My Ducks Don't "Quack"
And I don't think it's only my ducks who can't seem to pronounce "quack." It seems to me that ducks are universally incapable of enunciating the "qu" sound. Instead they pretty much just say "wank wank wank," (heavy on the "wa" and light on the "k") which has led my husband and I to chide our ducks with the nomiker "wankers." Considering that this term has somewhat lewd connotations in the UK, I guess I can understand why biologists have agreed upon the term "quack." (As an aside: Here is a great chart listing the accepted mnemonics of bird calls).
Besides, it really wouldn't do to have a class of schoolkids in the UK singing "Old McDonald Had a Farm" with a "wank wank here and a wank wank there, here a wank, there a wank, everwhere a wank wank," eh?
Note: I posted this photo of a mallard drake because I really love it. This photo was brought to you courtesy of PD Photo.org.