Oh, and Easter Island is a special case. It is a terribly degraded environment, and it would be no surprise at all to discover that there were many other birds there before humans arrived and started breeding, breeding, breeding.
Easter Island was densely forested, but when man arrived, the trees were cut down one by one, the birds and their eggs were eaten, and eventually there were no trees left at all.
With no wildlife left to eat, and no wood left with which to build boats to go out fishing either, starvation set in on a massive scale. The only convenient source of protein that remained was human flesh. Society collapsed, the human population crashed to a fraction of its former size, and Easter Island now hosts just a handful of species.
It's a fair bet that there were any number of endemic plants that are now wiped out, and who knows what birds lived in the forests?