Bald
Eagle Nests
Contributed by Peter Nye
When you
look at the migration map, remember that there are many, many more eagles
than the handful of birds I am tracking! Biologists estimate that about
16,000 bald eagles winter (counted each January) in the lower 48 states.
Some of these obviously stay to nest in the lower 48 come spring, but
the vast majority probably are migrating back into Canada to nest. That
is a lot of breeding eagles and a lot of breeding habitat. What do you
suppose it all looks like?
What Does All Eagle Habitat Have in Common?
I'm certain eagles in Canada occupy many different types of habitats,
as they do down here in NY and elsewhere in the US, from deserts to deep
forest; from totally isolated, undisturbed places to locations very near
human-habitation like roads or buildings. In all cases, I can imagine
these eagles on or very near WATER! Their common element. I can also picture
each and every one of them, regardless of their surroundings, with their
gleaming white head, bright yellow beak and serious eyes looking absolutely
stunning against a powder-blue summer sky.
For example, Eagle #F43 has neighbors, and part of the reason she hurried
back northward was probably to stake a claim on her territory. There is
also a very strong, and natural, instinct for eagles (and all life!) to
breed annually. While we're not certain yet whether male and female pairs
over-winter together, we do know that they both appear on territory within
a short time of each other. And the work begins!
Just as an aside, I strongly suspect that most pairs do not over-winter
together, as our observations and tracking indicate that this birds are
leaving alone and migrating alone. Recent work I have been involved with
tracking migrating ospreys shows the same thing; they do not leave or
migrate together, or winter together. And, in most cases, the males do
not go as far from their nesting territories as the females, perhaps in
order to get back to their breeding site first and "secure"
it.
Home Again, Home Again! (and The Work Begins!)
Eagles usually don't lay eggs until several weeks after arriving at the
nest site. They spend the first days dealing with their neighbors, if
any. In the most dense breeding areas I've visited, in Alaska, eagles
often nest within 1/2 mile of each other. In most other areas, eagle nests/pairs
are nowhere near so dense, and are often spaced 1-3 miles from each other.
In some places with numerous small lakes (100-300 acres), only one pair
of eagles may occupy the lake. Eagle "territories" vary in size
depending upon the amount of food available and the density of eagles.
They will often position themselves at the edge of their territories as
a signal to others to stay out. When an eagle inadvertently flies to close
or through an occupied territory, the resident adults with give chase,
and in extreme cases, engage in aerial tumbling and even talon-locking.
This latter behavior can (and has) be fatal to one or both eagles. Adults
on territory are often particularly aggressive towards immatures that
wander into their area, which is why you can find gatherings of immature
birds in "safe zones" sometimes, where no breeders are present
and thus not a threat to them.
In addition to defending the area from intruders, both males and females
help to build the nest. If it's not their first year on the nest, they
just add to the same nest used in previous years. The branches used can
be up to 6 feet long and 2 inches thick. The biggest nest I've ever seen
was in NY, and was about 12 feet deep and 7 feet across. Since they add
to their nest each year, they can get very big and heavy. One nest in
Florida, which finally caused the tree to fall down, was reported to be
22 feet deep and weighed over two tons! With this kind of size, there
is no problem having most eagle nests easily hold a man's (or woman's!)
weight, and I regularly often climb right into the nest with the eaglets
while banding. That way I can get all my supplies out and organized, and
relax a bit!
Describing Eagle Nests: The Visit and the View
The one common denominator just about all eagle nests have in common,
is the view: the view from a eagles nest is one of the most spectacular
on earth - they certainly know how to pick the prime lookout spots. The
nests themselves, once eaglets are born and a few weeks old, are completely
flat across the top, contrary to what you might think of as a "bowl
nest." They contain soft vegetation, and often fresh greenery such
as white pine sprigs or some other leaves; cattails and cornstalks are
also a big item here in New York!
Once incubation begins, the male and female take turns sitting on the
eggs, but the female does the bulk of the work. The incubation period
lasts for over a month (34-36 days).
Eagles typically lay 2 eggs, sometimes 3. The first egg is laid a day
or two before the 2nd, and sometimes the third is a full week behind the
first. Because incubation begins as soon as the first egg is laid, this
means the young hatch at different times. This may ensure that at least
one young will survive and fledge, as it will get the first food and be
the strongest. If food is not a limiting factor in a particular year,
more of the eaglets will likely survive. In some species of eagles, only
one young typically survives, but in bald eagles it is very common for
2 or even 3 to survive and fledge.
Fresh Fish Deliveries Attract Flies to Nest
Adult eagles obviously bring in a lot of fish for the 3 months their young
are in the nest, and that can be quite an attraction to flies and other
insects! It can also create quite a smell ! Occasionally, insect loads
are so bad up in a nest, that a chick or two may die, due to stress and
simply being bitten to death; I've seen this with black flies in our Adirondack
region where bugs and insects are intense (I would imagine many parts
of interior Canada are the same). Eagles might get some relief from this,
by putting their nests up very high, and thus keeping up and away from
insects as well as benefiting from more regular breezes, which act to
keep bugs down. To help keep nests clean, eagles continuously add fresh
vegetation to their nest, covering up old food. I have yet to see them
actually "clean-up" their nest, however, by removing old fish
scraps. Some nests get pretty gross with old, maggot-filled fish parts
stuck in the sticks of the nest, baking in the hot sun! Eagles often build
"alternate" nests, within their territory near their other nest,
likely in an attempt to let one nest sit unused for a year or two, to
allow it to "weather" and clean itself out.
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