Our Water Hawk - The Osprey
by Charlene DeVol
It has been a very interesting and unusual 2005 season at the Wild Bunch Refuge. Diana O'Connor rehabilitated 25 Ospreys, which is extremely unusual. Bordered by the Rappahannock and Potomac rivers and the Chesapeake Bay, the Northern Neck area provides a large habitat for these magnificent raptors. The majority of these birds that come into care are either found after falling out of a nest, injured, or unable to get themselves out of water when fledging. Others are found tangled in fishing lines.
It is not an easy task to rehabilitate Ospreys, and can be dangerous as well as expensive. They tend to be very high strung and stress very easily. Teaching them to self feed is also not an easy task. In captivity, these birds can be very aggressive and will often charge when their cage is entered. Their sharp talons and strong beak can do serious damage. Ospreys must be fed fresh fish every day, which is costly and not always easy to obtain. We were lucky to find a local grocer that would sell fresh perch (their favorite) at a discount.
Diana is the only rehabilitator in the Northern Neck area of Virginia that rehabilitates Ospreys. Because there are such a small number of rehabilitators in Virginia that handle Ospreys, calls that come into the Wildlife Center of Virginia, as well as Kent Knowles at the Raptor Conservancy of Virginia are often referred to her. Diana is tremendously knowledgeable about Ospreys and has excellent release sites that she carefully selects. Diana admits that Ospreys are her favorite bird of prey as she admires their feistiness and attitude.
The Osprey is one of the most widely distributed birds in the world. They are found to winter or breed on every continent except Antarctica. They are particularly abundant in Scandinavia and the Chesapeake Bay area in Maryland. Bones from an earlier, but very similar species, found in California and Florida, show they have been in existence for approximately the last 13 million years. There are four subspecies of Ospreys, which are separated by geographic region. One of the two migratory subspecies breeds in North America and in the Caribbean and winters in South America. The other breeds in Europe, North Africa and Asia and winters in South Africa, India and the East Indies. The two non-migratory subspecies reside in either the Caribbean or Australia and the southwest Pacific.
The Osprey (Pandion haliaetus), whose species name is derived from the Greek hals (salt or sea) and aetos (eagle), is the only bird of prey, active during the daytime, that feeds primarily on live fish. The Fish Hawk, Sea Hawk or Fish Eagle, as it is also known, is a medium to large raptor with a body length of about two feet, a wingspread of 5 to 6 feet and long sharp talons to aid in holding onto slippery fish. They weigh from 2-5 pounds, with the female generally being larger than the male. The Osprey is very distinctive from other raptors. The back and long arched wings are brown with white plumage covering the breast and upper legs. They have distinctive dark brown patches at the bend of each wing and dark brown stripes through the eyes. In flight, it commonly soars or hovers with a distinct crook in the elbows of the wings.
Ospreys are extremely successful because they are able to live almost anywhere where there are safe nest sites and shallow water with abundant fish. Their habitat includes lakes, rivers, swamps, bogs, reservoirs and salt marshes. Ospreys typically choose structures that can support a bulky nest and are safe from ground-based predators.
Nest sites are made safe from predators either by being difficult for predators to climb to or by being over water or on a small island. Over-water nest sites often used by Ospreys include buoys and channel markers, dead trees and artificial nest platforms. Ospreys have also been known to nest on various man-made structures, such as power poles, duck blinds, communication towers, buildings and even billboards.
Ospreys are vulnerable to predation from aerial predators, such as owls and eagles. In North America, Bald Eagles and Great Horned Owls are known predators of Osprey nestlings and occasionally adults. Raccoons, snakes and other climbing animals are suspected predators of Osprey eggs and nestlings. Probably, to afford greater protection from these ground predators, the majority of Osprey nests in many areas like the Chesapeake Bay are built over water.
The Osprey diet consists of 98 percent fish and 2 percent birds and rodents. Ospreys feed almost entirely on live fish, the species of fish is dependent on what is available in the area. Occasionally, they will eat dead fish and only very rarely other animals such as rabbits, rats, ducks, turtles, snakes and frogs. They do not generally need to drink water as fish flesh supplies sufficient amounts to meet their requirements.
The Osprey is particularly well adapted to its diet of fish, with reversible outer toes, closable nostrils to keep out water during dives, and backwards facing scales on the talons which act as barbs to help catch fish. It locates its prey from the air, often hovering prior to plunging feet first into the water to seize a fish. They can hit the water at speeds of 18 to 44 miles per hour and dive as much as 3 to 4 feet underwater. With a fish held securely in their talons, they pull themselves out of the water with one powerful stroke. As it rises back into flight, the reversible outer toes will turn the fish head forward to reduce drag. The feet are such effective tools for grasping fish that, on occasion, Ospreys have drowned because they were unable to release their grip on a fish that was heavier than expected.
Ospreys mate for life and generally start breeding at three years of age. In the Chesapeake Bay area, with a larger concentration of birds and great competition for nesting sites, it is more common for these Ospreys to start breeding at five to seven years old. The spring mating ushers in a five month period when both male and female work together to raise their young. Both sexes collect material for the nest, but the female does most of the actual nest building.
Nests are typically constructed of sticks and lined with softer materials such as seaweed, kelp and grasses. A wide variety of other materials may also be used in the nest, including fishing line, cardboard and plastic bags.
Females lay three to four eggs by late April, and rely on the size and design of their nest, with its pronounced depression at the center, to conserve heat. Eggs are laid at intervals of several days until the clutch of three to four is complete. The eggs are similar in size to very large chicken eggs, mottled and the color of cinnamon. The eggs are incubated by both male and female for approximately 5 weeks. Osprey chicks normally fledge between 48 and 76 days of age. After fledging, young Ospreys begin to hunt on their own. However, they often continue to return to the nest to receive food from their parents for up to eight weeks after fledging. Because Ospreys migrate individually, juvenile Ospreys must be fully independent of their parents by the time the southward migration begins.
When food is scarce, the first chicks to hatch are the most likely to survive. In areas of low fish abundance, often only one Osprey nestling will survive. In highly productive areas, two or three may survive. On average, 40% to 60% of all newborn Ospreys die in their first year. It is not known how long the average Osprey lives, but they are thought to be a relatively long lived bird species. The oldest male ever recorded in captivity was 25 years old and a female 23 years old.
Ospreys nest at a range of densities, from very solitary to loose colonies. Colonies may form because the presence of established nests is a signal of suitable habitat to arriving individuals, or because good nest sites are often clustered together, such as on an island or along a power line. Grouping of nests is uncommon in raptor species because most raptors defend the feeding territory around their nest. Ospreys, however, only defend their nest or nest site and not the surrounding territory. They are often observed hunting in groups, which is also unusual for raptors. Ospreys are known to travel from their nest during hunting forays almost 9 miles away. They are able to cover such a large area as they fly, on average, 40 miles per hour.
There are no real known negative impacts of Ospreys on humans. It was thought, in the past, that they competed with fisherman for fish but studies have shown that they consume a very small portion of all fish harvested.
Ospreys can be valuable indicators for monitoring the long term health of large rivers, bays and estuaries because of their fish eating lifestyle and their known sensitivity to many contaminants. This can help in detecting habitat destruction, dwindling fish populations and contamination of the environment. For many years, the Osprey was a marker for the decline of the Chesapeake Bay. Although the overall health of the Chesapeake Bay continues to be of serious concern, Ospreys, in recent years, have made a dramatic recovery. In the 1960s the number of Ospreys in the Bay region decreased dramatically due to the widespread use of pesticides such as DDT, which caused the birds to lay eggs with extremely thin shells that resulted in lower hatch rates. With the US ban on DDT in 1972 and the reversal of the Coast Guard's policy of dismantling Osprey nests on channel markers, Osprey numbers began to rebound. Active nests can now only be removed from channel markers and buoys with a permit. Ospreys still face problems resulting from exposure to pesticides, habitat loss, and being shot; however, today it is estimated that more than 2,000 pairs currently nest in the Chesapeake Bay area, the highest concentration of Ospreys in the world. We, at Wild Bunch, are pleased to have so many of these fascinating raptors as our neighbors.