WESTERN Bald Eagle Migration Update: April 6, 1998
Today's Report Contains
News from Biologist Jim Watson
Here's the news we've been waiting for:
Notice how many miles Eagle #16 flew last week, between March 24th and April 3rd! But how do you interpret the
news that haven't left Washington yet? What are the possible reasons? Take a look at the migration data below,
and read today's interview with Jim. Then see if you think you know where ALL the eagles intend to nest this season:
(To respond to this question, please follow the instructions at the end of this report.)
Migration Routes to the North
Interview With Jim Watson
We interviewed Jim about the routes the eagles are taking. Here's what he had to say:
Would you comment on the 2 routes your eagles seem to take, the coastal & the interior corridors?
Both migration corridors provide ample opportunities for eagles to feed as they move to and from breeding areas. Inlets and rivers on the coastal route provide seabirds and fish, such as salmon, in the fall, and herring in the spring and summer. The Fraser River and its tributaries on the interior provide salmon in the fall, but also a variety of other fish and birds throughout the spring.
The coastal migration route is a logical flight corridor for eagles nesting in southeast and interior Alaska, and northern Yukon Territory. However, we also had 2 eagles in the spring of 1997 that flew up the coast to southeast Alaska, and then back east to interior British Columbia and Northwest Territories.
This may have been a result of the fact that lakes and rivers in the northern interior were still be frozen by late March--by using the coastal route, these eagles can get a good part of their migration done, and by the time they reach southeast Alaska waterways are thawed out in the interior. For eagles nesting in central and southern British Columbia, the interior route is closest to their breeding areas.
How is food availability different/same? Why?
There may be some differences in food availability (how much is available to eat) along these flight corridors, but nobody has collected or compared this information since that would be difficult. Notice that the coastal route probably provides marine fish, seabirds, and even marine invertebrates (i.e., crabs, oysters, clams) to migrating eagles, whereas the Fraser river and associated lakes and streams provides freshwater fish, waterfowl, and mammals to migrating eagles. The fact that these routes both provide food makes them more attractive to migrating eagles than flying over land.
What other factors might influence the route they take?
Whether or not the eagle is a breeder and has a specific location to return to is likely a key factor determining the exact route the eagle uses in migration. If the bird is not breeding, the presence of other migrating eagles likely influences where and when it moves. However, even non-breeding eagles eventually return to their natal areas as we have seen from studies in Washington. An eagle's choice of the exact route it takes is certainly influenced by the local topography and prevailing winds on a given day. Ideally, these factors will allow the eagle to fly on "cruise control" and expend less energy.
Is this a learned pattern, so that maybe populations of eagles go along
There probably hasn't been enough research done to say for sure how much of an eagle's directional sense is learned and how much is innate. However, it seems that juvenile eagles during their first migration have a definite sense of which direction to fly. In Washington, we found that juveniles migrate north from the nest area without the adults. After they leave the nest area, they probably follow other eagles as they migrate through the same area. Over time, these younger eagles likely remember the flight route from visual cues, much the same as we do when we take our annual vacations.
It would be an interesting experiment to take an eaglet out of a nest in a northern latitude where the breeding
eagles migrate southward (i.e., Yukon Territory), and place it in a nest in Washington where the breeding eagles
migrate northward. Would the eaglet migrate south or north when it fledged?
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