Examples abound of cooperative feeding in animals – a pack of wolves that work together to chase down a deer, birds that distract the parents while their comrades raid a nest – the list goes on.
Recently I watched eared grebes (Podiceps nigricollis) on Eagle Lake (Lassen County CA) cooperate while feeding. In the fall small tui chub (Gila bicolor) minnows cruise the shore of Eagle Lake. American white pelicans, western grebes, American coots, and pied-billed grebes, among other waterfowl, feast on these little fish.
Often the eared grebes swim along the shore singly or in groups of two or three, diving every so often for a minnow. I watched these small groups gather together about fifty feet off shore until there were approximately twenty or thirty grebes. The grebes would then form a loose arc and swim toward shore. As the eared grebes approached the shore the water would roil before them with lots of bubbles and fish jumping out of the water. The grebes were herding the tui chub into the shallows at water’s edge. Once the fish were concentrated, the grebes began to feed. With almost every dive each grebe returned to the surface with a minnow. With time, the group of grebes would either disperse or reform out on the lake to begin herding the fish again.
By cooperating, the eared grebes were able to concentrate their prey and obtain more food with less expenditure of time or energy than they could alone. Pretty smart! On other days I watched pelican groups do the same thing. They used a slightly different tactic to herd the fish, however the result was the same.
One picture shows a group of eared just as they began to feed. The frantic minnows are creating the bubbles in front of the grebes. A tui chub jumping and a school of chub under the water show the grebes’ prey.
Pictures of pied-billed grebes eating fish can be seen here in an earlier post.