Biologist Peter Nye's Notes About Eagle Migration and Weather
I've listed some general statements about eagle migration and weather below. When you make your predictions you should keep these things in mind. However, although much has been written about this, I'm not sure how much is speculative. Also keep in mind one major principle I've learned over the last 20 years working with bald eagles:
Just as soon as you predict their behavior, they'll break the "rule" and do something different!
Some of the most important factors to watch for:
Other factors to keep in mind:
I guess the main thing to keep in mind, whether it is migration we're talking about or some other behavior, is to remember that bald eagles are individuals just like you and me, and while many may behave in a certain common way, there will always be some who will behave in a different manner. This is just a fact of life. So, don't get upset if your predictions don't follow the "expected" patterns!
Also just another preface to this discussion of "migration". Some biologists do not consider or characterize bald eagles as true "migrants", preferring to describe their movements away from and back toward their breeding territories as "seasonal movements". This is because almost all bald eagles only move away from their nesting areas as far as they need to to survive, meaning in order to find the food they need to survive. A great many bald eagles (i.e. along the coasts and in more temperate areas like the southeast U.S., never leave their general breeding areas because they don't need to, and remain there year-round.
Eagles from some areas, however, like the ones we are studying that overwinter in New York State, do have to leave their nesting areas because they totally freeze up in winter and the birds cannot find enough food to survive. Thus, these birds "migrate", but even then depending upon the severity of the winter, they may not follow the same path each year or even go to the same areas each year. For instance in a very mild year, an eagle that moves 1000 miles down to New York to winter from Labrador Province, may only need to move 400 miles to the St. Lawrence River, if they find much open water and food available there. So, keep in mind that bald eagles are not necessarily fixed "migrants" like many other species of birds, that move based upon other cues not related to food availability.
Now, all of the above discussion really relates to adult bald eagles; immature bald eagles move to different, largely unknown cues, not as described above. Probably the best way to describe movements of young eagles, at least during the first 3 years of their life, is semi-directed "wandering". During these years, young eagles are learning their way around, learning to survive, and learning what's out there. Very often their movements will be based upon other eagles; they will see and key in on other eagles they encounter along their route and check them out to see what they are up to, thus getting exposed to a variety of habitats and places during their young life. Primarily they are constantly in search of the easiest food source they can find, and extensively wander both summer and winter. Somehow, however, they are quite able to find their way back to their natal area (the area where they were born). This is a topic for discussion at some other time, but they likely do this based upon distinct geographic features they recognize.
Generally, eagles are much more leisurely in their southward fall movements then during spring. In the fall, much more suitable habitat and food is generally available, and there is typically no big rush to get to a particular wintering site. During spring, however, eagles (adults) move north rather rapidly and despite conditions at their breeding grounds because their internal biological clocks are telling them breeding time is near. Thus many adult eagles wintering in New York, return to their northern nesting sites while much of that area is still frozen and quite inhospitable. The hormonal drive to initiate nest building and courtship overwhelm the difficulties in finding food during this period.
So, the major question facing us, and you, concerning eagles wintering in New York State, is what conditions/circumstances prompt them to initiate their northward move in late winter early spring each year? Unlike fall migration that is directed by food shortages, the spring movement is believed to be mostly driven by photoperiod, or day length. Increasing amounts of daylight is believed to be the primary cue for eagles to move north. Within this framework, other factors are known to influence eagle movements once they decide to move.