A bald eagle sits just above its riverside nest in the Fox Valley. / Submitted by Patricia Fisher
It seems like it was only yesterday that bald eagles were gathering by the dozens along any open water they could find on the Fox and Wolf rivers.
Now, just a few weeks later, most of these birds have moved on. Adults of breeding age have been nesting in our area for several weeks already, while bachelor birds continue to move about, sometimes to the dismay of their parents.
Bald eagles in east-central Wisconsin began nest repair and construction last month, with several birds already sitting on eggs in massive nests of branches and small logs constructed in the crotches of towering hardwood trees.
Like great horned owls that began nesting even earlier, bald eagles must incubate and begin rearing their young early in the year in order for them to mature in time for fall and early winter migration.
While snow still blanketed their breeding territories, bald eagle pairs began to reclaim these precious and traditional nest sites during late winter, perching on and around the nests and fighting off any other birds that dare get too close. This includes their young from the previous year, who sometimes have a hard time cutting the cord.
Eagle pairs often will just perform some light winter housekeeping on last year’s nest if they are still together, while new breeding birds will construct nests from scratch once they find and begin defending territories.
The majority of the eagles spotted here along open water during midwinter are migrant birds from northern Wisconsin into Canada. These birds are drawn to the availability of fish, on which they primarily feed, as well as waterfowl such as wintering ducks and geese.
Open water was so lacking this winter that often eagles were spotted far inland feasting upon deer carcasses along roadways and in snow-covered fields.
Now that breeding season is underway, one of the pair usually takes up incubation duties while the other feeds or roosts nearby. Not wanting to draw attention to the nest, but eager to keep a close eye on his mate, the male often perches up to a mile away from the nest site, keeping close eye on any potential threat.
Built to last
Bald eagle nests can be gigantic. The largest nests, constructed of large branches and even small logs, can hold at least one full-grown man.
Bald eagles are heavy birds, the largest females weighing nearly 15 lbs. With two or three hungry eaglets, rapidly growing inside the nest, the structure has to be able to withstand all the weight, in addition to windstorms and other weather conditions.
Nests are usually constructed in large trees, often quite high, and along the edge of open water. However, there are always exceptions. Bald eagles may also nest on rocky cliffs, stumps or buildings in some locations. They have also been discovered quite a distance from any obvious open water source.
The family tree
Eggs are incubated for about 35 days. Both birds will take turns on the nest. Normally, two eggs are laid each season, though sometimes there may be larger clutches.
Depending upon where they locate their nest, winged predators such as crows, ravens, gulls, even herons, are a big threat and the pair is constantly on the lookout. Squirrels are also threats to the precious eggs.
By the time they hatch, food source availability has increased and eagles are able to provide a wider arrange of prey items for the young. Upon hatching, the young are hungry, and both birds take on parenting chores.
The pair care for the young throughout spring and early summer, as the young feed voraciously, begin to test their wings and finally leap into their first majestic flight over their homeland.
By late-summer and into fall, the young eagles are sometimes even larger than their parents, colored a rich, deep brown with white or light gray patches.
This is the plumage the immature eagles will carry for up to three years before acquiring the symbolic, glistening white head and tail of a full-fledged breeding adult.
A record year
This year, the number of nesting pairs of bald eagles in Wisconsin could surpass 2,000, a modern record. The story of the comeback of this magnificent bird is miraculous.
In the early 1970s, before the ban of DDT, which caused eggshell weakening, preventing the eggs from hatching, there were just over 100 bald eagle nests in the entire state.
A slow and steady comeback ever since has brought a greater number of bald eagles into the area each season, and, as more and more birds reach breeding age, typically their fourth year, that number will continue to grow.
Along the many rivers and lakes in our area, bald eagle numbers continue to build, with new nest sites showing up each season. These nest locations are easy to spot in late winter and early spring when the trees are bare, but quickly become hidden as the canopy fills in during mid to late spring.
Young birds that have not reached breeding age will spread out and explore new areas, sometimes alone, sometimes in groups. Because bald eagles may live up to 30 years in the wild, the eagle pairs currently residing in our area will continue to bring more and more of these stunning birds into the skies of the Fox Valley.
— Rob Zimmer, Post-Crescent staff writer, writes about nature every Tuesday in his Nature Calling column. He is reachable at 920-419-3734 or firstname.lastname@example.org. Follow him on Twitter @YardMD.