Osprey expert talks about migratory patterns of raptor
The meeting room at the library was filled to near capacity last week for Dr. Rob Bierregaard’s talk on osprey migration. The ornithologist has been studying ospreys in southern New England since 1971.
According to Bierregaard, there were more than 1,000 pairs of ospreys in the area prior to World War II. The largest concentrations of the birds were along the Connecticut River and in Gardiner’s and Mount Hope bays.
The use of DDT in the postwar era decimated the osprey population. At one point there were just over 100 pairs remaining, with as few as four birds left in Rhode Island. When DDT was finally banned for agricultural use in 1972, the population began a revival. Today, ospreys are prospering, and as many as 115 pairs spend their summers in Rhode Island.
Osprey tracking has become much more accurate in recent years. According to Bierregaard, ospreys are trapped near their nests, and hoods are placed over their heads to keep them calm. A backpack with a transmitter is mounted on the bird and it is released.
In the Pacific Northwest, ospreys are tracked as their migration routes take them south to Mexico. Birds along the East Coast tend to migrate to Florida, Cuba and Hispaniola. After some trial and error when they’re juveniles, the birds tend to remember their routes as they get older. Females leave the northern climate in late August, with juveniles and males leaving in September. The experienced birds fly over land as much as possible since they can’t rest on the water.
The annual trip south takes about one month. The birds remain in their winter homes for about five months before the month-long trip back to the North. Many factors help guide their migration. Along the way, the birds sense the Earth’s magnetic field, along with the sun and the moon. A pigment in their eyes helps them to see to the north and south, and crystals in their beaks help with east and west navigation.
Trackers in the past have relied on the transmitters for information. These days, GPS and cellular towers help gather much more data than was ever possible. Where it used to take days to get information on a bird’s location, real-time data is now available.
Bierregaard said that the osprey population is in great shape, and they’re almost on the verge of becoming pests. Approximately 60 percent of the birds nest on man-made structures, and utilitycompany crews spend a lot of time removing the nests from power transformers and providing alternative nests.
According to Bierregaard, the decimation of New England fisheries in recent years does not pose a threat to the osprey population, although they may never again reach the numbers prior to World War II.
“They’re flexible enough in what they eat that there are always going to be ospreys with us,” he said.
It’s a good thing that ospreys will be around, said Bierregaard, because they are a sentinel species. It was ospreys, along with peregrine falcons, brown pelicans and bald eagles that alerted mankind to the dangers of DDT. Although the insecticide isn’t used anymore, there may be other chemicals presently in use that can harm aquatic ecosystems.
“If all of a sudden we see the osprey population going down, that will alert us that there’s something wrong with the environment again, and it behooves us to go figure out what it is,” Bierregaard said.
Marine biologist Chris Powell, who founded the Conanicut Island Raptor Project, said that he was pleased with the turnout for Bierregaard’s talk, calling the crowd the largest he has seen for such an event.
“I think that people learn to appreciate what is around them,” Powell said. “People on the island are very aware of the environment and very aware of the animals who live here.”
Prior to Bierregaard’s presentation, there was a special appearance from Varia, a 9-year-old barred owl. The owl was accompanied by Kim Calcagno, a refuge manager for the Rhode Island Audubon Society. Varia was injured in a fall from a nest when she was a fledgling, and as a result she can’t fly. She seemed just as content, however, to observe the crowd, as they observed her.
Unlike other birds, said Calcagno, raptors hunt for food with their feet, and not their beaks. She said that female raptors are generally bigger than males, and that the barred owl’s large eyes and ears are crucial in helping it find prey.
Earlier in the day, Bierregaard and Calcagno made a similar presentation to 218 students at Lawn School. At that talk, Calcagno brought a great horned owl.
“The kids had a lot of fun,” said Bierregaard. “When I go to a school group, I may just tweak the imagination of a kid who wouldn’t necessarily be interested in birds, and the next time he comes to school, he may look up and see that osprey on the light tower above the athletic field. So maybe we can get one or two more people connected to nature and caring about it.”