Bald Eagle
(Haliaeetus leucocephalus)
# Legal status in U.S.: Threatened
# Legal status in WI: delisted
# 1991 numbers in WI: 900
# Length: female 34-43 inches male 30-35 inches
# Wingspan: 7 feet
# Weight: 8-14 pounds
Description
Bald eagles are well-known as our national symbol. The adult eagle's most distinctive characteristic, its white head, is described by both its common name, "bald," and scientific name, "leucocephalus," which means "white head." An adult bald eagle's white head and tail contrast strikingly with its dark-brown body. Its large bill, feet and eyes are yellow.
Immature bald eagles are harder to identify. They are dark-brown with only spatterings of white on the underwings and tail. Head and tail feathers don't turn white until the birds are four or five years old. Immature eagles' feet are yellow, but unlike the adults, their bills and eyes are brown. Thus, it is easy to confuse immature bald eagles with large hawks or golden eagles. Bird identification guides describe ways to tell these birds apart.
Food
Bald eagles feed primarily on fish. In Wisconsin, the most important species are suckers, northern pike, muskellunge and bullheads. Eagles scavenge dead fish along shorelines and capture live ones with their feet. Their talons are strong and sharp, good for grasping prey. Occasionally, eagles eat waterbirds or mammals. Often these are carrion, like roadkilled deer.
Breeding Biology
Bald eagles are sexually mature when four or five years old. A courting male and female will engage in aerial displays (soaring, chases, dives, siderolls) that are thought to help cement the bond between them.
During the breeding season, bald eagles establish and defend territories. Mated adults usually return to the same breeding territory each year. Thus, a newly mated pair must find a territory that is unoccupied. If one member of a pair dies, the other will remate. The territory must have a suitable nesting site (4 large tree) and be near water with an adequate supply of fish. Eagles also require isolated areas, for they are easily disturbed by human activities.
In February or March, Wisconsin bald eagles begin building a nest or repairing one they built a pervious year. They usually build in a tall tree, often a live white pine. Nests are made of large sticks and other vegetation, the sticks forming the shell and more flexible greens lining the inside platform. The average nest is four feet in diameter and three feet deep. Eagles will build more than one nest in their territory, although they use only one each breeding season. If the pair reuses an old nest, they add new materials to it. A nest may become so big and heavy that it makes the tree susceptible to breakage during storms. Sometimes the entire nest will blow down.
The female eagle usually lays two white, three-inch long eggs in late March or early April. Both adults incubate the eggs starting when the first one is laid. The first egg hatches about 40 days later. Both parents feed the downy white chicks. They carry fish back to the nest, tear off bits with their beaks and place the food in the chicks' beaks. Eggs and chicks are vulnerable to the effects of severe weather and predation from great horned owls, ravens and raccoons. Many chicks don't survive the first year.
When three months old, the young eagles are able to fly. They exercise their wings and soon leave the nest. For several weeks, they stay near the nest site and the adults as they become familiar with the area and how to find food.
Distribution
Bald eagles live only in North America. Historically, they inhabited the entire continent wherever there were adequate nest sites and an abundant supply of fish. However, due to human activities, bald eagle populations have declined dramatically throughout most of the species' range. While small numbers of eagles presently nest in many regions of North America, the largest breeding populations are in Alaska and Canada. In Wisconsin, bald eagles nest along the shores of inland lakes and rivers. Their largest breeding concentrations are in the northern third of the state.
Bald eagle distribution varies with the seasons. In southern states, eagle breeding and wintering range can be the same. Eagles that breed in northern states, however, move south as northern waters freeze. They often congregate and share communal roosts and feeding grounds. While biologists don't know where Wisconsin's bald eagles overwinter, they suspect the birds move south where there is open water, concentrating along the Mississippi and Lower Wisconsin rivers.
History in Wisconsin
Until the 1800s, bald eagles bred throughout Wisconsin. As the state was settled, eagle populations began to decline. Habitat disturbance and destruction, and shooting of eagles were major causes. With the passage of the Migratory Bird Treaty in 1916 and the Bald Eagle Protection Act in 1940, it became illegal to shoot bald eagles. But enforcement of the law was weak. By 1950, eagles no longer existed in the southern two-thirds of Wisconsin.
Eagle populations remained stable in northern Wisconsin until the 1950s, when use of pesticides like DDT became common. Pesticides accumulated in animals that fed high on the food chain (like eagles, which eat fish). High pesticide levels caused physiological changes in eagles that had serious impacts: eggshells became thin and broke under the weight of incubating adults and parenting behavior changed. The bottom line was that eagles were unsuccessful at rearing chicks. They were unable to produce enough young to offset adult mortality, and the population plummeted.
In 1972, bald eagles were placed on the Wisconsin Endangered Species List. The same year, the federal government banned the use of DDT and other organochlorine pesticides in the U.S. But eagle populations were slow to recover. The number of bald eagles breeding in Wisconsin has gradually increased from 82 pairs in 1970 to 414 in 1991.
Other factors still cause bald eagle mortality. Wisconsin's eagles are sometimes caught in trap set for other animals, shot illegally by misinformed people who dislike birds of prey, electrocuted when they perch on power lines or poisoned by lead ingested when they eat waterfowl that have lead shot pellets in their bodies. Recent passage of a Wisconsin law banning use of lead shot should help reduce the problem of lead poisoning. Habitat destruction and disturbance also remain major problems for bald eagles.
Current Status in Wisconsin
Bald eagles were placed on the Federal Endangered Species list in 1973. However, since Wisconsin's eagle population is higher and more stable than that of most other states, the federal government listed Wisconsin's eagles as "threatened" in 1978. Bald eagles are protected by both state and federal laws. Violators face penalties of up to $20,000 in fines and/or 1-5 years in prison.
In 1991, 414 active territories were located. The recovery goal (360) was exceeded. The eagle's state status was upgraded to "threatened" in 1989. Eagles will still receive the same protection as other listed species.
Research and Management
Since 1963, the U.S. Forest Service has made annual surveys of bald eagles on federal forest lands in Wisconsin. Other annual surveys by the National Audubon Society (1967-1973), the U.S. Fish and Wildlife Service (1973-1978), the Wisconsin DNR (1978 to present) and the National Wildlife Federation (1979 to present) have provided information on the status and distribution of bald eagles in Wisconsin.
In 1986, a Bald Eagle Recovery Plan was approved by the DNR. The plan's main objective is to increase the self-sustaining population of bald eagles in Wisconsin to 360 breeding pairs by the year 2000 with an average annual productivity of at least 1.2 young per occupied nest. The goal has been reached through work to study current population and habitat status; determine the population and habitat required to achieve recovery; protect, enhance and increase bald eagle populations and habitats; and establish communication and education networks to coordinate recovery efforts.
SOURCE:
http://www.dnr.state.wi.us/
Linear Mode
