Tracking Vineyard ospreys: where they've been, and where they're going
The Vineyard is a popular summering spot for creatures other than tourists. From two pairs in 1969 to more than 60 pairs today, Vineyard ospreys have annually checked into their summer homes for some South Shore fishing.
"I spent summers in Edgartown from 1961 to 1969, and all I knew about the Island was what I could get to on my bicycle," Bob Bierregaard, an ornithologist at the University of North Carolina at Charlotte, said in a talk at the Oak Bluffs Library last week. "In the late 1960s, I became a falconer, and I looked up the membership list of the North American Falconers' Association to see if anyone on Martha's Vineyard was a falconer; I found this guy with three first names: Gus Ben David."
Mr. Bierregaard described for his audience the beginnings of what became a 40-year study of the Island's osprey population.
First, he gave Mr. Ben David a call, talking at length as falconers are apt to do about hawks trained and rabbits caught. Mr. Ben David and his group of volunteers are responsible for many of the 100-plus Vineyard osprey poles. In the spring of 1969, the osprey aficionados met.
"He showed me the rest of the Island and got me going on a long series of years in which I studied the birds of prey nesting on the Vineyard," Mr. Bierregaard said, referring to Mr. Ben David.
Before the harmful insecticide DDT was banned in the US in 1972, he said, only two pairs of ospreys visited the Vineyard. The chemical weakened the shells of the ospreys' eggs, severely diminishing their rate of reproduction. When Mr. Ben David learned that power line crews had been knocking osprey nests off transformers to protect power lines, he asked them to give him a call before they did so. He erected poles near the transformers taller than the power lines to entice the raptors to transplant their nests; ospreys, apparently, are mad for height when it comes to nesting spots.
Photos by Alex Bell
"The reason we now have so many young on the Island is that Gus Ben David and the notorious 'osprey crew' ran around the Island in the seventies, eighties, and nineties, and even still today, putting up poles for the ospreys," said Mr. Bierregaard. "As best we can figure from historical records, there were probably never more than five to ten pairs of ospreys on Martha's Vineyard."
Once Mr. Bierregaard obtained a banding permit in 1971, he climbed up to his first osprey nest in a dead pitch pine tree in Mink Meadows. Then, he used colored bands to track birds. Now, he uses $4,000 solar-powered GPS transmitters. Despite the change, catching an osprey still remains a challenge.
Mr. Bierregaard, whose mother owned a needlepoint shop in Edgartown, uses his own homemade "noose carpet." Laid on top of the nest, the trap is made of chicken wire with more than 100 monofilament slip-knots tied to it to catch the ospreys' talons. He hopes the menhaden popsicles he leaves in the nest will entice the hungry young to land for breakfast. Sometimes, the birds take their sweet time returning to their nests. "Hours of boredom, and then 15 minutes of panic - that's the way wildlife biology works," he said.
Once the bird is trapped, Mr. Bierregaard scurries up to the nest on a ladder or in climbing spurs. He hoods the birds to calm them and then puts them in straightjackets, while he frees them from the monofilament slipknots.
If he catches a bird he was after - right now he's looking for juveniles - then he attaches a GPS transmitter to its back. Two straps go over the wings, two go under, and he stitches them together over the bird's chest (with a dab of super glue for good measure).
"My mom taught me well," he told a curious child among a crowd of spectators at Long Point last week, where he tagged a young female osprey he named Penelope.
"If this isn't natural selection in operation, I don’t know what is," Mr. Bierregaard said about these maps. Ospreys get a mix of migration genes from both of their parents, and some mixes are better than others. Ospreys have occasionally been sighted as far east as Bermuda; those ospreys probably won't survive the next leg of the trip. If an osprey doesn't have a good first migration, it will join the 80 percent of ospreys that don’t live long enough to migrate twice.
Maps courtesy Rob Bierregaard
"This process doesn't hurt the birds," he said. "They're a little bit stressed-out, but they get over that quickly." He added that adults wearing transmitters tend to die sooner than birds without transmitters.
Next year, he said, he will tag adults but take the transmitters off after one migration cycle. "[The transmitter] only weighs an ounce, but the birds are not as aerodynamic with those things on their backs as they would be."
If Mr. Bierregaard catches an osprey that he's not interested in, he uses a special lock-on metal band with no transmitter to tag the osprey's leg, then enters that band's number into a database.
A typical migration of East Coast birds after the summer season tracks south to Florida, to Cuba, to Hispaniola, and then usually to South America, where most birds winter - and adults go back to the very same place every winter.
"The birds are basically finding the same tree about 3,500 miles away from home," said Mr. Bierregaard.
But, how do juveniles know where to go on their first migration?
Four years ago, Mr. Bierregaard started tagging juveniles, almost by accident. About 80 percent of juveniles don't make it past a year to see their first "hatchdays," so few people would risk a $4,000 transmitter on such odds.
In 2004, the British Broadcasting Corporation funded Mr. Bierregaard with four transmitters for a series on animal migrations. After being able to tag only one adult osprey, Mr. Bierregaard, not wanting to disappoint his sponsors, put the three remaining transmitters on juveniles that he knew came from very productive nests. The juveniles made for a dramatic twist in the show, and opened up new doors to understanding animal migrations.
"They're probably following a very special sort of computer program: go south, and stay over land if at all possible." Because juveniles can't use navigation to find a certain spot, they must rely on their instincts for orientation.
Ospreys feed almost exclusively on fish, but don't care all that much what type. They can dive as deeply as three feet below the water's surface to catch fish from six to 18 inches long.
The New England population of ospreys is entirely migratory. Females head south around mid-August, while males make their exit mid-September. Pairs mate for life, but migrate separately during the winter. "Those two facts are probably not unrelated," joked Mr. Bierregaard.
If an osprey's mate from the previous year returns, they mate right away; if the partner doesn't return, it will be replaced. Nests can be active for decades because of this serial replacement.
When identifying an osprey's sex, researchers look at the banding on its chest. Females tend to have darker bands across their chests than males and also tend to be larger. Juveniles can be identified by white scalloping on their wing and back feathers.
Manmade structures for ospreys are nothing new. Mr. Bierregaard said that since the early 20th Century, New England farmers have been putting up old wagon wheel platforms for ospreys to nest on, because the raptors keep redtail hawks away. Natural osprey nest sites have become increasingly rare on the East Coast. "Nowadays, if an osprey nests in a tree, it's big news," he said. He cited one renegade group of three pairs of ospreys in Tashmoo nesting in trees.
This year has been a good one for ospreys, with 121 ospreys fledged on the Vineyard alone; some years this number has dropped to 60. Dick Jennings, a tour guide at the Trustees of Reservations' Cape Poge Wildlife Refuge, is the "captain of the census team" on the Vineyard, according to Mr. Bierregaard.
Mr. Bierregaard recently added three Vineyard birds, as well as one in Woods Hole, to his roster of tagged ospreys. His website, which may be found through a search of his name, features maps for users to follow each bird's path from Martha's Vineyard, around New England, and to South America.