Now I understand that babies have limited repertoires as to what they can say — and this particular baby's utterances are limited to probably twenty words, half of which are Japanese, so I know the mother meant well and was just trying to reinforce to her kid a word she was probably already familiar with.
But still, it was as if to the mother all birds on the water of a certain size were ducks, even if they didn't have webbed feet, or even if their feeding and breeding habits were quite different from aforesaid quacking bird.
This, then, was how our "conversation" went:
Me to the mother, opening iPhoto: "Look at the Moorhens. They're doing a dance."
Mother to baby: "Look at the ducks. Can you say duck. Duck!"
Me (under my breath): "They're moorhens."
She (louder): "Duck!"
But it's not like the word is hard in Japanese, though I doubt that many people know it: ban (バン). I guess it was the lack of curiosity in the mother to even find out how or why a moorhen wasn't a duck that bothered me; that she didn't give me wink to say "yeah, you and I know that's a moorhen, but for now, for her, it's a duck."
I was all alone with a very primitive tribe of mountain Papuans, who were excellent hunters. I sent them out every morning with their guns, and for every specimen they brought back I asked, "What do you call this one?" I recorded the scientific name in one column, and the native one in another. Finally, when I had everything in the area, and when I had compared the list of scientific names and the list of native names, there were 137 native names for 138 species. There were just two little greenish bush warblers for which they had a single name (Mayr, 1955, p 5. taken from James C. King's The Biology of Race, 1981.)The biological species concept, put forward by Mayr, states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." Literally hundreds of alternate species definitions have been proposed in the past forty years, but I think we can take Mayr's definition as a good starting point, especially since I doubt many of my readers are biologists.
To recap then: Ducks and moorhens are separate species, because ducks and moorhens don't interbreed. They don't even hang out together, mitigating the risk of a mistake. I'm pretty sure they know their own kind.