This morning at breakfast, Betsy and I watched a flock of about 50 Red-breasted Mergansers hunting and displaying to one another in back of our house. They were diving and coming up with small fish, sometimes the whole flock disappearing underwater for half a minute or so. They were also performing mating displays, which were a lot of fun to watch. The males extend their heads as far forward and upward as possible, then they rock their body forward submerging the base of the neck and elevating the rear end. At the same time they let out a "cluck" sound with a wide open beak. The females do the same thing, though not as frequently.
We have also had a small group of 2 to 6 Common Mergansers in the "backyard" lately. It's wonderful to see the two types of mergansers right next to one another for comparison. The males are easy to distinguish from one another, but the females are a bit more difficult. When you see them together, though, it's quite clear that the white patch under the chin of the female Common Merganser is a good field mark to distinguish them from the Red-breasted females.
Another interesting aspect of these large groups is that mergansers hunt cooperatively. You will see them all headed in one direction, like a pack of wolves. Suddenly they discover some prey ahead – perhaps a herring ball. They paddle furiously forward to a certain point and dive underwater as a group. A short time later they pop back up to the surface, most of them eating small fish they just captured. We see similar hunting behavior in groups of Neotropic Cormorants in Arizona.
