The islanders used the word BIrdman exactly in that form, 1000 years ago and continue to use it today. Half bird, half man, birdman. There is no confusion there, it's an accepted transaction in all English history books. We do use the word in the original language - the language is RongoRongo, the glyphs from that Island and the name of the deck. The word is prominently featured on backs - it's a character with a "big butt."
The cult of BIrdman is well documented in English books since 1750, when the term first appeared in the first English book.
What you are suggesting is for people to stop using the names of hybrid creatures like Centaur, Faun or Siren, or using them in their original Greek language to avoid a confusion with a name of a car wash which just opened down the road. English is a West Germanic language, so if we follow that logic all English words would revert back to their original form.
Actually, I suggested using the term in the original language not solely to prevent confusion with some 20th-century cartoon character but also because it would seem culturally relevant, since you are using this culture as your inspiration.
FYI: Rongorongo is a glyph-written script of either the Rapa Nui language or a language as yet undiscovered and probably lost. Calling Rongorongo a language as you have above is like saying you speak Alphabet and your best friend speaks Kanji while his brother speaks Braille. (Sounds harder than speaking Klingon!) The language uses the term "Rapa Nui" to refer to Easter Island itself, but I'll be using it here as the name of the language as well for expediency.
The script has never been successfully translated as there were few if any people left who could read it at the time of its discovery by a man named Eugène Eyraud in 1864, with the first Western-world written reference to it appearing in a report by him filed in 1866. Further hampering the translation effort is the fact that so few examples of the writing exist with which to compare. Many people have attempted to translate it, but in the end ended up with gibberish or phrases that make no sense, especially when compared with Rapa Nui language structure. So speaking with any authority about calling any of the characters "bird-man" would be pretty off-base, unless you lived on Easter Island a few hundred years ago and were taught to use the script. In Rapa Nui, the term "rongorongo" translates approximately as "to recite, to declaim, to chant out." "Rongorongo" is not even the name used by the original writers of the glyphs - it's a modern name for the language.
"Accepted transactions" aside, of course...since there is no such term when it comes to history texts, at least not in English. If you're talking about accepted
translations, well, you hit that stumbling block again of Rongorongo never having been deciphered, making it unlikely that any of the glyphs are properly known as "bird-man".
As far as "bird-man" being the term for anything, it's the name of the winner of a traditional annual contest held on Easter Island - he becomes the "tangata manu", which translates into "bird-man". There are some religious elements to it, but overall it sounds more like an Ironman competition. The winner of the contest is considered "sacred" for only the first five months of the year in which he receives and carries the title.
http://en.wikipedia.org/wiki/Tangata_manuUnless he or she was speaking Rapa Nui-glish, I'd think it unlikely that a native Rapa Nui speaker would use the word "bird-man" over the name "tangata manu". This is especially true a thousand years ago, centuries before anyone on Easter Island was exposed to English, despite your claims to the contrary.
http://en.wikipedia.org/wiki/Decipherment_of_rongorongohttp://www.omniglot.com/writing/rongorongo.htm"Hey, kids, for just a small fee, you can be writing Rongorongo in no time! First hundred callers get a free batch of Sea Monkeys®!"
http://www.myfonts.com/fonts/deniart/rongo-rongo/a-regular/
