Common mergansers (Mergus merganser) are year-round residents where Leonard and I reside in Northeastern California. Also called a fish duck, the diet of a common merganser consists mainly of fish, although they also eat some invertebrates, frogs, plants, small mammals and birds. They find their food by sight.
Recently I saw a single male common merganser sunning itself on a log at Crystal Lake (Shasta County CA). Eventually the merganser slipped into the water and began to behave in a strange manner. First the merganser would swim with its head slightly submerged, then suddenly shoot forward creating a large wake. After about 50 yards (I am terrible at judging distances!) the merganser would stop and lift itself out of the water while flapping its wings before beginning the sequence again. I watched the merganser repeat this behavior continuously for at least 25 minutes until I had to leave.
My first thought was that the merganser, with its head partially submerged, was looking for fish or other foodstuffs under the water. My second thought was that he was “herding” small fish into a group with his splashing before submerging to eat. But the merganser never completely went underwater or appeared to be eating anything. Perhaps this was some courting behavior? Common mergansers have courting rituals where they swim in circles and employ other antics to gain the attention of a female. But look as I may, there were no female common mergansers to be seen on the lake or, as far as I could determine, along the shore. Besides, several males often participate in courting displays and this was a solitary male. Here was another unexplained, confusing behavior.
A account of male merganser courting behavior written by Dr. Charles W. Townsend in 1916 described perfectly what I observed – “. . . one [merganser] with swelling breast and slightly raised wings sprints ahead at great speed. . . . The birds suggest swift motor boats by the waves which curl up on either side. . . ” Only this narration referred to several birds competing together.
Perhaps there was a female nearby and the male merganser was attempting to impress her. Maybe he was “practicing” for courting at a future time. Or perhaps I could not discern that he was simply feeding. Maybe someone with more knowledge about common merganser behavior can shed light on my confusion.