Like Water Off a Ducks Back

All birds seem to spend an inordinate amount of time preening, but what is true of all birds is even more true of water birds. Waterfowl distribute oil from glands over the surface of their feathers for the purpose of waterproofing and, since they have to do it with the tip of their bill, it seems to take up a large portion of their day.

Distributed properly, water cannot penetrate the oil in the feathers and, hence, we get the saying, "Like water off a ducks back."

You can see in this photo of a male American Black Duck how well this works. This species is closely related to the Mallard and often interbreeds, creating hybrids that can make identification difficult.

Here is a Mallard male for comparison.

The actions of this female Black Duck dunking herself was freaking out a school of fish over on the left of the photo where you can see some of them jumping out of the way. These two breeds - mallards and black ducks -  are not particularly spooky (at least when hunting season is past) and they are a lot of fun to watch.