest, not mentioning a celebratory feast or entertaining others, do mention other meats harvested and stored such as pork, bass, cod, & chicken.
Anyway, turkey works for me on this day of thanks.. I love it. And the leftovers are versatile.'
Happy Thanksgiving everyone!