We stepped out of the local grocery store the other day and glanced up to see five osprey overhead. It’s easy to forget that just a few decades ago we might not have seen a scene like that. And if we went back hundreds of years we might have seen even more!

There are multiple reasons why each spring, in many parts of Maine, we regularly see ospreys and bald eagles and great blue herons and other birds in relative abundance. One essential piece responsible for the rebounding of populations of large fish-eating birds was the banning of DDT; in some areas, this deadly chemical caused the complete reproductive failure of entire populations of birds. Another factor in a few special places has been the restoration of natural flows and the removal of migration barriers for the fish that are the key to so much of the cycle of life in rivers, lakes, estuaries, and interconnected marine ecosystems like the Gulf of Maine. Many fish species, including well-known ones like Atlantic salmon, alewives, and Atlantic sturgeon, spend much of their life in the ocean but must migrate back to freshwater rivers to lay their eggs—a cycle that is required in order for them to maintain their populations.

Other fish species may move back and forth within freshwater rivers, streams, lakes, and ponds to find suitable places to lay eggs. One species, the American eel, has one of the most incredible migration journeys on Earth. When adult eels reach maturity in freshwater rivers and lakes, they follow an ancient urge and somehow find their way to the Sargasso Sea, east of Bermuda, where they mate and release their eggs into the ocean. The young hatch and float around the sea for just shy of year before metamorphosing into the now well-known glass eel stage. These and the slightly older and more darkened elver form are what fetch the astronomically high prices (and are now closely monitored) for export overseas to be raised in fish farms. The baby eels, under normal circumstances, make their way into our lakes and ponds, and live quietly (and mostly nocturnally) for years until they make the trek back into the ocean and find their way to the Sargasso Sea.

The Kennebec River was one of the first in the country and certainly the first large river in Maine to see the removal of a major fish migration barrier when the Edwards dam, spanning the river in Augusta, was removed in 1999 more than 150 years after it was constructed. By the 1860s, within 30 years of its original construction in 1837, most of the commercial fisheries based on migratory fish in the Kennebec had already collapsed because the dam prevented the fish from reproducing. Later, the river became so heavily polluted by the many mills along its banks that a stench emanated from the river on warm summer days that could turn the stomach. When textile mills freely discharged their chemicals it is said that the river would run in different colors depending on the dyes that were being used on any particular day. Now the Kennebec River is clean and free flowing for much of its length, and the fish have been coming back literally in the millions.

And those beloved birds have come back, too—osprey, bald eagles, common mergansers, cormorants, great blue herons, belted kingfishers, and more!

Thanks to the efforts of Maine leaders like former Senator Ed Muskie and others, our nation now has laws in place and agencies like the EPA, which need to be kept strong and effective, to ensure our waters stay clean and healthy. There’s also, thankfully, and increased understanding that dams that completely block fish passage have a ripple effect in ecosystems and economies distant from their physical location. Several dams have been removed in the Penobscot River in recent years, and you can bet that the results will be similar, with a thriving, productive ecosystem reborn!

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology. 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 book, “Maine’s Favorite Birds.”