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

9. The Oxbow Bend

the start of their nesting, although last year’s nest still perched conspicuously on the top of a tall lodgepole pine, only a few dozen feet from a group of equally large and also uninhabited great blue heron nests. Few of the herons had returned to the area, and none had yet attempted to establish territorial rights. Thus, the eagles and the ospreys maintained almost exclusive fishing rights to this stretch of river. To be sure, a few common mergansers sometimes patrolled its edges, and a frolicking family of otters occasionally passed downstream, but there was more than enough fish for all. The bald eagle nest was a ponderous structure, about sixty feet up in an old, slowly dying cottonwood, which was being gradually undercut by the swift river. In March both eagles began adding to and refurbishing their nest. They collected broken branches on the ground, or more frequently flew full force against dead branches of standing trees, breaking them off with their feet and carrying them to the nest. The largest branches, as much as three feet long and two inches in diameter, were held by the talons, while smaller ones were carried in the beak. After establishing the new nest platform, the eagles added dead grass, weeds, and clods of earth that were incidentally attached to roots. Flying over the nest with a bundle of grass in his claws, the male would drop it into the nest while the larger female would work it into the interstices of the branches and twigs. Within a few days they had completed a mattress-like layer of grass and straw around the raised margin of the nest surface, and by the beginning of April the nest was finally ready to receive its first eggs. Each morning during that week the female perched for a time overleaf. Three bald eagles share a tree on neighboring perches above the Lamar River, Yellowstone National Park, Wyoming The OxbOw bend 99 on the tallest branches of the cottonwood . Soon the male would fly up to her, land lightly on her back, and mate with her while flapping his wings for balance. Then the female would return to her nest to further work the lining into a comfortable cup for the eggs. She laid her first egg April 4 and two more at intervals of three days each, so that on April 10 a full clutch of three dull white eggs were being warmed beneath her. After laying the first egg the female remained on the nest nearly constantly . Whenever she left it for any period, the male would harass and dive at her, driving her back to the nest. Soon, however, he began to help with incubation. Changeovers in nest-tending were achieved by attracting the mate to the nest with a simple chittering call. For more than a month the two eagles guarded the nest assiduously. The first egg hatched on May 9, thirty-five days after being laid. The second hatched two days later, but the third embryo had died during development. As the two eaglets dried, their smoky gray body down contrasted with whitish down on the head, chin, and underparts. Weighing only three ounces at hatching, the chicks opened their eyes a few hours later and almost immediately begged for food. When the first egg hatched, the male began to bring in food, primarily suckers and other rough fish. After dropping it into the nest both parents tore the fish into tiny bits and gently passed them to the hungry chicks. Within a week, both eaglets had grown substantially and had even begun to wave their stubby, down-clad wings. There was plenty of food for both eaglets, but the earlier-hatched bird, a female, was always fed first and soon was substantially larger than her broodmate. For the first few weeks after hatching, the female closely brooded her offspring each night, raking up the dead grass and leaves around her huddled body as the sun dropped behind Mount Moran. Her mate spent the night about twenty feet away on a higher branch of the same tree, his white head catching the last rays of the setting sun and the feathers tinted reddish in the evening afterglow. During the day each adult alternately foraged and tended the nest. By the end of May both eaglets were large enough to peer over the rim of the nest and watch...