Eagle Migration and Weather
Contributed by Peter Nye
some general statements about eagle migration and weather below. When
you make your predictions, you should keep these things in mind. Much
has been written about this, but I'm not sure how much of this is speculative.
Also keep in mind one major principle I've learned over the last 20 years
working with bald eagles:
soon as you predict their behavior, they'll break the "rule" and
do something different!
the most important factors to watch for:
- Any fronts
or weather conditions that provide strong uplifting winds and thermals
to facilitate soaring, combined with strong tailwinds, are ideal for
eagles. Clear, sunny days seem to really set up ideal conditions or
maximum thermal updrafts, which eagles really prefer for their movements.
to keep in mind:
- The ability
to soar high and far is absolutely dependent on the weather, particularly
on the wind and the sun.
- Wind is
very important to migrating eagles. They have the tendency to drift
with the wind, and may even deviate from a straight line in order to
take advantage of the wind; although too strong a wind can blow eagles
off their chosen course.
- If there
is no wind at all, eagles can't soar and are likely to stay put or
travel only very locally.
in spring won't move much on overcast days. Rain or snow or low clouds
can cause eagles to stop moving until conditions clear.
to our telemetry studies, migrating eagles can fly as many as 225 miles
in a day. The average distance per day of one of our eagles was 98
miles. Our eagles' migrations were completed quite quickly, in roughly
8 to 15 days.
are thought to migrate during the day, primarily between 8am and 6
pm, with most not starting until mid-morning or later, perhaps giving
the sun a chance to create more suitable thermals by mid-day.
- Watch for
the presence of physiographic features, such as mountain ridges, river
valleys, ocean cliffs, etc. In the case of our eagles, the Hudson River,
Lake Champlain and the St. Lawrence River may be significant.
eagles are nowhere near as weather dependent for movement as songbirds
are. Songbirds (and most other birds) migrate differently than eagles
and other raptors. Raptors migrate by soaring-gliding flight rather
than by flapping-flying flight as in other birds. This obviously conserves
tremendous amounts of energy, but also indicates that ideal weather
conditions for migrating are different for the two groups. Many other
species of birds also migrate at night, while bald eagles move strictly
during the daylight hours.
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!
They Really Migrants?
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.
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.
Eagles Do Their Own Thing!
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 patterned on 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. As they constantly search for the easiest food source
they can find, they 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 to discuss at another
time, but they likely do this based upon distinct geographic features they
Fall, Hurried Spring
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
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.