Decoding Bald Eagle Migration

Approximately 200 pairs of bald eagles reside in Colorado, with the majority of breeding pairs remaining in the state year-round. These birds rear their young in the spring and summer, and then migrate to more northern latitudes in the late fall and winter. Why, then, does the Colorado bald eagle population surge to well over 1,000 birds during this period?

The most obvious answer to the question of why some birds migrate and others do not is that migration is a complex phenomenon. The question arises as to why some eagles migrate while others do not. The concept of migration is typically understood in terms of north or south, with the direction depending on the time of year. For a significant proportion of birds, including many species of birds of prey, this is indeed the case. In the case of bald eagles, however, the situation is not as straightforward. The decision of whether a bald eagle will migrate or remain in its current location is likely influenced by a number of factors, including the age of the bird, its habitat, and the availability of food sources. These variations in the nature and timing of migration are referred to as "movement strategies."

We will commence with the subject of age. A review of studies that have fitted satellite transmitters to bald eagles has revealed that young birds are more likely to travel significant distances during migration and to depart their home range at an earlier stage than migrating adults. It is during this initial migration or two that young eagles are likely to establish a pattern in their annual movements. Bald Eagles that are migrating often retrace their previous migratory routes with each journey. However, the distance they travel tends to decline with each passing year until they reach maturity at approximately five years of age. At this juncture, the location and distance traveled from the home range become relatively consistent.

It is now necessary to revisit the fact that some breeding-age eagles migrate, while others do not. First, it is necessary to distinguish between breeding-age eagles and birds that are actually part of a breeding pair that will attempt to nest in a given year. The former are designated as "floaters," while the latter are classified as "breeders." Individuals designated as "floaters" typically exhibit a tendency to wander within the geographic region where they hatched. Additionally, they engage in seasonal migration along routes that were established during their juvenile years. This wandering behavior is likely associated with the search for suitable nesting habitats and potential mates. Indeed, when a member of a nesting pair of eagles dies or otherwise fails to return to the nest, floaters have been observed to move in swiftly to assume the role of the missing mate, with the process sometimes occurring within a few days.

What about the latter group, the breeders? In certain areas of their range, such as Colorado, breeders typically remain in close proximity to their nesting territories throughout the year. In other regions, the species exhibits migratory behavior in accordance with seasonal changes. This discrepancy in behavior is contingent upon the availability of sustenance. In the northern reaches of the bald eagle’s range, prey becomes exceedingly scarce due to the extreme cold, deep snow, and the dearth of open water from which to catch fish. It is noteworthy that the eagles do not necessarily migrate southward; rather, they tend to head toward the closest reliable food source. For some, this may entail undertaking a westward or northwestward migration to reach the more temperate regions of the Pacific Northwest, where they can find open water and salmon in large numbers during the fall run.

The eagles that spend the winter in Colorado each year are likely from a multitude of locations and have arrived for a plethora of reasons, as previously discussed. One unifying factor among these eagles is their ability to locate food sources during the winter months. While numerous lakes and reservoirs in Colorado are covered with ice for a portion of the winter season, they remain accessible for longer periods and thaw earlier than the regions from which the eagles are likely to have originated. The eagles have a variety of prey available to them during the winter months, including large numbers of migratory waterfowl, abundant cottontail rabbits, and prairie dogs. Although migration confers numerous advantages upon birds undertaking the journey, it is not without inherent risks. These dangers may be of a natural or man-made origin. Natural hazards include storms or other extreme weather, as well as the potential that the food source driving migration fails to materialize on the wintering grounds. Man-made hazards can take many forms, including collisions with wind turbine blades, electrocution from electricity generation and distribution, exposure to lead, and even direct persecution.

It is possible to reduce the impact of these risks through the collaboration of industry and the implementation of regulations designed to safeguard eagles. The bald eagle is afforded protection under the Migratory Bird Treaty Act (MBTA) and the Bald and Golden Eagle Protection Act (BGEPA). The recent proposed amendments to the MBTA could result in an increased risk for hundreds of migratory species. Eagles are fortunate in that they have an additional layer of protection through the Bald and Golden Eagle Protection Act (BGEPA), whereas the majority of other migratory birds lack such additional legal protection. In conclusion, it can be stated that robust legislation is a prerequisite for the conservation and protection of North American migratory bird populations.

On a more positive note, there is still an opportunity to observe wintering bald eagles in Colorado. Barr Lake State Park is an excellent location for this. On any given day from November to March, there may be as many as 100 bald eagles soaring above the lake, perching in the giant cottonwoods on the shoreline, or loafing on the ice. If you do visit, it is advisable to keep an eye out for the variety of plumage stages that you are likely to see. This will enable you to gain insight into the bird's previous locations and the reasons behind its current presence.