My reason for wondering about this is the stereotypical manner in which the flock of five mergansers circled the lake, sometimes clockwise and at other times against the clock. Almost always, one or two would lead the group, and the rest tailed off a little ways behind.

I displayed a photo of the leading duck, eyes under water, in yesterday’s blog post, which I reproduce here:



As they passed by our rear patio, the group coursed along quite rapidly, with heads or even half their bodies underwater. Suddenly, they would stop, and they would converge on a school of fish. The leading birds, which appeared to overshoot the school, would then retreat back in to join the others at the shoreline. (Of course, as I also noted, various combinations of the three species of herons would join them at that spot). The manner in which the trailing birds followed the leader was what made me realize that the group may not have simply been encountering random schools of fish. In fact, they often lined up in an irregular diagonal line, with the rearmost bird closest to the shore. Could they have been “herding” the fish?

Here are the two trailing mergansers in the group of five, mostly under water, and quite close to the shoreline. The Tricolored Heron is hurrying to keep up:

As I typed this, I thought of asking Google about it. I entered the search term “cooperative feeding in birds.”  To my great surprise, one of the first hits was Cooperative feeding behavior in Red-breasted Mergansers,  The Auk, October, 1965, Volume 82, number 4, Page 635, “General Notes,” in which Bayard H. Brattstrom of California State College at Fullerton made a similar observation. He stated that this behavior, while reported previously in cormorants, had never been described in mergansers.

“On the morning of 10 April 1963, at San Carlos Bay, near Guaymas, Sonora, Mexico, we saw seven Red-breasted Mergansers (Mergus serrator) behaving in a manner that suggested they were fishing cooperatively. The seven birds were feeding in a shallow arm of the bay in water less than 24 inches deep on an incoming tide. They swam in a loose line…, moving slowly, more or less in the same direction with their faces either under the surface (hunting?) or above it for short periods…

“When a hunting bird discovered a fish, it immediately gave chase, flapping its wings and running on the surface. The positions of the fish were apparently determined by the birds peering under the water at frequent intervals. As soon as one bird began a chase…, the others joined in the pursuit…, the nearby birds flapping and running along the surface, those more distant flying. In the few seconds it took the last bird to arrive, a semicircle was formed by the birds with the pursued fish in the center of the arc…

“As the birds chased the fish, one or another of them dived under the surface in pursuit, surfacing at about the time the others reached that spot. This diving continued in each case until one of the birds caught the fish. Immediately upon surfacing that bird would eat the fish. As soon as a fish was caught, the rest of the birds would begin to disperse and to hunt again. Presumably greater efficiency in fishing results from this type of cooperative feeding behavior, since a chased fish has a reduced number of escape routes, and more than one bird has the fish in vi ew at any given time…”