Laura Erickson's For the Birds

Bald Eagles

(Recording of a Bald Eagle)

Spring in the Northland means the Bald Eagle is nesting once again. Eagles were decimated in the United States thanks to pesticides and shooting, but they’re making a notable comeback in Wisconsin and Minnesota, thanks to the banning of DDT and laws that place a heavy penalty on eagle killing. Now eagles are no longer on the endangered species lists in our states–they’ve been reclassified as threatened here.

The Bald Eagle’s scavenging and thieving habits caused Benjamin Franklin to write that “like those among men who live by sharping and robbing, he is generally poor, and often very lousy.” Eagles are excellent at catching fish, but they do seem to prefer scavenging or stealing their food from Ospreys. And I watched one harass a Great Blue Heron in the Boundary Waters last summer, possibly to steal its meal. Up here Bald Eagles eat a lot of roadkills and killed and crippled ducks during hunting season–the lead shot in these ducks sometimes poisons the eagles. Bald eagles have lived longer than 48 years in captivity, but they don’t normally live that long in the wild. During the nesting period, the young are vulnerable to lightning and wind on their high perches. Nests along Lake Superior and the St. Louis River have a far lower rate of nesting success than nests elsewhere in Minnesota and Wisconsin. Fish in these waters are often contaminated with heavy metals, PCB’s, and pesticides, and eagles can’t read the fish consumption limits the D.N.R. issues. So the egg shells may be thinner than they should, and young may suffer direct poisoning as well. Power lines, guy wires on towers, automobiles, and shotguns also take a heavy toll.

Eagles mate for life, and pairs add on to their nests year after year, producing the largest structures of any North American bird. Pairs of European sea eagles, a close relative, have been known to use a nest every year for at least 80 years. One sea eagle nest that crashed down in a storm weighed 2 tons. The largest Bald Eagle nest known in the United States was in St. Petersburg, Florida–it was 20 feet deep and 9 1/2 feet wide. Most eagle nests are about five feet wide and three feet deep.

The Bald Eagle is the largest eagle native to North America. It’s about three feet long, with a wingspan seven to eight feet across. Although eagles reach full adult size within a couple of months of hatching, it takes about 5 years for them to assume the white head and tail of adult plumage. Immature eagles have dark heads and tails, and are sometimes confused with dark hawks or Golden Eagles.

As with just about all birds of prey, female eagles are bigger than males. A fully grown male eagle weighs in at a measly 8 or 9 pounds, but females tip the scale at 10-14 pounds. Back in the less hectic era of the 1950’s, an ornithologist counted, one by one, all the outer feathers of a 9-pound Bald Eagle. The total was 7,182 contour feathers in all, which weighed more than a pound–fully 14% of the eagle’s body weight was in his feathers. The same eagle’s skeleton weighed less than half as much as his feathers.

Adult eagles in the Northland are nesting now. Eagles usually lay two eggs. It takes about a month before they hatch, and another two-and-a-half months before the young are full grown and ready to fly. Unlike most birds, eagle parents actually starve their babies into making the first flight, which makes sense since some eagle nests are 150 feet high. Any sensible eaglet would need massive motivation in order to make that kind of a leap.

Baby eagles are extraordinarily aggressive to each other, and, sadly, the smaller nestling often doesn’t survive the competition. The many stories of eagles carrying off human babies are entirely unfounded. Experiments show that eagles can only carry about four pounds or so, and couldn’t possibly manage a baby. Anyway, eagles have enough trouble keeping peace between their own babies without lugging in more trouble.

(Recording of a Bald Eagle)

This is Laura Erickson, and this program has been “For the Birds.”