Some ospreys migrate seasonally, but not all. Non-migratory populations breed and winter in the same location, though they may wander several hours from their nest during the non-breeding season. These populations begin breeding between December and March. Migratory populations generally breed where winters are cold enough to drive fish into deep water where they are inaccessible. These populations begin breeding in April or May.
Courtship in ospreys centers on food and nest sites. In migratory osprey populations, males and females arrive at the nest site separately, the male often arriving several days earlier than the female. Male ospreys sometimes perform a conspicuous aerial display near the nest site. This display usually occurs during early courtship, and may serve to attract potential mates or to threaten an intruder. Both sexes collect materials for the nest, but the female does most of the arranging of materials at the nest. Osprey 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 into osprey nests, including fishing line, plastic bags and nearly anything else that an osprey might find and can lift. Osprey pairs use the same nest year after year, but must spend some time each year repairing it and adding materials before eggs can be laid.
Once a pair has established a nest, the male begins to deliver food to the female. This feeding continues until the young fledge or the nest fails. 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, and occasionally from neighboring males if they are not well fed by their mate. Males may protect their paternity by feeding their mate. They may also protect their paternity by guarding their mate from other males and copulating frequently when she is most fertile (several days before egg laying).
Ospreys are generally monogamous. However, polygyny can occur in rare instances where nest sites are close enough together that a male can defend two nests. When this occurs, the first nest usually experiences higher reproductive success than the second because the male devotes more resources to that nest.
Mating System: monogamous ; polygynous
The breeding season of ospreys differs between populations. Non-migratory populations breed in the winter and spring, laying eggs between December and March. The breeding season of migratory populations occurs in the spring and summer, with egg laying in April and May. Two to four eggs are laid over a period of several days, each 1 to 2 days apart. Both the male and female incubate the eggs, which hatch after approximately 40 days. 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.
When osprey chicks hatch, they are covered in white down with brown streaks on the face, back, and wings. This is replaced by charcoal-colored down after approximately 10 days. Feathers begin to replace the down at approximately two weeks. By one month after hatching, chicks have reached 70 to 80% of the adult size. Osprey chicks fledge between 48 and 76 days old. Generally, chicks in migratory populations fledge sooner than those in non-migratory populations. 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. Because ospreys migrate individually, juvenile ospreys must be fully independent of their parents by the time the southward migration begins.
Ospreys are sexually mature at approximately 3 years old, but may not breed until age 5 in areas where nest sites are scarce. Migratory ospreys in both Europe and the U.S. exhibit a pattern of behavior that is unusual in raptors. Rather than returning to the breeding grounds in their first summer, yearling ospreys almost always remain on the wintering grounds throughout the year. They then return to the breeding grounds the following summer when they are more likely to be able to breed successfully. This strategy allows young ospreys that are too physically immature to breed to avoid an unnecessary migration.
Breeding interval: Ospreys breed once yearly.
Breeding season: The breeding season lasts for approximately 2.5 to 4 months. Breeding begins between December and March in non-migratory populations. In migratory populations, breeding begins in April or May.
Range eggs per season: 1 to 7.
Average eggs per season: 3.
Range time to hatching: 32 to 43 days.
Range fledging age: 48 to 59 days.
Range time to independence: 7 to 17 weeks.
Average age at sexual or reproductive maturity (female): 3 years.
Average age at sexual or reproductive maturity (male): 3 years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization
Average eggs per season: 3.
Average age at sexual or reproductive maturity (male)
Sex: male: 1095 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 1095 days.
Both male and female ospreys care for their young. Ospreys provide parental care by protecting their young from from predators and weather, and by feeding them. During incubation and the nestling stage, the male osprey provides food to the female and the chicks. This entails delivering 60 to 100 g of fish to the nest per daylight hour (3 to 10 fish per day) during the nestling and fledgling stages. When a fish is delivered to the nest, one of the adults rips pieces of flesh from the fish and feeds them to the chicks. Parents continue to feed the young until two to eight weeks after they fledge.
During the first weeks after hatching, osprey chicks are not able to control their body temperature well. The female parent broods the chicks almost constantly for the first two weeks. She continues to brood them intermittently during very hot or cool weather until they are approximately four weeks old. Both parents expend considerable effort protecting the nest from intruders, including other ospreys and potential predators.
Parental Investment: no parental involvement; altricial ; pre-hatching/birth (Provisioning: Male, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female)