The Illustrations are screen-captures from the Golden Gate Audubon Society nest cam, some of which I tinkered with in Photoshop.
from: University of Toronto
OSPREY - Pandion haliaetus
Infra phylum: Gnathostomata
Body length: 53-65cm, females are larger than the males.
Wing length: -
Bill length: -
The osprey is a large bird of prey about the size of a small eagle. The crown and upper nape are white, streaked with dark brown. The crown also bears a crest, which is not erectile but rides in the wind as the bird flies. The dorsal aspect of the bird's body is chocolate colored from the lower portion of nape, through the mantle, back, wings and rump, down to the tip of the tail. In contrast, the chin, throat, breast, belly, flanks, shins, undertail coverts, lesser underwing coverts and vent feathers are primarily white with sparse mottling, which is more pronounced in females than the males; males tend to be whiter on the underside than females.
A distinctive dark brown stripe runs from the lores, through the eye and back towards the lower nape to join the rest of the brown on the dorsal side. This stripe separates the throat from the nape.
There is a ridge of dark feathers over the eye, which is believed to reduce the glare of sunlight while hunting over sunlit waters.
In juvenile ospreys, the brown body feathers have lighter margins, thereby giving them a scaly look, which makes them easier to distinguish from the adults.
The iris of a juvenile osprey ranges in color from red to orange, which changes to bright yellow on reaching maturity at the age of 3 to 5 years. Its eyesight is eight times sharper than that of humans.
The osprey's nostrils are long and slitted, and may be capable of closing during dives. The cere and nostril area is dull blue-grey and so are the feet, while the hooked bill and sharp talons are black. The bills and talons grow continuously to compensate for the wear and tear during hunting. The footpad and the pads under the toes are covered with sharp spiny scales, which help the osprey grasp the slippery fish. The osprey's talons are capable of snapping shut within two hundredths (2/100) of a second (this may be the result of a tactile reflex rather than a voluntary one). Moreover, the outer reversible toe (the fourth digit) rotates posteriorly to give the bird a better grip of its prey, while its long legs and tarsi increase its underwater reach by upto 3 feet.
The preen gland, at the dorsum of the base of the tail, secretes a pungent oily substance, which keeps the feathers from being soaked in water during diving. The odor of this secretion lasts in the plumage of laboratory specimens, even after decades of storage. As the bird preens itself, it picks up the secretion from the preen gland and spreads it all over its feathers, with its beak, to make them waterproof. However, prolonged exposure to water, as in heavy rains, soaks up their feathers, rendering them unable to fly.
Unlike many migratory birds, ospreys don't molt all of their feathers together, they rather molt a few feathers at a time, thereby remaining capable of flight, and thus hunting, all of the year. The molting process stops prior to migration, and in males during the breeding season, since during that period, they have to hunt for their mates as well as their young which demands greater then normal flight efficiency.
When perched, the osprey's wing tips extend beyond the tip of the tail.
In contrast to the streamlined, graceful figure of a typical perched raptor, a perched osprey appears rather untidy with wings drooping rather than held tightly against the body.
The flight colouration of osprey is unique among raptors. The ventral aspect of both the remiges (wing feathers) and the rectrices (tail feathers) is dirty white, cross-barred with brown. The outer five primaries are either black in colour, or tipped with black.
The primary underwing coverts are dark brown; the secondary underwing coverts are white, tipped or streaked with black, while the lesser underwing coverts (except the wrist area), the belly, flanks, vent and the undertail coverts are all white. The underwing coverts in the wrist area are jet-black, producing distinctive, rectangular, black wrist patches.
Another important feature expressed during flight is the noticeable bent at elbows, producing a diagnostic M-shaped wing silhouette, in contrast to the +shaped wing silhouette characteristic to eagles; moreover, an osprey's wings are also narrower than those of eagles and hawks.
Ospreys usually soar with wings slightly bent, wingtips pointing downwards and back, and wrists held above the body level. However, ospreys are the only raptors, which can hover over a fixed point during flight.
Male and female ospreys can be distinguished from each other both in terms of size as well as coloration. Unlike the male osprey's mostly pure-white under parts, the female's breast is streaked and mottled with brown, the streaks often taking the form of a mottled necklace. The dark streaks on the crown are also more pronounced and denser in the female than the male.
Moreover, the female osprey is also larger and heavier than the male, which is a confusing phenomenon, since it is the male, not the female, who does most of the hunting and defending the territory from intruders. One possible explanation might be that the female during breeding season eats more and flies less, since she is fed by her mate, therefore she has more time and protein to grow larger.
Although some scant fossil remains (only a few claws), from the Eocene epoch (50 million years ago), are suspected to belong to some paleo-osprey; however, more complete, obvious and widely authenticated remains of osprey (i.e. wing bones), date back to mid-Miocene epoch about 13 million years ago. These remains were found in California and the species was christened Pandion homalopteron by Warter in 1976, who questioned the authenticity of the Eocene remains, as fossil evidence of osprey's origins.
Some other fossils ranging from mid to late Miocene include an egg from Austria and hind limb bones from Florida. This suggests that ospreys were well distributed in Northern Hemisphere some 10-15 million years ago. The Florida fossils are complete enough to suggest that late Miocene ospreys were similar to modern ospreys, although not so robust.
Some fossils from Pleistocene epoch (2 million years to 10 000 years ago), found in Western Europe, North and Central America are very similar to the modern ospreys. This suggests that ospreys were well established in their modern geographic range by that time.
Habits & Habitat:
Geographic Range & Distribution: Ospreys are cosmopolitan, they are common along the shores and banks of bodies of water in all the biogeographical regions (namely Nearctic, Palearctic, Neotropical, Ethiopian, Oriental and Australasian) except the Antarctic region. Ospreys are found in a wide variety of biomes such as tropical rainforest, temperate rainforest, deciduous forest, coniferous forest, mixedwood forests, mangrove forests, salt marshes, freshwater swamps, freshwater lakes, estuaries and lagoons etc.
In Canada, they are confined to the wide belt of coniferous forest, stretching from the Atlantic coast to the coasts of British Columbia. Coastal nesters are abundant along the Atlantic sea-board in the provinces of Nova Scotia and New Brunswick, where shallow bays and islands encourage breeding colonies; but the greatest majority of Canadian ospreys colonize the shores of inland lakes and rivers.
However, their range in Canada does not extend as far North as in Fennoscandia, where their range extends as far North as the tree line. The reason might be that bald eagles, the chief competitors of ospreys as fishing-raptors, have contained the ospreys from spreading further North in Canada, since they themselves are well established in the Northern latitudes up to the tree-line.
Ospreys are not very choosy when it comes to their diet. They will feed on any fish, which falls within the optimal range of size and weight they can carry, that is 150-300 gms (Poole 1989); although the weight of prey may wary from 50-1200 gms (Cramp & Simmons, 1980: Prevost, 1982). However, the prey must be within their striking range of 3-feet from the surface.
Their preferred targets are either slow moving, bottom feeding fishes such as suckers, bullheads, catfish, carp, fallfish, flounder, perch etc., which dwell shallow waters; or surface feeders like herring and menhaden, which form large schools thus offering easy pickings to the osprey.
What is on the menu also depends on which fish are available during a particular season, since in a specific area, during a particular season, some fish are more plentiful than the others are. Ospreys take full advantage of such seasonal bounties, which come in the form of fishes such as herring, pollock and smelt, which come to shallow coastal waters to spawn. Inland ospreys however, have more rigid menus all year round.
In the Great Lakes region, their diet consists mainly of brown bullhead, rock bass, small and large mouth bass, pumpkinseed, blue gill, white sucker, carp and yellow perch.
Besides fishes, ospreys are also recorded to prey on other animals such as small birds, turtles, frogs, rodents, crustaceans and mollusks like conchs, and in one recorded instance, also a small alligator; however, such prey items rarely comprise more than 2% of the bird's diet.
Hunting and Feeding:
Ospreys search for food either by scanning the water surface from a perch, relying on their sharp eyesight to locate potential prey; or on the wing, soaring over water, looking for food.
Wintering ospreys prefer to hunt from a perch than on the wing, since they have only themselves to feed. When not hunting or feeding, ospreys spend their time perched on a branch over water, droop winged, preening and fluffing their feathers and shaking to dry to them prior to the next hunt.
When hunting on the wing, the osprey begins its search for food by soaring over water at an altitude between 50-100 feet. When it spots a potential prey, it stops and hovers over the spot, tail spread and wings fanning rapidly as it judges the depth and positions itself precisely for the plunge.
Osprey -- the ultimate fisher2 min. 51 sec.Most dives are made from the altitude of 65-100 feet at an angle ranging from 45 degrees to nearly vertical. When an osprey finally prepares for a dive, it folds back its wings and free falls, it then positions its feet right under the head so that it can precisely aim at the prey with its outstretched talons. It may launch a preliminary dive to make certain the prey is the within the range and of the right size, even at this point it may abort the dive, if the prey turns out to be undesirable or far from its effective range of 3 feet or less underwater. When it plunges in, a hunting osprey disappears completely under the water and it may take several seconds for the bird to reappear on the surface. If the dive is successful (which certainly is in about 40% of the cases) and a fish is caught, the bird rests on the surface of the water, briefly while securing its catch. It then slowly lifts itself above water using deep almost horizontal stokes of wings. After being airborne it rearranges its prey so that the head points forward and one foot of the bird is ahead of the other, this reduces the air resistance and speeds up the return flight. On its way back, the bird flies low over water to avoid any heavy gusts of wind and it also shakes off excess water in mid air.
This hover-plunge technique is the standard hunting practice of ospreys. However, some observers have also reported seeing ospreys swooping down to skim the water and snatching prey from the surface, just like fishing eagles (Haliaetus sp.) do. This technique is more effective for smaller prey, species such as sardines, shad, etc. which swim to the surface in large schools. Besides the hover-plunge and swoop &snatch techniques, some other seldom-practiced hunting techniques have also been observed.
When the osprey returns to its perch to feed on its catch, it first waits for the prey to die and then uses it strong tough beak to tear off chunks of flesh, usually starting from the head. When feeding, they usually tear off small chunks but sometimes they may swallow large pieces of flesh with skin, scales and even bones. They regurgitate indigestible matter in the form of pellets, similar to those of an owl, but mostly their food passes normally through the gut, thanks to their specialized intestine.
There are two distinct populations of osprey with respect to lifestyle, the Residents and the Migrants. Residents are the ones, which breed in the subtropical latitudes in winter; they either stay in the same area all year round or move only locally during the non-breeding season. The other ones are the Migrants, which nest in the temperate latitudes and fly off each autumn to spend the winter in the tropical regions.
North American ospreys start migrating south to their wintering grounds in Central America and in the northern parts of South America, around mid-August. By early September, most nesting colonies in the northern latitudes are empty. The numbers of migrating ospreys observed at selected watch points along their southward route are at their greatest during mid-September and early October.
They reach their wintering grounds by late November. During winter, the only thing an osprey does is to eat, rest, recover from the stress of the previous breeding season and prepare for the next breeding season. It has been observed that during winter, they become more tolerant of each other as they congregate in loose flocks and are even known to hunt together in small groups, a few birds hovering over water, looking for fish might also stimulate others to join in the frenzy.
Adult ospreys start leaving their wintering grounds in early March, by late March, more than half of the wintering ospreys have left the wintering areas, for their natal nesting grounds.
Yearling ospreys stay on their wintering grounds for another eighteen months, returning to their natal nesting grounds at the age of about two years.
Two-year-old ospreys however, leave the wintering grounds much later than the adults, and may reach their natal nesting grounds as late as June, by then; they are already too late to breed. However, pre-adult ospreys may become a nuisance to the breeders, because they try to take over nest sites and may interfere with the breeding pairs, as they explore and search for nesting sites of their own. They are also sometimes known to replace a partner in a breeding pair if it dies or is incapable of performing its duties.
Ospreys arrive back at their natal nesting grounds by early May.
Besides the courtship call of the male, the osprey vocalizations have been classified into three main types.
1) Guard calls comprise a slow series of whistles and are emitted when an intruder (usually another osprey) or potential threat approaches too close for comfort. These calls are meant to indicate that the bird is aware of the threat and will attack if it persists.
2) Alarm calls range from a series of whistles to high pitched squeals and are emitted when a threat becomes obvious and inevitable. This also alerts other birds in the colony, which may join in the chorus too, resulting in a frenzy of loud calls, which eventually drives off the enemy.
3) Begging calls are emitted by the females as they beg to their mates to feed them, since during the breeding season, they spend most of their time at the nest, incubating the eggs and caring for the young. Begging calls of nestlings are similar to those of the females'.
When an osprey nest is approached, aside from uttering guard calls and alarm calls, they will also respond by making a threatening posture with erect stance, neck extended, back feathers erected and wings partially opened and beating slowly. The guarding bird will chase off any airborne intruders, while those approaching from the ground are dived at and struck with talons.
The arrival of ospreys at their natal nesting grounds in early May marks the start of breeding season. Males tend to arrive a few days ahead of females, to stake claim of suitable nesting sites. Older, experienced breeders are the first to reach the nesting grounds, followed a few weeks later by younger ones.
Established pairs always return to their previous nesting sites, while the new arrivals or those which have lost their nests, may spend weeks searching for an adequate nesting site. Only large trees can bear the weight of an osprey's nest, made of sticks, twigs and dead branches of trees. Secondly, it must be close to water, 3-5 km from water is an adequate distance. The long narrow wings of an osprey are poorly developed for turning tight corners, therefore there must be open space around the nest site for the bird to reach and land. The tops of isolated, often dead but sturdy trees are the preferred nesting sites; if a live tree is chosen, its top is generally flat.
In recent times however, man-made structures such as power poles light towers and special platforms are slowly replacing natural nest sites. These artificial sites are also usually difficult for nest robbers (such as raccoons) to climb, which is an added advantage.
Courtship and Nest Building:
Older, established pairs start repairing and rebuilding their previous nests as soon as they arrive. The younger, new arrivals however, have to first find a nest site and a partner to get started. When a single male arrives at the nesting ground, he searches for a suitable, unoccupied nest site; and when he finds one, he stakes his claim to it and defends it from other marauding bachelors.
The male begins his aerial courtship display by emitting a high pitched, continuous courtship call, as he flies with legs dangling and with a fish or nesting material clutched in his talons; although some males also display without anything in their talons.
These displays are performed near the nest and in clear weather, the flight pattern during the display is either of the two distinct types. The first one is an undulating flight in a wave pattern with brief hovers at the crest of each wave. The second courtship flight pattern is the one in which, the displaying male rises vertically upwards like a helicopter and falls down repeatedly, with wings fluttering all the time.
If a passing female is impressed by the location of his nest site or by his aerial display, she responds by landing at the nest or nest site and begging for food. The male in turn feeds the female and thus a pair bond is formed. When it comes to courtship feeding, older, more experienced males are more generous at feeding their mates than younger males, which are reluctant to do so. Mated pairs also perform these courtship rituals, in order to strengthen their bonds.
Nest building begins soon after the partners bond to each other. The nests are usually made of sticks and twigs and are furnished with seaweed, kelp, grass, paper, fishing nets etc. When actively at work, the pair may make
over a hundred trips a day, to gather nesting material; both male and female gather nesting material, but it is the male who gathers most of the nesting material, and the female who arranges it into the nest. Small adornments and furnishings continue to be added to the nest throughout the breeding season.
The pair bond in ospreys is life long. The successful pairs assemble at their nest sites year after year. However, if one of the partners dies, the other will find a new mate. The only instance when an osprey switches its mate, despite its previous mate still living, is when the pair had failed to breed successfully, the previous year. A female might leave her mate if he provides no food.
Copulation and Laying:
After the pair has established itself on the nest site, and the nest is complete, the female spends most of her time at the nest, begging for food to her mate rather than hunting for herself. Copulation usually occurs at the nest, sometimes the female takes the initiative by tilting forward with raised tail and drooped wings; but most of the times, the male starts copulation by mounting the female and tarsi resting along her back, vigorously flapping to maintain its balance. If the female is willing and receptive, she tilts forward and raises her tail, allowing the male's tail to slide under hers, thus facilitating cloacal contact and transfer of sperm. She usually maintains this position even for a few moments after the male dismounts.
The success of copulation depends largely on the forward tilt of the female, since if the female is not willing or unreceptive, she will either maintain the horizontal posture or will tilt back on her tail, causing the male to slide off her back. Young pairs are less likely to copulate successfully than older, more experienced pairs. One possible explanation to this low success rate might be the reluctance of young males to pass on food to their mates, since courtship feeding is a key stimulus in making the female receptive.
The earlier copulations do not fertilize the egg, since the females have not become receptive by that time; these copulation attempts rather stimulate the growth of eggs within the female's ovary and serve to strengthen the pair bond. However, copulations attempted a few days prior to laying are more critical in fertilizing the eggs. During these few days, when the female is at her peak of fertility, the male guards her jealously at all times, even accompanying her during flight; swooping low over her and gently stroking her back with closed talons as if urging her to return to the nest.
Some male ospreys are known to keep and defend two different females, either at the same nest or at two different nests. Polygyny is common where male mortality rate is high.
After a few days of copulation, the female lays 2-4 eggs. The egg laying is the second landmark event in the breeding season, which happens after 10-30 days of the arrival of the first breeding adults in the nesting colony. The eggs range in color from dirty white to light brown, mottled or blotched with red or dark brown. The eggs weigh 60-80gms each, about the same size as chicken eggs. The eggs are laid one or two days apart, with the first egg being the largest and the later ones subsequently smaller.
Incubation and Hatching:
When incubation starts, the birds spend most of their time at the centre of the nest, where the eggs lie in a shallow depression, rather than perching on the edges as they would normally do.
Both male and female incubate the eggs and both have brood patches; the heavily vascularized areas of skin which lose their down feathers during the breeding season to allow the bird's body heat to transfer to the eggs. However it’s the female who performs the bulk of incubation duties and always takes the night shift.
The eggs hatch about five to six weeks after being laid and in the same sequence as they were laid. If the first clutch is stolen or destroyed, a second clutch of eggs is laid about three weeks after the failure of the first one.
One or two days prior to hatching, the developing chick starts tapping the egg shell with its beak. A small outgrowth on the tip of the upper mandible, called the egg tooth helps the chick break free of the shell; at this stage, faint peeping calls of the chick can also be heard. The neck is the strongest part of a newly hatched chick's body, which drives its beak to break through the shell.
The osprey chicks are halfway between altricial and precocial; they are described by biologist Alan Poole as semi-precocial. Their eyes open just hours after hatching, their body is covered with down feathers; and they can actively pick chunks of food from their parent's bill, rather than fed bill to bill by the parent. The chicks however, are not mobile at birth. They start begging for food at any movement on the nest's edge, standing weakly with shaking heads, open bills and necks extended high.
The chicks are kept warm by their buff colored plumage of down feathers called the first down. The crop develops within the first week of their life, which stores food so that the chicks don’t need to be fed very frequently. They usually double their weight in the first week of their lives.
At ten days of age, the chicks become fairly mobile, quickly approaching the female when she feeds them, fighting with their siblings when food is scarce and backing up to eject feces over the rim of the nest. It is around this time that their first down is replaced by a dense, wooly, dark colored, second down, which lasts another 10 to 15 days. A conspicuous light brown streak runs along their spine, the feet turn bluish-grey and the beak and claws, black.
At two weeks of age, rusty golden feathers start replacing the down on the head and neck. Darker feathers appear on the rest of the body slightly later, while the primaries, secondaries and the rectrices appear at the age of 20 to 25 days.
By the age of 30 days, the chicks have already gained 75% of their adult body weight; the growth of the body slows down at this stage, while the feathers are growing rapidly.
Studies have shown that the culmen of a chick grows at a steady rate, therefore its length may be helpful in determining the chick’s age. At the age of 20 to 35 days, the females, which are heavier as adults, start gaining weight much faster than the males. Thus body weight is a reliable criterion for sexing the chicks more than 30 days old. Young in the regions with good food supply grow faster, than those in the regions with poor food supply.
When food is scarce, the siblings fight for access to the mother, distributing food; it is usually the oldest chick that dominates the feedings and is the first to feed. The smaller, younger siblings don’t get a chance to feed until the dominant chick is satiated. If a subordinate chick tries to snatch food from the parent before the dominant chick has had its fill; the dominant chick pecks the subordinate viciously until it crouches submissively at the edge of the nest. In times of scarcity, the subordinate chicks do not get enough to eat and slowly starve to death. Parents don’t interfere in such squabbles, because it is easier to raise one or two well-fed and healthy chicks than three or four weak and undernourished chicks.
As the chicks grow, the quantity of food delivered to them slowly increases, but becomes constant when they are 30 days of age. This quantity drops just prior to fledging at 40 to 55 days of age; this decrease in the food supply by the adults is possibly meant to encourage the chicks to leave the nest, after which, it rises again.
During the last 10 to 15 days prior to fledging, the young regularly exercise their wings to develop their flight muscles. Finally, when the young are about 50 to 55 days of age, they leave the nest. A fledgling might take its first flight by catching the wind, while exercising its wings. The first flight is brief and awkward and ends soon with the fledgling landing on a nearby perch.
In the nesting colonies where nests are close to each other, the fledglings might switch nests during their preliminary flights. The adults either tolerate these young intruders, or are unable to distinguish their own young from the others, (which is unlikely). They even feed these intruding chicks, over-run by their parental instincts. Subordinate chicks, which do not get enough food to satisfy them, at their parents' nests, are more likely to switch nests, seeking a nest with younger chicks, where they can dominate at feedings.
Even after leaving the nest, fledglings continue to depend on their parents for food, for about 10 to 20 days after leaving the nest. This period between fledging and becoming independent is critical for the survival of the individual, which depends on whether or not it learns to hunt for itself. During the time, a fledgling is acquiring and perfecting its hunting skills, food from the parents acts as a vital back up. However, it must soon learn to live on its own.
It was at first believed that youngsters acquire hunting skills from their parents, since parents often encourage them to hunt by dropping fish in mid-air for them to dive for and snatch. However, experiments have shown that hand-raised young, if released into the wild, can also hunt successfully, after 3 days to 3 weeks of their release.
After becoming independent, some youngsters stay at their nests, a week or two after their parents' departure, but most young leave their nesting grounds with their parents. By early September, all of the adults as well as the young have deserted the nesting grounds for their wintering habitats, not to return until next spring.
Predators and Enemies:
Ospreys have few natural enemies. Mostly the eggs and the young are victims of predation, but sometimes, even adults especially incubating females are snatched from their nests by owls, under the cover of darkness. Eagles are another avian threat to osprey chicks.
Although adult ospreys fiercely defend their nests, a determined raccoon, fox, skunk or a similar land based predator might raid the nest to steal eggs or chicks, if it is within its reach.
Ospreys tend to avoid these perils by building their nests on isolated spots such as islands, secluded trees, or on top of man-made structures such as nesting platforms, electricity poles and other, similar, hard to reach spots.
Fishing eagles worldwide are the main competitors of ospreys for the ecological niche of an aerial, diurnal, fish-eating, raptor.
Different fish eagles compete with ospreys in different parts of the world. In Asia, the white-tailed eagle (Haliaetus albicilla), white-necked eagle (H. leucoryphus), and the Steller's sea eagle (H. pelagicus) are its competitors; while in Africa, it faces competition from African fish eagle (H. vocifer). In the New World, competition exists in the form of bald eagle (H. leucocephalus).
These eagles often rob ospreys of their catch (kleptoparasitism), and might also drive them off good foraging and nesting areas, or kill their young. However, such unfortunate events occur only sparsely.
Sometimes smaller birds such as gulls or ravens also try to pirate osprey kills, but are less likely to succeed.
|fish in = fish out...|
Ospreys are very important as an umbrella species, which reflects the health of an aquatic ecosystem.
Ospreys are good indicators of the health and abundance of the fish stocks they hunt, and can alert us to the impending threats to that fish stock in the form of harmful pollutants such as DDT and DDE, because the ospreys also suffer from the same pollutants that threaten their prey. The shells of osprey eggs become weaker and fragile, if the fish they feed on are contaminated with DDT & DDE. Such fish are certainly not fit for human consumption. Thus ospreys act as canaries in a coal mine, warning us of the impending environmental hazards.