Wikipedia says:

It has been suggested by a number of sources over the years that the pear tree is in fact supposed to be perdrix, French for partridge and pronounced per-dree, and was simply copied down incorrectly when the oral version of the game was transcribed. The original line would have been: "A partridge, une perdrix."

Some misinterpretations have crept into the English-language version over the years. The fourth day's gift is often stated as four "calling" birds but originally was four "colly" birds, using another word for a blackbird. The fifth day's gift of gold rings refers not to jewelry but to ring-necked birds such as the ring-necked pheasant; or to "five goldspinks" - a goldspink being an old name for a Goldfinch. When these errors are corrected, the pattern of the first seven gifts all being birds is restored. There is a version of "The Twelve Days of Christmas" that is still sung in Sussex in which the four calling birds are replaced by canaries.