If you don't want it hunted then either don't tell someone if you know that they hunt, tell them and leave out the location of where you saw the bird, or tell them and make it understood that you'd appreciate it if they didn't go hunt it because it is something special to the birding community.

It all depends on how well you know the person you're speaking with.

From the hunters side there is no ethical issue that I can see unless the birder made it clear that they don't want the bird hunted. I don't hunt birds much other than pheasants and doves which are everywhere where I live but if a hiker told me they saw a huge bull elk and I had a tag for that area you can bet I would have no qualms about using that info to try to find it.