Osprey Feel the Heat

Osprey in flight. Photo by Jennifer DelgarmoOsprey in flight. Photo by Jennifer Delgarmo

Osprey are an iconic part of the Chesapeake Bay. These raptors with their curved talons and piercing cries are seemingly everywhere on the Bay, their nests crowning channel markers and other landmarks.

The epic migrations of osprey from South America to the Chesapeake Bay mark the seasons, with their return in March a traditional sign of spring.

Populations of osprey in the Bay have quadrupled since the 1970s because of a federal ban on the pesticide DDT. But, increasingly, the migratory schedule of osprey is being altered by a new environmental problem: climate change.

Dr. Bryan Watts, Director of the Center for Conservation Biology at the College of William and Mary and Virginia Commonwealth University, said that in recent years he has seen 15 to 20 osprey living in the southern Bay all winter, instead of migrating south to Brazil, Venezuela, or Columbia. Dr. Watts and his colleagues conduct visual surveys of bird populations from airplanes, and have seen the resident osprey around Virginia's Chickahominy River, among other locations.

"There are more and more reports of osprey overwintering in the Bay," Dr. Watts said. "That may be a function of a warmer climate, and their continued access to fish, which is the main issue for why they leave."

Osprey eat a lot of menhaden—small, oily, bony fish that normally migrate out of the Bay in the winter to the Carolinas. But with milder winters, increasing numbers of menhaden also appear to be remaining in the southern Bay, or at its mouth, off Cape Henry, Virginia, Dr. Watts said.

With warmer waters, fish might also be remaining higher in the water column during the winter, which could encourage fisheating birds to stick around instead of migrating south.

"As fish go," Dr. Watts said, "osprey go."

If this is true, and the apparent trend continues, osprey would not be the first species of bird to adapt their migrations to a warming globe.

Several species of song birds, including warblers and vireos, that normally fly south in the winter are instead loitering around the Chesapeake region in the cold months, Dr. Watts said. In addition, some waterfowl, such as mergansers and diving ducks, that summer in Canada and normally fly down to the Chesapeake for the winter are arriving in diminished numbers, likely because the northern lakes they live on are no longer freezing as often.

"Some of these species are shifting their range as the climate allows them to do that," Dr. Watts said. "The evidence is fairly clear that climate change is happening, and has been going on for some time. One doesn't have to rely on a bunch of statistics to know this. We can feel it going on around us every day."

Other researchers said Dr. Watts' observations about the timing of osprey migrations match what they've seen.

Glenn Therres, a biologist and an associate director at the Maryland Department of Natural Resources, said that while osprey do not appear to be overwintering yet in Maryland's portion of the Chesapeake Bay, he has noticed osprey migrating south later than normal in the fall.

"There are some late birds leaving the Chesapeake Bay in November, as opposed to September," Therres said. "And we have noticed a few early returning (osprey) migrants in the spring."

Normally, osprey return to Maryland in mid-March, Therres said. But now some are showing up about two weeks earlier.

Dr. Alan Poole, Editor of the Birds of North America project at the Cornell Lab of Ornithology, said that Dr. Watts is "absolutely right" to link the change in osprey migrations to warmer winters and earlier springs. Dr. Poole said he's also noticed osprey returning to Massachusetts earlier in the spring than usual.

But Dr. Poole said the shift raises a perplexing question: If osprey are returning to America's East Coast earlier in the spring, how do they know—when they are on a different continent—that they should fly back earlier because the weather is warmer?

"If you are sitting on the equator, and it's 100 degrees, you are not going to know if it's warmer in Massachusetts this year than it was last year, are you?" Dr. Poole asked.

The answer might be, Dr. Poole said, that osprey adjust to weather conditions as they fly, and move along their traditional migratory routes faster when the weather is warmer.

If that is the case, the change is only the most recent example of osprey proving their adaptability.

The Chesapeake Bay has been called the "osprey garden" because it has the most concentrated populations of the birds in the world. But osprey are a global species that have adapted to an extraordinarily wide range of habitats. Osprey nest on lakes in Siberia, on the shores of the Red Sea, in the Boreal forests of Canada, as well as the salt marshes of the Delmarva Peninsula.

The banning of the pesticide DDT by the U.S. Environmental Protection Agency in 1972 allowed osprey populations to rebound, according to Dr. Watts and others. After that, residents and government agencies around the Chesapeake promoted osprey breeding by building nesting platforms, and by encouraging osprey to build nests on channel markers.

Osprey readily adopted these new homes, Dr. Watts said, because they offered more protection for their young from predators, such as raccoons. Historically, about 90 percent of osprey nests were built in trees along shore. But today, that is reversed, with 90 percent of nests atop manmade landmarks, Dr. Watts said.

Whether or not osprey will feel so much at home atop their Chesapeake channel markers that they will start celebrating Christmas there...that may be up to the climate.

Tom Pelton

The Osprey at a Glance

APPEARANCE: Osprey are raptors with dark brown and white feathers and a white stomach.

SIZE: Osprey weigh two-and-a-half to four-and-a-half pounds and their wingspan can reach five to six feet.

DIET: Fish make up almost 99 percent of the osprey's diet.

INTERESTING FACT: The osprey is one of the most widespread birds of prey and can be found on every continent except Antarctica.


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