The bald eagle (Haliaeetus leucocephalus) is one of the largest raptors in North America. This bird of prey is well recognized as the national symbol of the United States and is identified by its bright white head and tail, its large, dark brown body and wings, and its yellow bill and feet. In addition, fish comprise a large portion of the bald eagle's diet, and so it is no surprise that bald eagles are often found near lakes, reservoirs, marshes, rivers and coasts.
The changing seasons bring many challenges to a bald eagle's habitat and food availability. This is a similar story for many birds, and bald eagles are just one of many migratory birds species found in Colorado. Some birds arrive and stay for the summer. Some birds just migrate through, not even staying for a whole season. But the bald eagles will generally spend their summers in Alaska and Canada but shift south and spend their winters in the lower 48 states.
Bald eagles have complex migration patterns that depend on age, breeding location and food availability. Adult bald eagles in Alaska and Canada will begin their fall migration when northern lakes and rivers freeze and food becomes scarce. Compared to their spring migration northward, bald eagles seem to travel at a leisurely pace in the fall, sometimes remaining in an area for a week before continuing their journey. Adult bald eagles do not migrate with juveniles who are actually entirely dark brown in color and do not get their "bald" white feathers until their fourth or fifth year. Young eagles will migrate before their parents, and some will wander in a wide range during their first few years.
To conserve energy during migration, eagles soar on columns of rising air called thermals. After using a thermal to reach higher altitudes, an eagle will glide in the direction of its migration until it finds the next thermal to repeat the process.
During winter, bald eagles gather in large numbers at communal roosts and feeding sites. In Colorado, look for them hunting around unfrozen lakes, reservoirs and rivers. According to Colorado Parks and Wildlife, there are anywhere between 400 and 1,000 bald eagles that spend their winter in Colorado.
However, this has not always been the case. Although they have been protected in the United States since the Bald Eagle Protection Act of 1940, bald eagle populations have significantly declined as a result of habitat loss, chemical pollutants and illegal shooting. Perhaps the most notable impact on bald eagle populations occurred after World War II with the increased use of DDT on American farms. This pesticide was deposited via runoff into lakes, streams and rivers and then gathered in fatty tissue of fish. Many eagles feeding on these fish were unable to reproduce while other eagles produced unhealthy eggs that were so weakened by the pesticide that they cracked under the weight of nesting adults. In the 1960s, there were approximately 417 eagles that survived in the lower 48 states, and bald eagles were declared an endangered species.
Fortunately, with increased awareness and conservation efforts, including the banning of DDT, bald eagle populations have made a successful comeback. In 2007, populations had significantly recovered with nearly 10,000 bald eagles nested in the lower 48 states, and the bald eagle was delisted as an endangered species.
In the Eagle Valley, keep a look out for bald eagles that have migrated here for the winter, especially in the tree-lined areas near the Eagle River! Let their presence remind you that despite how fierce and strong life may appear to be, existence is a fragile thing and our human impact does not go unnoticed by the natural world.
Theresa O'Halloran is a naturalist at the Walking Mountains Science Center.