I mentioned this phenomenon on PABIRDS and got a huge response so I decided to go into more detail here.
On Sunday at Moraine State Park (Fine Weather For Ducks) I heard a frog-like sound coming from the area where the ducks were swimming. Using Cornell Lab of Ornithology's website I figured out the sound came from the hooded mergansers. The male is pictured at left, the female at right.
According to Cornell Lab, hooded mergansers are silent except in courtship when the male gives a deep rolling frog-like sound, recorded here at the end of the clip.
Amy Taracido says their voice sounds like "merg merrrrrrrrg" - a perfect description and one that makes me laugh when I say it.
Besides the sheer amusement of hearing a bird make this noise, it's curious that this is considered a courtship sound. We generally assume birds don't court in the winter, so from our perspective the hooded mergansers cannot possibly be courting on December 9th. But why not? Why does courtship have to be confined to spring?
The flock was large and about 50/50 males and females. On migration southward, the hooded mergs might be in "dating" mode, especially the young ducks who haven't bred yet. Flocks would be the perfect place to meet the opposite sex and decide on a mate. Mergansers might even need some get-acquainted time before they make a choice.
So I guess Lake Arthur is The Bar Scene for ducks.
(photos by Chuck Tague)