Osprey

Ospreys are large, distinctively shaped hawks whose diet and affinity for coastlines have earned them the nicknames “sea hawk” and “fish hawk.” Their slender bodies and long legs mean that, even with a wingspan reaching nearly six feet, these birds only weigh up to 4.5 pounds. The wings have a characteristic bend at the wrist joints in flight, making an M shape. Their plumage is dark brown on top and bright white underneath, with dark brown patches at the wrist joints, a mottled dark brown necklace, a dark stripe through each eye, and a distinctive white crest on the head. While there is much variation, females often have darker plumage and a more defined necklace than their male counterparts. The feet of this species are pale blue-gray, the beak is black, and the eyes are yellow. Females are also about 20 percent heavier than males, and have a 5 to 10 percent greater wingspan. Because of their plumage coloring and flight shape, ospreys are most often confused with gulls when seen from below.

Look for ospreys around nearly any body of fish-filled water. They have a worldwide distribution, wintering or breeding on every continent except Antarctica. That’s because they can eke out a living almost anywhere there are safe nest sites and shallow water with abundant fish. They occupy a broad range of habitats, ranging from mangrove islets of the Florida Keys to Alaskan lakes, from New England salt marshes to the saline lagoons of Baja California. Rivers, lakes, reservoirs, estuaries, lagoons, swamps, marshes, and even coral reefs are all fair game. There is only one species of osprey, but there are four subspecies:
• Pandion haliaetus carolinensis breeds in North America and the Caribbean, and winters in South America;
• Pandion haliaetus haliaetus breeds in Europe/North Africa/Asia and winters in South Africa/India/East Indies;
• Pandion haliaetus ridgwayi is non-migratory and resides in the Caribbean, from the Bahamas and Cuba to southeast Mexico and Belize;
• Pandion haliaetus leucocephalus is also non-migratory; its range includes Australia and the southwest Pacific.

There is some morphological variation between the subspecies, mainly by region. Tropical and subtropical individuals tend to be smaller than individuals that breed at higher latitudes. Cuba and Hispaniola are key migration hubs for our local ospreys hanging out on the east coasts. In fall, these migrants funnel down the Florida peninsula, hop to Cuba, then move east across Haiti and the Dominican Republic. From there they cross the rest of the Caribbean to wind up in South America. Ospreys on the west coast don’t migrate as far, and spend less time crossing ocean. They typically winter in Central America. Ospreys migrate through Texas from March through May and again from September through November. Some overwinter in Texas, but only a few breed here.

An osprey may log more than 160,000 migration miles during its lifetime. Scientists track ospreys by strapping lightweight satellite transmitters to the birds’ backs. The devices pinpoint the bird’s location to within a few hundred yards and last for two to three years. During thirteen days in 2008, one osprey flew 2,700 miles—from Martha's Vineyard, Massachusetts, to French Guiana, South America.

The osprey is the only hawk on the continent that eats almost exclusively live fish, and they have several morphological adaptations for this unique lifestyle. These adaptations include relatively long legs (for a raptor); barbed footpads called spicules; long, sharp, curved claws, and a reversible outer toe to aid in gripping slippery fish. That special reversibility means their toes can be held with three forward and one back, or with two forward and two back, an arrangement seen in owls but not in other diurnal raptors. In addition, ospreys have dense oily plumage and efficient nasal valves that prevent water from entering the nostrils when the bird dives to catch a fish. Since they can only dive up to about three feet, they tend to stick to shallower fishing grounds, though they’ll venture out over deeper water if fish are schooling near the surface. Ospreys hunt on the wing (as opposed to from a perch), gliding up to 130 feet above the water. They often hover briefly before diving towards the water surface. Just before hitting the water, the osprey swings its legs forward and sends its wingtips towards the sky, plunging feet-first into the water. It then has to power itself and its prey from the water. Once airborne, the osprey rearranges the fish so that the head is facing forward. Presumably, this makes the fish more aerodynamic and easier to carry.

In North America, fish account for 99 percent of the osprey’s diet. The type of fish varies by region; they eat a lot of mullet along the Texas coast. However, they have been observed eating other non-fish prey on occasion, including birds, snakes, voles, squirrels, muskrats, salamanders, conchs, and even a small alligator. Reports of ospreys feeding on carrion are rare, but also not unheard of. They do not generally need to drink water as fish flesh supplies sufficient amounts, although there are reports of adults drinking on hot days.

Ospreys are excellent anglers. In medieval times, fish were thought to be so mesmerized by the osprey that they turned belly-up in surrender. In slightly more recent studies, ospreys caught fish on at least 25 percent of their dives, with success rates sometimes as high as 70 percent. The average time they spent hunting before making a catch was about 12 minutes — something to think about next time you throw your line in the water.

Though ospreys don’t have many predators as adults, bald eagles will sometimes chase them and steal their catch. In Africa, Nile crocodiles sometimes kill ospreys bathing or roosting near water. Raccoons, snakes, and other climbers are suspected thieves of eggs and nestlings. Osprey parents try to discourage these nest predators by choosing elevated nest sites with open surroundings. They’re not too picky about the base, as long as it’s close to fish – snags, treetops, utility poles, channel markers, duck blinds, cliffs, human-built platforms, etc. are all acceptable. In fact, artificial platforms have become an important tool in reestablishing ospreys in areas where they had disappeared.

Osprey courtships center on nest sites, aerial flight displays, and food. One notable display is the “sky dance,” in which a male carrying a fish or nest material alternates periods of hovering with slow, shallow swoops as high as 600 feet or more above the nest site. Sustaining this display for ten minutes or more, he utters repeated screaming calls while gradually descending in an undulating fashion to the nest. A variation involves the pair circling high together between sky dances. In migratory populations, males and females arrive at the nest site separately, the male often arriving several days earlier. Both sexes collect materials for the nest, but the female arranges most of the materials. Nests are typically constructed of sticks and lined with softer materials such as seaweed, kelp, grasses, or cardboard. A wide variety of flotsam and jetsam may also be incorporated, including fishing line, plastic bags, and nearly anything else that an osprey might find and can lift. Nests on artificial platforms, especially in a pair’s first season, are relatively small, less than 2.5 feet in diameter and three to six inches deep. However, pairs use the same nest year after year, and spend some time each year repairing it and adding materials before eggs are laid. After generations of repeatedly adding materials, ospreys can end up with nests ten to thirteen feet deep and three to six feet in diameter, easily big enough for a human to sit in.

Once a pair has established (or re-established) a nest, the male takes over grocery shopping, and continues until the young fledge. Generally, females that receive more food are more receptive to mating attempts by the male, and are less likely to copulate with other males. Females beg for food from their mates, but will implore neighboring males if they are not well fed by their mate. Nesting ospreys defend only the immediate area around their nest rather than a larger territory. Though generally monogamous, polygyny can occur in rare instances where nest sites are close enough together that a male can defend both. When this occurs, the first nest usually experiences higher reproductive success than the second because the male devotes more resources to that nest.

Two to four eggs, creamy white with brown splotches, are laid over a period of several days, each one to two days apart. Both parents incubate the eggs, which hatch in about forty days. Because incubation starts when the first egg is laid, the eggs hatch in the order in which they were laid, each one or two days apart. Chicks that hatch first are larger and have a significant competitive advantage over their younger siblings. If food is abundant, chicks share meals in relative harmony; in times of scarcity, younger ones may starve to death. This happenstance, common in raptors, is called brood reduction. Once the chicks hatch, the female remains with them most of the time. The male typically provides three to ten fish per day, which the female tears up into tiny bites and feeds to the chicks. A male who is also providing food for a mate and offspring during the breeding season will typically consume at least part of the fish before delivering the remainder to the female and chicks. Ospreys do not cache their prey. If a fish is larger than the family can consume, it is discarded.

Newly hatched chicks are covered in white down with brown streaks on the face, back, and wings. Charcoal down replaces the white in about ten days. Feathers begin to come in after about two weeks. By one month after hatching, chicks are three quarters the size of their parents. They fledge between 2.5 and 3.5 months old. 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 two to eight weeks after fledging. Generally, chicks in migratory populations fledge sooner than those in non-migratory populations since juveniles must be fully independent of their parents by the time the southward migration begins. Juveniles resemble adults, but have a somewhat speckled appearance due to buff-colored feather tips and a less well-defined necklace. Juveniles also have an orange-red eye, rather than the yellow that is typical of adults. Juvenile plumage is replaced by adult plumage by eighteen months of age.

Ospreys are sexually mature at approximately three years old, but may not breed until five in areas where nest sites are scarce. Yearling ospreys of migratory populations in both Europe and the U.S. almost always remain on the wintering grounds throughout the year, skipping the breeding migration altogether. This strategy allows young ospreys that are too physically immature to breed to avoid an unnecessary migration. The oldest known osprey in North America was a 25-year-old male in Virginia. He was banded in 1973, and found in 1998. The oldest known female lived to 23 years. However, very few live to this age. Chances of survival from one year to the next varies between populations, but are estimated to be approximately 60 percent for ospreys less than 2 years old and 80 to 90 percent for adult ospreys.

These large, rangy hawks do well around humans and are a conservation success story. Osprey numbers crashed in the early 1950s to 1970s, when pesticides poisoned the birds and thinned their eggshells, but after the U.S. banned DDT in 1972, populations rebounded. As natural nest sites have succumbed to tree removal and shoreline development, specially constructed nest platforms have become vital to their continued recovery. Sadly, a growing cause of death for young ospreys is entanglement at the nest. The adults incorporate baling twine and other discarded lines into their nests, which sometimes end up wrapped around a chick's feet, injuring it or preventing it from ever leaving the nest. Ospreys are also still shot during migration and on their wintering grounds by fish farmers, even though studies have demonstrated that ospreys take a very small portion of all fish harvested and are not serious competition for commercial and recreational fishing. Indeed, they can be economically beneficial for local economies by boosting ecotourism. Additionally, they are a valuable indicator species for monitoring the long-term health of large rivers, bays, and estuaries. Their piscivorous lifestyle and known sensitivity to contaminants make them well-suited to this role. They are also relatively easy to study because they have conspicuous nests and are tolerant of short-term disturbances by researchers. Really, there’s no downside to these fishy birds!

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