Saturday, August 22 2009 @ 09:44 PM EDT
Contributed by: davidh
The Annual Movements and Migration of Bald Eagles -- a Quick Overview
Some researchers don't consider all bald eagle populations as migratory. Some of the eastern populations, and maybe a few others, basically disperse north after the breeding season, probably an effective way to abandon the fledglings and have them head out on their own. Then, almost immediately, the birds seem to randomly move about and then move back into their breeding territory. Other eastern populations seem to more traditionally move north and south or south and then back north -- depending upon the population and the historic availability of foods.
On the west coast, the real home of the bald eagle!!!, at least in the sense of the dominant numbers, the bald eagle spends at least 6 months living off dead salmon. On the southern west coast of Vancouver Island, the relatively wilderness living eagles, those not associated more recently with urban living, have a seasonal movement pattern very directly associated with the availability of dead salmon. This pattern has likely paralleled the retreat of the glaciers so that the southern nesting eagles have to go continuously farther and farther north to adjust to the earlier spawning salmon.
The almost complete inability of young eagles, at least juveniles of the first year, to catch live prey makes them totally dependent upon finding spawned out salmon. Sure a few will find road kills, beached fish or seal carcasses, but the reliable food source is associated with salmon spawns. For the vast majority of our southern eagles this may mean a flight of 500 to 1200 miles to find the first reliable meal. For the more northern eagles the fledging season more closely parallels the local spawning of salmon and the eagles can scavenge the carcasses often in their own nesting valley.
With the melting of the glaciers and the penetration of spawning salmon into the more northern rivers, the southern eagles have farther to find that first meal. It is my hypothesis that the huge wide and long wings of the young eagles is to facilitate that long flight to the first scavengable meal. Then as the north freezes up in the late, and sometimes early, fall, the carcasses are either all eaten up or frozen under the frozen rivers. The mass emigration of eagles leaves these northern rivers for the milder and still thawed southern rivers. Here in southern British Columbia we historically had the latest runs of salmon and consequently offered the season's latest feeding bonanza.
While a few of the northern rivers have spawned out carcasses as early as July and August, our southern rivers don't develop meaningful quantities of carcasses until early to late fall. The eagles dependable movements, quite reasonably defined as an annual migration, is driven by first the dates of salmon carcass availability, it is also driven by the opportunistic nature of the bald eagle. If other food sources are available they will be utilizing it.
This opportunism takes advantage of rangeland winter kills, road kills and garbage dumps -- particularly those associated with fish processing plant discards.
On top of this many eagles along the west coast, particularly the none nesting birds, at least those not fixed to their nesting territories too far distant, can utilize the other incredible west coast phenomena - the herring and oolachican spawns. Again, hundreds of tons of these little fish are available to the gatherings of gulls and eagles in March and April -- the otherwise difficult times for inexperienced hunters and fisherman -- the first and second year eagles. The eagles' biology and breeding cycle, not maturing until the fifth year, and often not entering the breeding population until the sixth year, suggests it takes that long for sufficient experience to have accrued for eagles to be able to effectively hunt a small territory near a nest and support themselves and their young. Likely this huge time investment before breeding is that it takes that long to guarantee to develop sufficient hunting and fishing skills to support staying in a small area -- the area that must provide nearby food that can be transported up to a nest for adults and young.
Where the bald eagle reigns supreme, our west coast, their survival is closely tied to the historic migration patterns of the salmon. If our salmon go so do our eagles -- our orcas -- and us. And at the rate at which we humans despoil habitat and polute our oceans, the question is not the validity of that statement, but when.