Yellowstone Wildlife: Ecology and Natural History of the Greater Yellowstone Ecosystem

12. The Cirque

yon, gaining strength from innumerable rivulets leaping down the mountainsides. A massive glacier once bulldozed down the same canyon, crudely gouging it into a broad, U-shaped valley. Now, however, the stream is only a few feet across in many places and choked with boulders and fallen trees. Small waterfalls commonly toss the stream into spray and foam. One of these is a free fall of about ten feet where a rocky outcrop slowed the cutting edge of the creek. A small gray bird occasionally burst out from behind the streaming water like a shot from a hidden cannon or just as surprisingly flew full force into the misty spray from downstream. The pair of dippers that had built their nest behind the falls ranged up and down a half mile of Cascade Creek, strenuously defending their territory from other pairs upstream and downstream from them. They were intimately familiar with every twist and turn of their part of the creek, but virtually everything beyond it was terra incognita. The dippers invariably followed the winding stream when flying up or down it and rarely flew more than a few feet above the swirling waters. During winter the dippers moved downstream toward Jenny Lake, but in spring they returned to their previous territory. Even before returning, the male began singing exuberantly . Indeed, both members of the pair sang loudly from January on, usually while perched on a rock in the middle of a stream, but the male was especially vocal while in the presence of his mate. At times they engaged in spirited chases, the male singing loudly while flying closely behind the female, which twisted and turned as if trying to escape, but which goaded the male to chase her whenever he seemed to tire of the game. Sometimes they collided in The cIrque 124 midair and tumbled together into the water, but mating occurred on a sandbar after the pair alighted there normally. Their nest had been in the same location for several years. The dippers simply repaired it each year by adding grass and moss. It was a simple globular structure about the size of a football , with a downward-pointing entrance hole that opened toward the falling water. In fact, the outside of the nest was constantly moistened by the spray. But within this soggy structure was a cup of coarse, water-resistant grasses and a dry inner lining of leaves. Within a few days of replacing the inner lining, which the pair had removed the previous summer as soon as the young had fledged, egg-laying began. One egg was laid on each of four successive days. Throughout this time, both dippers spent most of their time foraging in the rushing stream by flying headlong into the icy waters. As they entered the water they threw their wings out and back slightly and swam quickly to the stream bottom. They worked their way upstream by walking and using their wings, picking among the rocks for stonefly and mayfly larvae and occasionally also taking small fishes. When they had trouble finding food, they sometimes remained submerged for fifteen to twenty seconds but often popped back up to the surface after only five or ten seconds. After completing her clutch, the female abandoned her food searches and incubated full time. The male was forced to work ever harder to obtain enough food for himself and his mate, since he fed her on the nest while she incubated. With the hatching of the young after some sixteen days of incubation, the situation changed. The female gradually assumed the initiative in food-getting, although she also brooded a good deal of the time the first week after hatching. With room for only one adult on the nest, the female would rise on her legs far enough to let the young poke their heads out from beneath her to receive their food when the male appeared at the nest opening. Frequently he carried a fecal sac away from the nest, but within a week after hatching the young birds began to extrude their capsules directly out the nest opening into the water below. By then the young were extremely vocal, calling for food so loudly that they could be heard over the roar of the waterfall. Their dark, beady eyes watched every movement through the peephole in their nest. They had been in the nest for three weeks and...