Soaring Above Us
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Across the country, the population of ospreys (Pandion haliaetus) has been on the rise after their precipitous decline starting in the 1950s and peaking in the early 70s due to the use of DDT as a pesticide. Like its brethren the bald eagle and many other birds such as herons and falcons, osprey populations dipped dramatically when ingestion of this toxic pesticide through the food chain caused the birds to lay eggs with extremely thin shells. With the use of DDT banned in the United States in 1972 in no small part thanks to publicity of the DDT issue in Rachel Carson’s seminal book Silent Spring along with a change in the U.S. Coast Guard’s policy of removing osprey nests from channel markers, ospreys began to make a comeback. Today, the Chesapeake Bay harbors the largest community of osprey in the world, boasting nearly 3600 pairs. We are fortunate to have several nesting pairs on Nantucket which can be seen all around the island.
The osprey was one of the many species described by Carolus Linnaeus in his 18th century work, Systema Naturae, and was originally placed into the falcon genus as Falco haliaeetus. The genus name Pandion was described by the French zoologist Marie Jules César Savigny in 1809. Savigny was not having a good day when she named this creature Pandion, which she intended to be based on what she thought was the story of the mythical Greek king Pandion of Athens. Pandion had two daughters who became embroiled in a tragedy involving marriage, rape, imprisonment, weaving, infanticide, and even grimmer pursuits with Tereus, a Thracian and the son of Ares. All of these ill-fated people (except Pandion) were changed into birds, but no one was changed into a hawk and Pandion just had the bad luck to be the patriarch who makes some bad decisions. An excellent and completely diversionary tale of Pandion’s many woes can be found at www.paleothea.com/Myths/Procne.html.
Getting back to etymology, the word haliaetus is derived from the Greek words “halos” for “sea” and “aetus” for “eagle.” “Haliaetus” in this instance was misspelled by Savigny when it should have been spelled “haliaeetus,” which is the genus name for actual “sea eagles” such as the American Bald Eagle. The origins of the word “osprey” are a little tougher to figure out; the word itself was first recorded around 1460, and is most likely derived from the Anglo-French ospriet and the Medieval Latin avis prede "bird of prey.”
Ospreys can be found around the world on every continent except Antarctica, although the populations in parts of South America and Indo-Malaysia are considered non breeding populations (migrants) according to the Animal Diversity Web site. There are four subspecies of ospreys, which are separated by geographic region. Pandion haliaetus carolinensis is the species we see on Nantucket and it breeds in North America and the Caribbean, and winters in South America. P. h. haliaetus breeds in Europe, North Africa and in Asia and winters in South Africa, India and the East Indies. P. h. ridgwayi is a non-migratory subspecies with a range that extends from the Bahamas and Cuba to southeast Mexico and Belize. Another non-migratory subspecies is P. h. leucocephalus who occupies a range that includes Australia and the southwest Pacific.
Osprey are creatures that exhibit sexual dimorphism, which is a fancy way of saying the sexes look different and, in this case, are different sizes with the females being larger and typically having a more pronounced brownish “necklace” of color on their chests. They are large raptors, reaching 60 centimeters (24 in) in length with up to a 1.8 meter (6 ft) wingspan. Their long wings have a characteristic bend at the carpal ("wrist") joints. The osprey is generally dark brown on the back (dorsal) side) and wings, with white on the top of the head and extending from under the chin down the belly. A blackish-brown stripe runs through the eye to the nape. Its brown eye stripe looks like a “Zorro” type mask to me. The feet of this species are pale blue-gray, and the beak is black. Osprey lack the supra orbital ridge (bony plate over the eyes) that hawks and eagles have. In flight, the osprey can be distinguished from the bald eagle by the white belly and the crooked wings with dark wrist patches. Osprey’s irises change colors from orange-red to yellow as they get older and they have unique nostrils that are usually long, slit-like, and closeable. We’ll see why that is important soon. Juvenile ospreys resemble adults, but they have a somewhat speckled appearance due to light tan-colored tips on their dark brown upper-wing and back coverts (cover feathers) and a less well-defined necklace. Juvenile plumage is replaced by adult plumage by 18 months of age.
Ospreys are diurnal (daytime active) birds. Ospreys are able to begin breeding at three years of age and breeding takes place between March to September. The first pairs start to arrive on Nantucket around Easter. Nests are made at the tops of trees (usually coniferous, if available), on rocky pinnacles, or on artificial platforms. Scientists have found that in many cases, nests that are built on artificial structures such as nest platforms and power poles are more stable and fledge more chicks per breeding season than nests on naturally-occurring structures. High nesting usually occurs near waterway food sources and also protects the nests from land predators. Osprey will build their nests on islands and near marshes, as close as possible to good eating and safety.
Ospreys usually mate for life with one brood per year. Females lay 2 to 4 eggs of a creamy white to cinnamon color flecked with reddish brown spots with an incubation period of just over a month. Incubation and rearing young is shared, however males do most of the hunting while the females guard the nest. Breeding osprey may travel as far as 14 km (8.7 miles) on hunting forays although 3-5 km is more typical. Incubation takes around six weeks. It is interesting that the chicks hatch in sequence 3-5 days apart. Because incubation starts when the first egg is laid, the eggs hatch asynchronously in the order in which they were laid. Chicks that hatch first are larger and have a competitive advantage over those that are hatch later. If food becomes scarce, the smaller chicks are less successful in competing for food, and often die. This decrease in the number of chicks in the nest makes food more available to the surviving chicks, and increases their likelihood of survival. This process, common in raptors, is called brood reduction. The young are fully fledged at around 7-8 weeks after hatching. In the wild, between 40-60% of all the young die in their first year, those that do survive can usually live for 15-20 years in the wild; some have been known to live for more than 25 years.
Photo by Len Germinara
Nest building is very important skill in attracting a mate, as most females gravitate to the male who can provide the best nest for her young and the most food for the table (not all that different from humans). It is amazing how large some of the nests on island are (3-4 feet deep and 4-5 feet across are not uncommon) and the nest building materials include everything from diapers to plastic bags to ropes, floats, and balloons. Osprey will forage around the area of the nest and bring back a bewildering assortment of junk for the nest including a lot of dangerous material like fishing line. A four-year survey by the U.S. Fish and Wildlife Service from 2000 to 2004 found fishing line in 50 percent of nests along Chesapeake tributaries. They also found adults and young maimed or starving to death, enmeshed in the line - a reminder to anglers to dispose of their line with care. Chicks banded at the field station have been found essentially “hog-tied” in the nest by the ribbon attached to balloons, certainly reducing their chances to survive and fight for food morsels from the parents.
Ospreys feed almost entirely on live fish, the species of fish is dependent on what is available in the area. It prefers fish that weigh less than one pound and measure between 10 to 14 inches in length. Occasionally they will eat dead fish and only very rarely other animals such as rabbits, rats, ducks, turtles, snakes, and frogs. Very few things eat ospreys if their nests are built high enough to avoid mammalian or snake attacks on the eggs. In North America, great-horned owls and bald eagles are the only major predators of both nests and sub adults.
Pretty much every part of the osprey is designed to catch fish for a living. It is the only raptor whose outer toe is reversible, allowing it to grasp its prey with two toes in front and two behind. All of their toes are the same length too, which is unusual for raptors. The bottom of osprey feet are covered with sharp spines called spicules, which aid the birds in gripping their slippery prey. Their talons are also more curved than most raptors and they even have a long talon on the "little" or outer toe, unlike other raptors. Osprey carry fish "torpedo" style, i.e., headfirst which is more aerodynamic and makes it easier for the osprey to hang onto the fish or eel. They use both feet to hold all but the smallest fishes. Ospreys soar over water, sometimes hovering and scanning the surface until they see a fish. Upon locating a potential meal, they tuck their wings back and let the force of gravity pull them towards their prey. They plunge feet first into the water with their wings held up and back, sometimes hitting the water at a speed of 18- to 44-miles-per-hour and diving as much as three to four feet underwater. Dense feathers on their chest provide protection when hitting the water and their feathers are covered with oil that allows them to shed water. Ospreys have been known to drown, especially if they get their talons stuck in too large a fish and they can’t take off. And here is where the “nose valves” mentioned above come in handy, as their nostrils close when they hit the water just as nose clips help us during dives.
We have had some problems with chick survival, nest failures, and mortalities on the eastern side of the island over the past few years that Dr. Bob Kennedy of the Maria Mitchell Association is following; more information including recent banding and satellite tracking news can be found at http://www.mmo.org/natural-science/research-collections.html.
In order for a population to sustain itself, the number of recruits into the breeding pool must balance the losses due to adult mortality. In parts of the northeastern United States, biologists have calculated that a pair of ospreys must produce, on average, 0.8 young each year, for a population to remain stable. This break-even figure will vary with factors such as the availability of suitable nest sites, age at first breeding and mortality rates.
Some people worry that the nest failure is related to pollution, but here on Nantucket we do not have significant heavy metal sources although we do have mercury alerts for several of our ponds due to atmospheric input from coal-fired plants and other power plant emissions. Scientists have not found a substantial correlation between higher heavy metal concentrations and osprey and other large bird hatchling mortality. Scientists have documented that heavy rainfall, low temperatures, and strong winds during the early nestling stage can cause significant mortality amongst osprey broods, and often reduce the male's foraging success. Possible factors reducing the amount of chicks that survive on Nantucket may also include red tide outbreaks, reduction in prey species (less food for them to eat) due to factors like overfishing, frequent storms, and increases in human/osprey interactions. Another potential issue could involve eutrophication of the ponds that occurs as more nutrients flow into the ponds from fertilizers and septic systems and runoff which feeds algal blooms that can cause anoxic condition in the bottoms of some of the ponds and cloud the water with floating algae making it harder for osprey to see prey. Osprey need to be able to see their prey and both natural factors like turbidity from sand transport and ripples and waves and manmade causes like oil sheens can reduce an osprey's ability to find food.
Contaminants like PCBs and organochlorides are still a problem for raptors such as ospreys: www.allenpress.com/pdf/entc_24_315_617_629.pdf goes to a very thorough study looking at PCBs and pesticide concentrations found in ospreys in Delaware Bay. Many scientists consider ospreys to be the ideal sentinel species for evaluating the concentration and introduction of pollutants in the environment because of their wide distribution around the world and their place near the top of the food chain. In that manner, they are the larger, more tough looking canary in the larger coal mine that is the planet.
Here at the UMass Boston Nantucket Field Station we are lucky to have an osprey nest that you can observe by going to www.umb.edu/nantucket, clicking on the Eider Cam and then hit the preset to see the osprey nest. You can also see the nest from the lab camera. This week you can see the female osprey feeding her young.
