The names reflect the committee’s view that spelling should be consistent throughout the list. Easily stated and on its face obvious, this rule became difficult to apply where the same words have for centuries been spelled differently in different English-speaking countries.
The problem essentially involves British and American spellings, with some countries being on one side and some on the other. The gray/grey difference is the most pervasive and best known, but other variant words are color/colour, mustache/moustache, racket/racquet, ocher/ochre, somber/sombre, saber/sabre, miter/mitre, sulfur/sulphur, and perhaps others.
The committee decided to select one spelling for each variant word, because to state these words in the alternative in every case would produce a cumbersome list. The spellings selected represent a compromise. Grey is used because far more taxa have traditionally used that spelling than gray. The list likewise adopts the British spelling of sombre, sabre, sulphur, mitre, ochre, and moustache, and the American spelling of color and racket. This tilt to the British side is justified by the fact that both spellings of every one of these variant words is considered correct in typical American dictionaries, such as the unabridged Merriam-Webster Dictionary .
Global replacements are easy, and the spelling issues do not seem to be a major impediment to use of this list. Nevertheless we are exploring the suggestion to simply provide separate British and American spelled versions of the IOC World Bird List.
Meanwhile, we encourage each author and publisher to select whatever spelling of these words is deemed appropriate.
We hope this solution will find favor with most users of the list.