Bald Eagle Migration Update: May 1, 2001
Today's Report Includes:
Latest Migration Map and Data
Now that the migration season is coming to a close, what do you suppose is happening on the nesting grounds of the eagles you've tracked this spring? Peter Nye describes eagle nests--his visits, the view, and also the smell! Visit an eagle nest by remote, and take your own field notes. Then compare the duties of male and female eagles during the nesting season.
Field Notes from Peter Nye
Well, it is now officially May, and I suspect most of our birds are on eggs right now, and will hatch sometime later in May; young should fledge in July. And, if I visit any of these nests this summer, I'll write and let you know what I find!
We'll look at a few of the specific birds in a minute, but for now, think about eagles in general for a moment. When you look at the migration map, remember that there are many, many more eagles than the six birds I am tracking! Biologists estimate that about 16,000 bald eagles winter (are counted each January) in the lower 48 states. Some of these obviously stay 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 territory (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.
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.
Eagle #F43 has neighbors, and probably part of the reason she hurried back northward was 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.
Describing Eagle Nests: The Visit and the View
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!
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 NY!
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 Also Attract Flies
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.
I was in a helicopter all day (and am beat), looking at eagle nests in southern NY, determining stage of nesting, age of young, number, etc. I have another to do in northern NY as soon as I can fit it in. I also start NY nest visits and banding Wednesday in western NY. We'll probably have about 50 nests to visit here this year. Plus I still want to try to visit a couple of our migrant birds' nests after that. I'll be in touch.
Eagle Eye Nye
More About Bald Eagle Nests
Journey North science writer Laura Erickson provides this overview of bald eagle nests:
Let's Visit an Eagle Nest!
Unfortunately, you can only imagine what's happening in Nye's eagles' nests right now. But you can take a peak into another eagle nest. Here are is a live "Eagle Cam" in Alaska:
Spring Nesting Phenology of Eagles
Like a silent observer, visit the eagle nests above and read the Field Notes that have been recorded this spring. Look for key observations about the stage of the nesting cycle at each nest. Compare the contrast the timing and progress in Florida and Tennessee. For example, read for observations of:
Field Notes by Remote
1) Record your observations on this nesting phenology chart. (Or, if you're lucky enough to live near a real eagle nest, record your own observations here!)
2) How is the timing of the nesting cycle different in Florida and Tennessee? How do the dates of each event
Chore Chart Comparing Male and Female Duties
Does your family have a chore chart, where you keep track of the work everyone is expected to do? When it comes to raising their young, eagles know exactly what their duties are, without writing them down. However, you might enjoy learning about the duties of male and female eagles, and recording them on this:
For helpful background information about Bald Eagle reproduction, see the phenology chart above, read Peter Nye's descriptions and see:
The FINAL Bald Eagle Migration Update Will Be Posted on May 15, 2001.
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