May 8 Notes - Bald eagle surveys

For the last 25 years, Charlie Todd, eagle biologist with Maine Department of Inland Fisheries and Wildlife, has completed annual surveys of Maine's eagle population. (In fact, he's in the air flying as we write this!). Eagle surveys provide us with vital information on the size and distribution of the population. Survey data are crucial for conservation programs and are used to track the recovery of the population. Surveys also provide important information on the health of the population - how many young are born, where we might recover an unhatched egg for contaminants analysis, or where nests are located in relation to proposed projects.


Eagle surveys are done with and amphibious float plane twice each year. The first flight begins in March on the coast where eagles nest earliest. These flights extend into May in northern Maine where we often still have snow on the ground this time of year. Biologist develop a "search image" for eagles and their nests as indicated in the photo above. With practice, the white head and tail of the bald eagle can often be discerned in a white pine at quite a distance.

Eagle nests are large, and sometimes snow in the nest bowl in late winter will reveal the location of a new nest. Bald eagles most frequently nest in our state tree, the white pine. Sometimes eagles will take over an osprey nest. Eagle nests are usually under a canopy of branches, and biologist have to search carefully - a difficult proposition when flying at 120 miles an hour. Sometimes we observe eagles flying along a lakeshore or coastline. From a distance we'll circle in the airplane, watch the birds, and follow them to a new nest location.

Eagles mate for life, and many pairs use the same nest year-after-year. The flight plan always calls for checking known nests first. If a pair is missing, we may spend time searching the nearby stands of pine for a new nest. Nests are abandoned for a number of reasons - disturbance the year previous, a mate change, or a nest damaged by a winter storm. The public notifies the state and federal agencies of new nests and eagle activity in areas where eagles had not been observed before. These tips are very helpful in finding new territories. When "deadheading" from one territory to the next, we often ask the pilot to skirt a lakeshore. Every minute in the air counts, and often new nests are located in this fashion.

All nests with an adult pair present or incubating are considered "occupied." In the photo above, one eagle is in an incubation posture and the mate is perched nearby. In June all occupied nests are surveyed a second time by airplane to document how many chicks were produced. We try to time the second flight when the eaglets are about 8 weeks or older. At this age, the chicks are starting to get their brown feathers. Most of the chicks observed at this age will survive to fledge later in the summer. As we've observed with the web camera, attrition can occur between hatching and fledging.

Sometimes special flights are made to survey nests more intensively, especially when documenting the age of eaglets for banding or locating unhatched eggs. Several paper company in Maine are helping the state and federal agencies to monitor contaminants in nests close to their paper mills. These nests are flown every week early in the nesting season to search for abandoned nests with unhatched eggs. Biologists collect unhatched eggs to measure contaminants. We have to promptly retrieve unhatched eggs. Crows and other scavengers quickly consume eagle eggs not attended by the parents.

Flying is interesting, but hard work. State and federal pilots have to be well-trained and experienced to fly all day at low altitude and in tight turns around the nests. Biologists have to maintain their concentration and data-taking skills despite working under high G-forces in a loud, hot cockpit. Sometimes, on breezy or turbulent days, Charlie and I crawl out on to a lakeshore or tarmac for a lunch break and wish we could walk home rather than face an afternoon of more flying. On the positive side, we've seen nearly all of the state of Maine, and the scenery and wildlife sightings are incredible. The pastel blush of trees leafing out on the mountains and hills of Maine in May rivals the riotous fall colors.

Last year, Charlie found 385 pairs of eagles nesting in Maine. We use the ratio of young fledged per occupied nest as an index of the health of the population. In a good year, bald eagles should produce about one chick per occupied nest.

As our population continues to climb, there may come a time when we can no longer afford to fly every eagle nest annually. The U. S. Fish and Wildlife Service and U. S. Geological Service - Biological Resource Division are developing a new survey protocol that samples a random, representative sample of the population. If the bald eagle is delisted from its threatened status, we may begin this survey methodology in the future.

Charlie Todd, Maine Inland Fisheries and Wildlife
Mark McCollough, U. S. Fish and Wildlife Service