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

8. The Pond

As the pond became progressively ice-free in early May, the waterfowl and shorebirds that had been using Jackson Lake and the Snake River flocked to the food-rich waters of Christian Pond. Mallards, pintails, American wigeons, buffleheads, Barrow’s goldeneyes, and ring-necked ducks were suddenly everywhere and were quickly followed by coots and yellow-headed blackbirds. White-crowned sparrows took up territories in the peripheral sage-covered hillsides, while yellow warblers and common yellowthroats claimed the willow thickets. Yellow-rumped warblers sang from the small cluster of conifers at the north end of the pond, and cliff swallows swarmed over the pond by the hundreds. When not feeding, they collected mud along Christian Creek to construct their nests on the underside of the bridge that crosses the creek. The trumpeter swans, which spent most of the winter near Oxbow Bend of the Snake River only a mile south, moved onto the pond as soon as it started to become ice-free around the lip of the beaver dam. They immediately began investigating the dense stand of bulrushes at the southern end of the pond. Every year for more than two decades the swans had nested on the pond, usually on a mat of bulrushes from the previous year. They had raised dozens of young, which often remained in the same general area and sometimes tried to visit their parents’ nesting pond. Although the pair freely mixed with their offspring on the wintering ponds, they would not tolerate them on their nesting territory and greeted their recognition calls with threats or even chases. Even less tolerated were “stranger” swans that sometimes flew overhead or attempted to land on the pond. Immediately the male would utter a challenge overleaf. Trumpeter swan pair and cygnets, July, Madison River, Yellowstone National Park, Wyoming. The Pond 88 and take flight by running at full speed over the water toward the intruders. This was usually enough to force the birds into full flight, whereupon the male would circle back and land with a skidding stop beside its waiting mate. Facing one another in the water, they would trumpet loudly and wave their wings excitedly in a mutual triumph ceremony. By these displays the pair had managed to maintain a close bond that would last throughout their lives. As the female, or pen, began nest construction, she trampled the bulrushes into a well-packed mound. From this platform she reached out as far as her long neck would allow to gather additional bulrushes and add them to the mound. Soon she established a nearly bulrush-free moat around the nest as she added to its height, making it more conspicuous while providing 360-degree visibility. The male, or cob, assisted in a rather desultory way, generally swimming around the nest, gathering bulrushes from beyond where the pen on the nest could reach, and dropping them over his shoulder in the general vicinity of the nest. The pen then added them to the nest. Within a week the nest was essentially complete, just as the pond was becoming completely ice-free. Each morning the pair spent an hour or two foraging. They also usually mated sometime during that period. Indeed, to the average observer the mating preliminaries looked no different from foraging behavior, as the swans swam side by side and repeatedly immersed their heads and necks in the water. However, their behavior gradually became more and more synchronized and increasingly began to resemble bathing movements. Soon the female lowered herself in the water and the male moved on top of her, grasping her nape in his bill and lowering his tail to meet her uplifted one. As mating terminated, the pair rose strongly in the water, trumpeted loudly while spreading their wings, and then gradually settled back into normal swimming positions. The pen laid her first egg on May 10 and four more on alternate days thereafter. By May 20, the nest cup was filled with five white eggs. As egg-laying neared completion, the pen spent progressively less time away from the nest and began to add to it a sparse mixture of white down and breast feathers. Finally, after laying her last egg she settled down comfortably over the clutch and began her thirty-four-day incubation period. At the same time the male selected a mat of bulrushes...