The Law of the Mudflat

By Terry Morse

It's a jungle out there, as you can plainly see by watching the antics of birds on Oregon mudflats in August. Intimidation and thievery are rampant. Crows steal from crows, terns from terns, and gulls steal from everyone indiscriminately.

Caspian Terns, elegant white gull-like birds, with a dark cap and bright orange-red bill, have found a way to circumvent this larceny when feeding their fledged young. It starts when an adult tern flies up off the mudflat, faces into the wind, and hovers over shallow water scanning for finger-sized fish. Junior is parked among a swarm of gulls, terns, and shorebirds on the mudflat, mewling piteously. After one to several plummeting dives, a fish is snagged. Holding the fish crosswise in its bill, silver scales flashing in the sun, the successful hunter flies back with its catch.

The trick now is to transfer the fish to Junior without another tern or a gull stealing it. Around high tide, when most of the mudflat is under water, the mixed-species flock can become tightly packed. Attempting the exchange in the thick of the horde is an invitation to thievery. Instead, the adult tern repeatedly flies over the assembly in long circles, crying "ka-ka-kow, ka-ka-kow!" A few gulls and terns may take off after it, hoping to steal the fish.

Eventually, a juvenile tern, presumably its own offspring, rises up from the horde and follows the adult, whistling "THREE-you" after each of the adult's short, croaking "kaows." "Kaow!" "THREE-you!" "Kaow!" "THREE-you!" The pair flies away from the mass of birds, again often followed by several gulls and terns, until they finally shake their pursuers and land on an isolated spot of mudflat, or in nearby shallow water. The adult passes the fish to the young one and either returns to the flock to rest or resumes fishing.

The youngster then swallows the fish, unless, of course, (s)he drops it. Clumsy and inexperienced, juveniles occasionally lose a fish to a gull or adult tern, but this is not common. The system works pretty well. Junior then flies back to the thronging birds to await the adult's next success. All the while, other parent-offspring pairs have been doing the same. Since the adults rest frequently, only a few are actively fishing at any one time.

Thievery and its avoidance by terns can be seen on Oregon estuary mudflats from late July through early- to mid-September, when adults with young stop over to feed before migrating south for the winter. Either side of high tide is the best time to observe it, since the gulls and terns are concentrated in a small expanse of mudflat and the adults are fishing close to shore. Extreme high tides are not so good, because the birds are widely dispersed offshore.

When large areas of mudflat are exposed at lower tides, the birds spread out and the reduced risk of thievery allows the exchange to take place wherever the juvenile happens to be. If you don't have Caspian Terns where you are, similar behavior has been reported in related species, including the Common Tern, so you may still get to see it.

It may not have trees and tigers, but the mudflat is still a jungle.

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