April 9 notes: precious eggs near hatching?

We are all eager to detect hatching at the nest this coming week. The adult eagles have faithfully tended their egg(s) for nearly 5 weeks. The incubation period for bald eagles is "about 35 days!" Very young nestlings may not be visible initially from our camera angle so watch eagle behaviors closely. There is a subtle difference between the prone posture of an eagle incubating eggs and the slightly raised (but very low) position of one brooding an eaglet. If there is more than 1 egg, hatching dates are days apart identical to the interval between egg-laying dates. In other words, an adult in the nest has to perform both tasks in this challenging time frame!

The first clear evidence of a successful hatch will either be a glimpse of the young eaglet or observations of feeding motions to an unseen, tiny hatchling. Look for food morsels being passed "beak to beak" from the adult to an eaglet.

We have discussed the impacts of disturbance and inclement weather on nesting eagles, but there are many other risks. One that once jeopardized bald eagles across most of their range in the continental U.S. was chronic breeding failures from environmental contaminants. DDE (a by-product of the insecticide DDT) once caused widespread eggshell thinning and breakage before hatching. In the 1960s and 1970s, only 30% of eagle nesting attempts in Maine yielded eaglets. At the time, levels of DDE were higher than any other findings in wildlife tissue sampled in the U.S.

The photo above shows the 1974 transplant of eggs not impaired by DDE into a nest in midocast Maine. Frank Gramlich and Paul Nickerson (both now retired from the U.S. Fish and Wildlife Service) were attempting to bolster the supply of eaglets where eggs failed to hatch at all local nests in the Kennebec River estuary during a 16-year period. It helped hold eagles in areas where the species was nearly extirpated. A limited supply of "precious eggs" was brought in a thermal suitcase from a captive breeding program in Maryland or healthy donor populations in Minnesota and Wisconsin.

Paul Nickerson, former endangered species coordinator in the Northeast, recalls egg transplants during 1974-76. Paul brought 3 eggs from Minnesota's Chippewa National Forest in 1974. Only one of 3 attempts succeeded that year. Addled eggs taken from each nest revealed high levels of contaminants. Egg transplants were about 40% successful. In later years, eaglets were fostered into nests with contaminated eggs. This technique was 90% successful. Desparate circumstances required extreme measures as the decline of Maine eagles continued well into the 1970s.

Our "eagle cam" was placed at this Hancock County nest partly because this location has the best nesting record of any nest in Maine since it was first used in 1995: 100% of ALL nest attempts here have been successful. Eighteen eagles have taken first flight here over the last 11 years: nearly double the average nesting productivity among Maine eagles.

Other nests are less fortunate, and some have continuing problems with environmental contaminants like PCBs and mercury. Another pair of eagles nesting only 30 miles away from the "eagle cam" represents the other extreme. Eagles nesting there have NEVER raised eaglets during 12 years of breeding at a remote location free from disturbances.

Most nests in Maine have nesting statistics somewhere between these two extremes. Since 1996, 60% of all eagle nesting attempts in Maine yielded eaglets that survived to fledging. Some of the 40% failures in the population are due to environmental contaminants. We hope that this problem does not arise at our "eagle cam" nest, and toxics never again threaten the population.

Management comments: Bald eagles are a top-level predator and therefore exposed to concentrated doses of environmental contaminants passed through food webs. Beginning in the mid-1940s, several raptors and fish-eating birds like bald eagles experienced dramatic declines for nearly 3 decades during the height of the DDT era. Banned in 1972, there are still residues in most eagle tissues in Maine, but the impact of DDE has diminished.

Mercury and PCBs still harm some Maine eagles. These contaminants can transport through the atmosphere exposing other regions. Maine is downwind from prevailing westerly winds in North America. Most people value bald eagles as our national symbol or a magnificent part of our wildlife heritage, but their role as an indicator of environmental quality cannot be understated. -- Charlie Todd, Maine Dept. of Inland Fisheries and Wildlife