On the margin of the lake where the feeder streams converge are a string of beaver ponds like tourmaline jewels. We paddle softly as we approach, portage neatly over the dam. We hardly notice its intricate web of mud and sticks, how with a minimum of materials it holds back the current and flattens it into a pool. We're not here to appreciate beavers (they're so secretive we rarely see them). We've come to the beaver ponds on this spring day to see ducks.
The trees sticking up above the water are just stumps. Reeds and marshgrasses make a mazy jungle, cut through by beaver channels and the old course of the stream. We punt with our paddles submerged, poling on the mucky bottom, portage another low dam. Red-winged blackbirds sing upp-sa-lee from the tips of cattails. The whistle of wings from behind—we whip our heads around to see a female wood duck skedaddle down the lake. A dead tree near the edge of the pond has a prominent hole that could be her nest. A rustle in the reeds—the dark bulk materializes into a goose, her head low and snaked along the water. A little weight on the paddle and our canoe glides into full sight. The goose is motionless, thinking herself unseen behind a tussock. We avert our gaze, push past, and then, when we're almost around the bend, look back: yes, she's tight on her nest, there along the streambank. We feel a little shiver, a frisson of pleasure, to have snuck past a sitting goose, to have disturbed nothing, yet seen so much.
Beaver ponds are fine places for waterfowl, a recent study funded by the Pennsylvania Game Commission and the Penn State Cooperative Wetlands Center has confirmed.
"The Game Commission was particularly interested in waterfowl broods—mothers and their chicks," says Diann Prosser, a graduate student in wildlife ecology who with Robert P. Brooks, professor of wildlife and wetlands, investigated beaver pond succession—the stages a pond goes through during its life span.
Although the last native beaver in the state was trapped in 1912, the familiar paddle-tailed rodents were reintroduced in 1917. Today there are 32,000 beaver, the Game Commission estimates, and, according to Prosser, 6,500 beaver ponds.
Beaver ponds are active for about 30 years. The first stage (which Prosser calls "new active") begins when a stream is dammed and a pond forms. The trees and bushes, their roots drowned, give shade and leafy cover. Eventually they die and rot (or are cut down and eaten, depending on their size and species). Then the beavers must travel further afield to forage, and the dam is widened and the pond enlarged, during this "old active" stage. Trunks and stumps dot the pond, but few shade trees remain except on the edges. The pond is carved with channels, a mix of open water and shrubby hummocks. After the beavers leave the "abandonment" stage—the dam eventually breaks and the water subsides. Grasses and shrubs recolonize the pondflats, and slowly it returns to woodland.
Prosser and Brooks surveyed beaver ponds in all three stages, looking not for beaver but for birds. They found all six of Pennsylvania's common waterfowl breeding on beaver ponds: Canada goose, wood duck, greenwinged teal, American black duck, hooded merganser, and mallard. "New active" ponds and "old active" ponds produced the most waterfowl. Geese seemed to prefer the older, more open ponds; while wood duck, hooded merganser, and black duck liked newer ponds with more cover.
Marsh and song birds also frequented beaver ponds. The American bittern and Virginia rail, both secretive waders, were found in older ponds, as were the alder flycatcher and redwinged blackbird. The Louisiana waterthrush and Acadian flycatcher visited active ponds; the swamp sparrow, common yellowthroat, and veery lived in all three pond habitats.
"A beaver pair's goal in building a dam is to create a pond where they can build a lodge, hide from predators in the water, raise offspring, and store food for the winter," says Prosser. "In the process, they are creating a variety of wetland habitat for waterfowl and other birds."
Diann Prosser received her master's degree in ecology in August 1998. Robert P. Brooks, Ph.D., is professor of wildlife and wetlands, School of Forest Resources, College of Agricultural Sciences, 301 Forest Resource Lab, University Park, PA 16802; 814-863-1596; rpb2@psu.edu. Their research was funded by the Pennsylvania Game Commission and the Penn State Cooperative Wetlands Center. Reported by A'ndrea Elyse Messer.