A Bird’s Tale

Sea-eagle “Invisibility” Shows Why Study of Bird Movements is Not Easy

Wed, 02/02/2022 - 8:00am

    The Steller’s sea-eagle has disappeared. At the time of this writing, the bird had not been reported since January 24, when it was back in Boothbay Harbor and on Southport Island after a few days over at Pemaquid Harbor.

    Of course, many people are still searching for it on a daily basis. But how far from the last place it was sighted should they search? This bird is clearly very unpredictable given it’s half a world away from where its fellow sea-eagles are at the moment, so it’s probably presumptuous for anyone to guess what it might be doing.

    But we did find a paper published back in 2003 that reported the results of a study in which 24 Steller’s sea-eagles were tracked, with satellite transmitters, from where they were raised in and around the Kamchatka Peninsula of Russia. The researchers found that the birds made their migratory movements to their wintering areas from September through December. All made stops along the way that lasted from four to twenty-four days. Once on the wintering range, the authors estimated the size of the wintering range to be up to a maximum of about 2,200 square kilometers.

    Imagine that the bird had settled on Boothbay Harbor as the center of its wintering range. Based on the 2003 study findings, we would expect that a circle extending to Friendship in the East, to Orr’s Island in the West, and up to Richmond and Damariscotta would cover an area of the size of the typical sea-eagle wintering range. Even many parts of that relatively circumscribed area would be difficult or impossible for anyone to access and scan effectively. The bird could be hanging out somewhere within a few miles of the many birders agonizing for a sighting of it, but essentially invisible to them for all practical purposes.

    It was not that long ago that all studies of the movements of birds had to be done essentially the way that birders are trying to track this amazing sea-eagle. Marked birds could be looked for but are usually only resighted occasionally so that a very rough map of movements and history could be pieced together. When we lived in Ithaca, New York, we well remember the many hours we spent trying to read the number and letter combinations on neck collars that had been placed on Canada geese to learn more about their movements and survival. Bird photographers occasionally are able to get high enough resolution shots of leg bands on larger birds in order to read the numbers and get a data point or two on the banded bird’s movements and longevity. Researchers capture banded or otherwise marked birds in mist nets, and of course, birds carrying leg bands are found dead or harvested by hunters, providing another source of occasional bits of information.

    All of these methods provide only a rough idea of the movement ecology of birds because, just like our sea-eagle, we don’t know where the bird was in between the occasional known points of its occurrence.

    That’s why the new technologies that send regular signals to radio or cell towers or satellites are so revolutionary in understanding more about the movements and ecology of birds. We have written in past columns about some of the cool discoveries that have been made using this technology including from the tracking of snowy owls through Project Snowstorm, the 38-hour nonstop journey of a Maine great blue heron, and the overnight flights of shorebirds from James Bay to the Maine coast.

    All of this new research is helping people understand how intricately connected all of the world really is. That means that we need conservation work to protect habitat for birds from north to south and everywhere in between. The land protection we do right here in Maine is an essential contribution to the biodiversity of the whole world across which birds and other animals make mind-boggling migrations.

    Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology and Vice President of Boreal Conservation for National Audubon. Dr. Wells is one of the nation's leading bird experts and conservation biologists and author of the “Birder’s Conservation Handbook.” His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization working statewide to protect the nature of Maine. Both are widely published natural history writers and are the authors of the popular books, “Maine’s Favorite Birds” (Tilbury House) and “Birds of Aruba, Bonaire, and Curaçao: A Site and Field Guide,” (Cornell University Press).