My Japanese friend uses the work ｢アヒル」when referring to ducks in a public park. However, another Japanese friend uses 「鴨」（かも）in similar situations.
In English, as far as my limited knowledge of the animal kingdom goes, we just have the word "duck". Can anyone explain the difference between these two words? Which is more common (and more correct) in the situation I've outlined? And do the rules change if we're talking about duck as a dish in a restaurant?