I used to work for a Professor whose academic homepage listed among his interests “sperm production”. If you want evidence that female animals can be as promiscuous as male animals you should have a browse through the literature on sperm competition.
In the case of ducks, that promiscuity is not necessarily the females’ choice, so they discriminate between mates by other means. One result of this is that male ducks have extraordinarily long bits. Sadly, no one believes a man when he talks about penis size, so here’s to a link to an article about woman who’s an expert on duck phalluses [requires free New York Times registration]. Some of them really are longer than the whole of the rest of the duck.