As in many other places in the mid-Atlantic, resident eagles can be seen year-round at Conowingo. But in the fall, the number of eagles can explode into the hundreds, as their northern cousins follow the Susquehanna down from its headwaters in Cooperstown in Central New York. Others follow the Atlantic Coastline, reaching the top of the Chesapeake Bay.
Adriaan Dokter, a research associate at the Cornell Lab of Ornithology in Ithaca, N.Y., said the eagles at Conowingo could be from as far away as Canada’s Newfoundland province.
“They are fishers, they are dependent on water,” Dokter said. “So when it freezes, they will look for open water.”
The water below the dam doesn’t freeze, providing open water for the eagles to spy their prey. And the dam blocks the way for fish headed upstream.
So far this fall, photographers at the dam have frequently outnumbered the eagles. This is largely credited to the relatively mild autumn weather.
“I wouldn’t be surprised if the birds will start to show up especially when the weather gets colder and inland water starts to freeze, such that they are forced to move to more open water,” Dokter said. “We haven’t had much frost yet this year.”
This winter should bring big temperature swings to the region, according to Capital Weather Gang’s winter outlook. Temperatures are expected to produce a colder than normal December, while January provides additional opportunities for snow.
Overall, though, the temperatures should work out close to average for the mid-Atlantic.
The National Oceanic and Atmospheric Administration’s Climate Prediction Center also called for above-average temperatures and precipitation for much of the United States. So the number of birds migrating from the northern climes may not be as large as in the past.
One of the best ways to monitor the bird activity at the dam is through the Facebook photography group Conowingo Bald Eagles. There are almost daily postings with images of the eagles’ acrobatics above the dam and fishing on the river below.
“People ask me all the time when will there be large numbers of Bald Eagles at Conowingo dam. The truthful answer is I don’t know,” the group’s administrator, David Lychenheim, wrote in a recent post.
Lychenheim said climate change has made the eagles’ arrival harder to predict. With warmer winters, there is less impetus for the birds to make the arduous journey, so the number of eagles wintering at the dam may vary considerably.
“Due to global warming many animals throughout the world have changed their ‘usual movements,’ ” he wrote. “In past years at Conowingo many ‘non-local’ birds showed up because the cold weather impacted their ability to fish in their local territories. … On a good year several hundred eagles can be spotted around the dam.”
Dokter agrees that climate change has had an impact on bird migration.
“There can definitely be quite a bit of year-to-year variation in when birds migrate, depending on the weather in a particular year,” he said. “Many birds have started to migrate earlier in spring as a result of the warming climate, but in fall the picture is more complicated, with some species migrating earlier and others later.”
The good news for photographers and other visitors is that many eagles live year-round near the dam. David F. Brinker, a regional ecologist with the wildlife and heritage service at Maryland’s Department of Natural Resources, said about half of the eagles seen at the dam during peak season are locals.
“There are birds that come down from up north,” Brinker said. “But the Maryland birds are permanent residents. They learn the dam is a good place to fish.”
Fish will go deeper into the water as the temperatures drop, so eagles from the northern Chesapeake and farther north on the Susquehanna valley will move toward the dam as wintry weather arrives, Brinker said. The dam is just easier fishing grounds.
The eagle population in Maryland has grown exponentially over the decades: In 1977, there were 44 nesting pairs recorded in the state; the estimate now is over 1,400 pairs, said Brinker. The population has grown so much that the state ended its survey of the birds in 2005.
Brinker credited the growth in the state’s eagle population to conservation efforts, such as the elimination of harmful chemicals like DDT.
“We literally have more breeding eagles in Maryland now than we’ve seen in our lifetime,” he said. “Even if you are a Maryland resident who is 100, you would not have seen as many eagles.”
The eagles’ numbers will start to thin out as the spring thaw arrives and the northern cousins start flying home to their mating grounds. But these same birds may return to the dam in the fall throughout their lifetimes because of the abundant fishing available.
“Birds are animals of habit. Once something works, they stick with it,” Dokter said.