Routes to the North
Interview Biologist With Jim Watson
Jim Watson about the routes taken by the eagles he's studied over the
years. 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
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 closes 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?
Do individuals go the same route each year?
Yes. Our information on the eagles studied in Washington shows invidividuals
use the same corridor from year to year; the amount of time they spend
at different points along the corridor varies, particularly in fall migration,
which is probably related to prey abundance.
Do eagles fly with their mates? Or with other related eagles?
I am unaware of any study that has tracked both adults from a territory
from breeding to wintering areas and back to the breeding area. We've
observed some nesting activity by migrant eagles in spring before the
moved north, and similar activity was observed in Colorado. In general,
when we see eagles migrate north from territories in Washington, the adults
do not leave on the same day. It seems unlikely they fly together, although
they may end up on the same wintering area. As I mentioned above, juvenile
eagles are independent once they migrate from their natal area and don't
have any association with the adults after that time. The juveniles we
have followed returned the following summer to within a few kilometers
of their natal area, but not to the nest site.
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