A variety of behaviors makes the Osprey an interesting migrant
Published: May 1, 2005
|If I had to choose only one migrant to study for the rest of my life, it would be the Osprey. I have observed it soaring in thermals over the hills of central New York and the eastern plains of Mexico at Veracruz. I've watched it coming in off the Gulf of Mexico in spring on Mississippi's barrier islands, as well as initiating flight across the chilly waters of Lake Superior. Ospreys can do it all. They can soar or use powered flight. They can migrate at 10 feet over the water or thousands of feet over land. They cross large bodies of water and can fly by day or night. From the perspective of behavioral plasticity, they may be the most interesting of all migrants.|
Although I'd watched Ospreys migrating when I was younger, I didn't really appreciate them until I studied them with tracking radar between 1978 and 1981 as they migrated over the Helderberg Mountains of east-central New York State. The radar I used was able to lock onto individual birds and follow them for a mile or more. I could determine the height, glide ratio, climb rate in thermals, flight speed, and other facets of their migratory flight that nobody had studied previously. I found that they sometimes soared to more than 4,000 feet above the hillsides, very different from the lower gliding flight along ridges or the powered flight along the Atlantic coast.
What I found most interesting was that Ospreys can climb vertically in thermals at a rate of about six miles per hour, meaning they are capable of ascending 1,000 feet in roughly two minutes, almost without flapping! They then glide at a rate of about 1,000 to 1,200 feet for every 100 feet of altitude that they lose. At an average altitude of about 2,500 feet, an Osprey can glide for nearly five miles before it reaches the ground - without flapping once. With a strong tailwind, its glide can be twice that distance, and it can add distance by incorporating a few flaps now and then.
Although radar was great for learning about the aerodynamics and mechanics of Osprey migration, I longed to know where "my" Ospreys were going. Thankfully, researchers Mark Martell of Audubon Minnesota and Rob Bierregaard of the University of North Carolina Charlotte are finding out. They follow Osprey movements using transmitters attached to individual birds. Locations are relayed to a satellite and back to the researchers, so the birds can be tracked practically anywhere, as long as their backpack transmitters continue to operate. The information that Martell and Bierregaard collect is priceless for understanding precisely where the birds go, when they migrate, and the pathways they use between destinations.
Martell, who has followed more than 100 Ospreys equipped with transmitters, and Bierregaard, whose studies of Ospreys migrating from Martha's Vineyard are being chronicled for an upcoming BBC documentary, have determined that eastern birds migrate much farther than western birds. Eastern birds fly either well inland or along the Atlantic coast for the trip to the Florida Keys, then they head south to Cuba, east to Hispaniola and sometimes Puerto Rico, and south to Colombia.
Recent studies of fall Osprey migration through Cuba have yielded surprises. Researchers now routinely count more than 100 Ospreys per hour moving eastward along Cuba's main mountain ridge. The rates of migration are greater than those from northern hotspots like Hawk Mountain, Pennsylvania, and Cape May, New Jersey. The spectacle continues for several weeks, mostly unseen by birders.
The final 450-mile, 15-hour leg is often done during the night, which is out of character for most raptors. Upon reaching South America, many birds continue inland to find a fishy place to spend the winter. Ospreys that nest in the Pacific Northwest fly pretty much non-stop over the western deserts. They have not been tracked migrating to South America, perhaps because there is a fairly large landmass in Mexico and Central America in which to spend the winter.
Martell and Bierregaard have made other wonderful discoveries about Osprey migration using satellite transmitter data. For example, individual Ospreys generally follow the same migratory path in successive seasons and usually return to the same location to spend the winter each year.
Not all Ospreys make the round trip each year, however. First-year birds that reach the tropics do not migrate back to their temperate natal areas right away. Instead, they simply stay in the tropics until they are nearly two years old, saving them thousands of miles of flight and the associated hazards.
Ospreys may migrate over water at night simply because they don't reach land during daylight hours. But the birds sometimes commence their flights late in the day or in darkness, so this answer isn't entirely adequate. We need to consider the physiology of bird flight to determine why they fly at night. Thermals are scarce over water, so Opreys must use powered flight to cross the sea. Powered flight is easier at night when the air is about 10°F cooler. This may be a primary reason for night flight by songbirds, shorebirds, waterfowl, and other species, so why shouldn't Ospreys have a similar adaptation?
Our knowledge of the night flight of Ospreys improved further during migration last fall. While watching night-migrating songbirds on the observation deck of the Empire State Building in New York City, ornithologist Bob DeCandido and his volunteers observed individual Ospreys on two separate nights. The birds were flying well after dark (about 10 p.m.) in late September and late October, and obviously, they were over land - a behavior never seen before. The birds were flying above the Manhattan skyline at altitudes of 1,500 to 1,700 feet.
Researchers Martell, Bierregaard, DeCandido, and others are making important contributions to our understanding of Osprey migration. Their observations show that migrating Ospreys do more than most other birds. Perhaps it would be better for researchers to ask, "What can't Ospreys do?"