How did turkey become the main course for Thanksgiving?

The historic descriptions of the pilgrims first celebration mentioned only fowl. While there may have been wild turkey, it was much more likely that the fowl mentioned was duck, grouse, goose and swan which were much more available and in quantity at the time.
Turkey became popular as the Thanksgiving celebrations became more and more of a gathering of family and friends. It was much easier to cook one large bird for the meal than several small chickens or ducks.