Wow. 181 million.
That's how many times the images provided by the webcam pointed at the Bald Eagle nest in Decorah, Iowa, had been viewed as of Friday morning. I know because I was one of the viewers.
The nest, in a cottonwood tree on private property in extreme northeast Iowa, has been producing happy, healthy eaglets for several years now. According to the Raptor Resource Project, which operates the webcam, two fledged in 2008, three fledged in 2009, three fledged last year, and this spring, it happened again. Three eggs laid in late February and early March hatched on three days in early April. Then, as millions watched on their computers on June 18, 20, and 23, the trio of young birds left the nest. They didn't go far.
I tuned in Friday morning because Independence Day is around the corner, and who doesn't think about Bald Eagles on July 4th? But mostly I wanted to see how the youngsters were doing, and I'm glad I did. I was treated to the sight of three large, dark-eyed, very dark juveniles calling to each other, hopping from branch to branch, and holding on for dear life as a strong Iowa wind whistled through the leaves and ruffled the birds' feathers.
I was curious how the birds were doing because I know they won't stay around long. In fact, in a few days, they'll do what all young eagles do -- they will fly away, and not just from branch to branch, but away from the cottonwood tree and its soon-to-be-darkened webcam, away from babbling Trout Run, perhaps away from Iowa altogether -- and commence what one eagle expert has described as a four-year-long "period of great exploration."
The period matches the amount of time it takes a Bald Eagle to attain its familiar definitive plumage -- dark brown body and wings, snowy white head, tail, and tail coverts. Four years is also how long it takes Bald Eagles to reach breeding age (4.5-5 years). So in addition to exploring, young eagles do a whole lot of developing after leaving the nest.
Before we attempt to say where they go, we need to understand the difference between migration and dispersal. Migration is an annual movement between breeding and wintering sites, usually between areas of different latitude. Dispersal is nothing more than a movement to new living areas. Birds disperse from their place of birth to the site where they make their first breeding attempt (natal dispersal); they move from breeding location to breeding location (adult dispersal); and they wander away from their place of birth in late summer (postbreeding dispersal).
In general, adult Bald Eagles migrate from their breeding areas only when food becomes unavailable. Adults from far northern interior parts of the continent move south between August and January and return as soon as weather permits, typically between January and March. Since food is available year-round in southern parts of the continent, adults that breed south of 40°N latitude (think of it as the Kansas-Nebraska border extended east and west) do not migrate; they simply move locally from feeding spot to feeding spot.
Most immature eagles, by contrast, do migrate, but distinguishing their movements from dispersal is tricky. Adults and immatures from northern California move north in early August to Canada and southeastern Alaska (some go as far as Chilkat) to feast on spawning Pacific salmon. Eagles fresh out of nests in Yellowstone and Glacier National Park in Montana disperse westward in early September for the same reason. (Or at least some of them do. Some Montana-hatched eagles prefer to move south or move locally in search of mountain whitefish, waterfowl, and dead deer and other ungulate carrion.)
Juveniles in Maine move to coastal areas and in general disperse south, while juveniles on Chesapeake Bay disperse either north or south but almost always stay in the bay. Which makes sense: Because of the abundance of food to be found in either location, neither the Maine birds nor the Chesapeake birds embark on long migrations; they disperse short distances.
Some Florida immatures do not migrate, but many disperse north between February and July. Birds banded as nestlings have been relocated in every state east of the Mississippi. Most move north along the East Coast, and a large number end up in the Great Lakes region, but one incredible Floridian was discovered 200 miles north of Winnipeg, Manitoba. Now that's a dispersal!
Texas juveniles are known to disperse northward, too; they move up the Mississippi River Valley beginning in April. And I read in a recent issue of The Journal of Raptor Research about radio-tagged juveniles from Arizona that dispersed northward as well. They departed 18-65 days after fledging (when they were 16-20 weeks of age) and ended up at lakes and reservoirs in Montana, Manitoba, Idaho, and Wyoming, on the shores of Puget Sound, along the Oregon coast, and at Lake Winnipeg. Most returned to Arizona by autumn or winter of the same year.
So what about those Decorah eagles? It's safe to say they won't go far -- at least for a while.
Decorah, remember, is located only 30-35 miles west of the Mississippi River and 32.5 miles exactly from Lock and Dam No. 9, also known as the Lynxville Lock & Dam, the largest federally managed pool on the Mississippi River. It restrains the reservoir known as Lake Winneshiek, which is part of 260-mile-long Upper Mississippi River National Wildlife and Fish Refuge. Since the refuge offers plenty of open water, plentiful food, tall trees for roosting and nesting, and minimal disturbance, eagles are year-round residents. It would be just the place for three famous immature eagles making a postbreeding dispersal.
Good thing the Raptor Resource Project is planning to tag one of the Decorah immatures soon with a satellite transmitter, so we can find out for sure. I can't wait. -- Chuck Hagner, Editor
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