Osprey: Breeding

Most Ospreys breeding in North America are migratory with a small percentage of year-round residents. The breeding season starts in the south with resident birds and continues towards the north as weather conditions improve. This article answers some of the most frequently asked questions about breeding Ospreys.

Where do Ospreys breed?

The Osprey breeding range is restricted mainly to North America and some Caribbean Islands. The vast majority breeds in the United States and Canada, with small numbers breeding in Northwestern Mexico, the Yucatan Peninsula, Cuba, and smaller Caribbean Islands north of Cuba. 

Breeding Ospreys are either year-round residents or migratory. Most birds migrate north in the spring to breed, then migrate south to spend the winter in the Southern United States, Mexico, the Caribbean Islands, and Central and South America.

Osprey populations of year-round residents in the east are found along the coast of Virginia, North Carolina, and Mississippi, most of South Carolina, Georgia, Louisiana, all of Florida, and Southeast Texas (see map). 

Year-round Osprey resident populations in the west occur in Southwest Oregon, the northern portion of California, and small populations scattered further south.

Mexico’s year-round residents live along the coasts of Baja California, Baja California Sur, Sonora, and Sinaloa. A smaller population breeds in a small area of the Yucatan Peninsula in the State of Quintana Roo and coastal Belize in Southeast Mexico.

In Cuba and other small islands to the north of Cuba, a distinct subspecies of the Caribbean Osprey breeds year-round. The Caribbean population is the southernmost breeder of the Ospreys in the New World.

In the last few decades, Ospreys have expanded their breeding range in all directions. This expansion may be due to the continued recovery of the once rare species during the period of heavy use of DDT. 

 

What time of year do Ospreys breed?

Ospreys in North America begin breeding at slightly different times. As a general rule, birds that breed in lower latitudes in the Southern U.S. begin to breed (lay eggs) earlier than Ospreys in higher latitudes further north.

Breeding Ospreys are composed of year-round residents and migratory individuals. Year-round residents begin to lay eggs as early as late November through January while migratory ones start to arrive in North America, from the wintering grounds, by December, beginning to breed shortly after arrival. 

Field studies indicate the following dates for the beginning of egg-laying/initiation of breeding:

 

How long is an Osprey nesting season?

Osprey’s nesting period varies depending on the temperature, day length, and food availability in the breeding region. Nesting seasons in northern latitudes are shorter than those in southern latitudes because the window of optimal conditions is narrow. Altitude may also play a role in delaying the nesting season.

The average breeding period or nesting season, defined as the time between first egg laid to first young fledged, lasts approximately 13.4 weeks or 94 days (3 months). After fledging, young Ospreys still depend on their parents for food and stay with them for the next 15 days or until the time all birds migrate south, about a month later. 

A study on breeding Ospreys in New England provides an overview of nesting activities and the approximate duration of the breeding season: 

• The average number of days between the first egg laid and the first egg hatched is 39 days ranging between 35 to 43 days.
• The average number of days between the first hatched egg to the first fledged young is 55 days ranging between 50 to 60 days (Poole 1989a). 
• The breeding period from the first egg laid to the first young hatched lasts approximately 13.4 weeks or 94 days (3 months).
• After fledging the nest, young Ospreys still depend on their parents for food. The time young Ospreys stay with their parents after fledging varies widely. The majority stays in the breeding area until the peak of Osprey migration south begins while other fledglings leave their natal areas as soon as 15 days after fledging. 

Day length and temperature trigger egg-laying 

The main drivers for egg-laying and initiation of breeding are day length and temperature. These two factors explain the south-north variation in initiation and duration of the Osprey breeding seasons from Florida to Alaska, and Labrador, Canada.

With the start of spring, longer days and higher temperatures are first recorded in the south and then progress northward.

In northern latitudes, an increase in temperature is crucial to arriving Ospreys since lakes, rivers, and other bodies of water where fish live must thaw to make their main food item accessible.

Conversely, changes in temperature at the end of the breeding season result in changes in lakes and rivers, prompting Ospreys to wrap up breeding activities sooner, resulting in shorter breeding seasons.

Where do Ospreys build their nests?

Ospreys build their nest in a variety of natural and man-made structures. Natural structures include dead or living trees, cliffs, and even on the ground, on predator-free islands. Man-made structures include communication towers, utility poles, and nesting platforms.

Ospreys have habituated to human activity around them. Nesting sites include busy highways, shopping malls, and light fixtures at sports fields among others. 

However, common characteristics of osprey nesting habitat include:

1. Proximity to water, especially good feeding areas, 
2. Openness, allowing easy access to the nest, 
3. Safety from ground predators, achieved by height or over-water location (islands; flooded trees, channel markers), 
4. Sufficiently wide and stable base to accommodate the large nest.

How far apart are Ospreys’ nests and nesting territories?

The distance between Osprey nests is highly variable and strongly affected by the availability of food and nest sites. 

Some pairs nest miles apart, while others nest close to each other. Nests of ospreys can sometimes be only 33 feet apart when tall trees are not available near highly productive lakes. Sometimes several pairs nest in the same tree and raise their young successfully.

In conditions where food is plentiful, Ospreys habituate and don’t attack each other when nests are close together.

Osprey usually mate for life

Ospreys usually mate for life. Males and females form long-term relationships that last as long as they live. If one bird dies or disappears, the remaining bird will find a new mate and continue breeding with the new partner. 

Occasionally, an intruding adult, usually a female, challenges the resident female for the territory, sometimes succeeding and taking over. 

Rarely, the couple splits for no apparent reason. Scientists have speculated that when a pair fails to raise young successfully, one of the paired individuals is more likely to leave the pair bond and try breeding with another mate.

Most Ospreys have only one breeding partner

Typically, ospreys are monogamous. Nevertheless, polygyny, a breeding relationship between one male and two females, can occur. 

Generally, one male and two female relationships occur when two nests are close to each other so that a single male defends both nests within what he considers to be his territory. 

Generally, the original nest has a higher success rate than the additional nest, mainly because the paired male puts more effort into feeding its initial nest than the second nest in the territory. 

Observations have shown that extra-pair copulations are rare among nesting Ospreys. However, when they nest close to each other, the number of extra-pair copulations increases and both males and females engage in extra-pair copulations.

Typical Osprey-nesting courtship behavior

During pair formation, nest building, and egg-laying, Ospreys may exhibit stereotyped courtship behaviors. Such behavior can be interpreted as a sign that two ospreys are forming or have formed a breeding relationship. Examples include:

• Aerial Sky-Dance Display.  Males perform it during the periods of pair formation, nest building, and incubation. Females rarely display the sky-dance display. Non-breeding males may perform this display as a practice run throughout the breeding season.

During the sky-dance display, a male Osprey gains an altitude of approximately 900 feet while holding a fish or a piece of nesting material in his dangled legs, making sure that it is visible. The bird performs an undulating flight while giving loud calls and screeches from high above the nest.  

• Hovering flight. It can be regarded as a variation of the sky-dance display. In this case, the male hovers in place while holding a fish or nesting material in its feet. It is performed at lower heights above the nest and is also accompanied by calls and screeches. The hovering flight display is also performed to defend the nest.  
• Courtship Feeding. The male starts offering fish to the female as soon as the pair is formed and continues to do so until the young ospreys leave the nest.
• Guarding of the female. Eggs are fertilized prior to and during the egg-laying period, as seen by the frequency with which the pair copulates. To ensure his patternity, the male closely follows the female so that she doesn’t stray for extra-pair copulations. 

What kind of nest does an Osprey build?

Nests of ospreys can be classified as platform nests. Most nests have a base of accumulated coarse nesting material about 2 to 4 cm in diameter and up to a meter in length. From the base up, the material becomes thinner and shorter. The top of the nest is a cup with a central depressed area lined with fine nesting material that includes grasses, seaweed, moss, and pieces of bark. 

Many nests of ospreys contain pieces of plastic, cardboard, fabric, and other materials that the nesting pair perceives as adequate soft material to line the center area where eggs are laid.

When finished, osprey nests have a nice round shape with a rim of sticks, which remains during incubation. Once the eggs hatch and the growing young ospreys step on the nest and soil it, it becomes a messy flat platform. 

The parents begin to add nesting material to the top of the nest after the young hatch, perhaps to keep the nest’s shape, but the nest ends up flat by the time the young fledge.

Mostly the male builds the nest

Ospreys have a well-defined division of labor when building a nest. The male collects nesting material while the female lays out the material the male brings. The female distributes and shapes the nest. She rarely helps bring nesting material. 

The male searches the ground for suitable nesting material not far from the nest. After identifying a piece, the bird descends and snatches it from the ground without landing. Also, the male breaks off dead branches and twigs from nearby trees by passing by without perching on the tree.

How long does it take an Osprey to build its nest? 

According to long-term studies, ospreys that are first-time breeders can take between 10 and 20 days to build a nest and lay eggs. Experienced breeders usually use the previous year’s nest, and it takes them only a week to add nesting material on top of the existing nest and begin laying eggs.

How big and heavy is an Osprey nest? 

Osprey nests are among the largest nests of birds in North America, along with the bald eagle and golden eagle nests. Every year, the nesting pair adds material on top of the existing nest, gradually expanding it. A large Osprey nest indicates that it has been used for a long time.

New nests start small the first year, measuring 27 in diameter and 3 to 6 in deep. The same nest enlarged over many years of use can reach 3–6 feet in diameter and 10–13 feet. 

A nest built on an artificial platform is constrained by the size of the platform itself, which lacks the support to grow taller, so it may topple over if it becomes too tall. Prior to the nesting season, they are often cleaned out.

What is an Osprey nest called?

Osprey nests do not have a distinctive name. An eyrie is the nest of an eagle, hawk, or other bird of prey. An Osprey’s nest can be called an eyrie since it is a bird of prey.

Where do Ospreys sleep at night while nesting?

An Osprey mated pair sleeps in nearby trees while building the nest, rarely on the nest tree or on the nest. When the female lays the first egg, she spends most of the day and night protecting the egg, but she does not necessarily incubate it.

Incubation begins after the second egg is laid, and the female spends the night in the nest while the male spends the night in a nearby tree. Once the clutch is complete, the female spends all night and most of the day (95% of the time) in the nest incubating while the male brings food and defends the nest. The male sleeps in a nearby tree.

Ospreys have diurnal and nocturnal nest predators

At all stages of the breeding cycle, Osprey nests are vulnerable to raids by predators. Due to their open nests and exposed locations, they are vulnerable to diurnal and nocturnal predators. 

The majority of Osprey nest predators are active at night when Ospreys are unable to defend their nests. The following birds and mammals prey on osprey eggs and young:

• The Great Horned Owl is a significant Osprey nest predator, killing mostly young birds and even some adults. Often, Great-horned Owls return to nests with young, taking all the young Ospreys over a period of time. Among 101 nestlings, 20 nestlings (20%) were lost to Great-horned Owl predation in a study on Osprey nesting biology.   
• Raccoons are known to prey on bird nests, and the nest of the Osprey is no exception. As sea and lake shores (Osprey nesting habitat) are populated by humans, raccoon numbers also increase, putting pressure on nesting Ospreys. Predation by raccoons is difficult to quantify because of their nocturnal habits. As a result of increasing raccoon predation, researchers suggest that Osprey may have shifted their nesting sites from trees to man-made structures. Raccoons steal eggs and nestlings.
• Bald eagles share the habitat and nesting sites with Ospreys and generally tolerate each other. However, given the opportunity, Bald Eagles may take nestling Ospreys. Adult Ospreys regard Bald Eagles as predators to judge by Ospreys’ reaction to the presence of Bald Eagles near their nests. 
• Crows are opportunistic nest predators taking eggs and very young nestling when a nest is left unattended.

 Do Ospreys abandon their nests?

Breeding and non-breeding seasons for ospreys are well defined, and in most regions, they disappear during the non-breeding season, giving the impression that they abandon their nests. 

During the breeding season, the female spends most of her time in the nest while the male is foraging for food or perched in a nearby tree, giving a sense of active occupancy. After the nestlings fledge the nest, the entire family leaves the area if it is migratory or stays in the region if it is a year-round resident, but they don’t spend much time at or near the nest until the next year. Leaving the nest after such a notorious and vocal presence creates the impression that the birds have abandoned the nest.

Ospreys tend to use the same nest every year

Ospreys use the same nest every year and have strong site fidelity. It is most common for them to return to the same nest when they have successfully produced young. 

In a study of Osprey nests in California, 68 nests were monitored for two years and 9 pairs (13%) returned and used the same nest (Judge 1983). A study of Ospreys nesting on artificial nesting platforms in Massachusetts found that, over a period of five years, approximately 95% of nesting pairs returned to breed on the same nesting platforms.

As long as the nest remains viable, ospreys will return and use it. In another study conducted in Florida, Ospreys returned to the same nest less often. 

In the Sunshine State, Ospreys nest in trees that decay or nests are blown down by the wind, causing resident and migratory Ospreys to construct new nests. At the Florida site, the return rate was about 70 to 80%.

Some Ospreys build multiple nests

Ospreys often build more than one nest beside the one they use to lay eggs and raise the young. According to a study on nesting Ospreys approximately 10% of males build additional or alternate nests. 

A male sometimes builds an alternative nest close to his breeding nest, where a single female settles. Due to this situation, the male with two nests in close proximity defends both nests and feeds and copulates with both females. Osprey biologists noted that males with two active nests put more effort into feeding and defending the original nest.

When nests are lost or a pair fails to breed, the male tends to build additional alternative nests. Some are completed, while others remain incomplete. The breeding pair is likely to return to the same area and choose one of last year’s alternate nests to breed.

The distance between an alternate nest and the original nest varies with the quality of the nesting habitat. When surrounded by productive sources of food, birds that lose their nest or fail to breed tend to build alternative nests near the original one. Additionally, a failed nesting attempt in poor nesting habitat may cause the pair to build an alternative nest elsewhere.

What do Osprey eggs look like? 

Among birds of prey, osprey eggs are among the most beautifully patterned and varied. Eggs are usually spotted, blotched, and smeared with reddish browns over a light brown or creamy background. 

Color patterns and intensity of color vary among pairs, from heavily dense spotting and blotching to creamy brown thinly spotted. This variation appears to be random and not necessarily associated with regions or locations.

How many times a year do Ospreys lay eggs?

An osprey lays eggs only once per year, which is a single breeding attempt. The pair will likely start another nesting attempt if the eggs are lost early in the nesting season. A pair is more likely to skip breeding that year if their eggs are lost during the middle or late stages of the breeding season.

How many eggs do Ospreys lay?

Ospreys lay anywhere from one to four eggs in a clutch, but the most common clutch size is three eggs.  It is also possible that the number of eggs that a female Osprey lays varies based on the availability of food. Female Ospreys tend to lay larger clutches where food is abundant. 

One egg clutches are rare. Osprey biologists believe that a nest with one egg may be from a pair that lost its first clutch and is trying again. It is also possible that the other eggs were stolen by a nest predator, leaving only one egg.

The clutch size depends on whether the Ospreys are migratory or year-round residents in the breeding region. Studies have found that year-round residents tend to lay smaller clutches than migratory Ospreys. Nevertheless, 3 eggs are still the most common clutch size for year-round residents and migratory Ospreys.

How often do Ospreys mate prior to laying eggs? 

Osprey pairs mate quite often. Copulating begins while they construct their nests or rebuild them if they happen to return to the same nest from last year. Observations show that pairs copulate between 88 and 338 times before laying eggs. The majority of copulations occur in the morning on the nest or nearby. In the days leading up to the female laying eggs, copulations become even more frequent.

Although the number of copulations may seem high, not all copulations result in cloacal contact, meaning the male often mounts the female for fake copulations. Field observations found that only 39% of mountings resulted in actual cloacal contact.

During the days leading up to egg-laying, the female may be most receptive to mating and egg fertilization may occur. On these days, the male stays very close to the female to ensure that he is the only one mating with her and ensuring paternity.

How long after mating do Osprey females lay eggs?

Female Ospreys begin to lay eggs 2 to 3 days after the period of most frequent mating. The female lays the first egg usually in the morning hours. Then she continues laying eggs 1 to two days apart until the clutch is complete. 

How long does it take for Osprey eggs to hatch?

The incubation period varies with latitudes taking longer in northern latitudes. Field studies indicate the average number of days between the first egg laid and the first egg hatched in 39 days ranging between 35 and 43 days.

Osprey eggs usually do not hatch at the same time

Depending on the mother’s day of incubation initiation, osprey eggs hatch at different times (asynchronously). Female Osprey females start incubating their eggs at different times. Most begin incubating after the second egg is laid, but some incubate intermittently from the day the first egg was laid. 

Osprey females lay their eggs about one or two days apart, so the first eggs have several days of incubation advantage over the last eggs. Consequently, the eggs hatch at different times.

Those eggs that hatch first have a competitive advantage over those that hatch last. During the first few days of life, young Ospreys grow so rapidly that the nestlings have doubled in size by the time the last egg hatches. 

Do male Ospreys incubate/sit on the eggs?

Osprey males do not incubate eggs very much. According to field observations of breeding Ospreys, the female spends all night and most of the day (about 95% of the time) incubating while the male brings her food and protects the nest. 

As the male brings food to the incubating female, she leaves the nest for a nearby tree to eat, allowing the male to take over incubation duties. After eating, she returns to the nest and resumes incubation. 

How long does it take for a baby osprey to leave the nest?

The time a baby Osprey takes to fledge after hatching varies with the latitude. Poole (1989a) reports that the average number of days from the first hatched egg to the first fledged young is 55 days, ranging from 50 to 60 days.

References:

• Bierregaard, R. O., A. F. Poole, M. S. Martell, P. Pyle, and M. A. Patten (2020). Osprey (Pandion haliaetus), version 1.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
• Bretagnolle, V. and J. C. Thibault. (1993). Communicative behavior in breeding Ospreys (Pandion haliaetus): description and relationship of signals to life history. Auk 110:736-751.
• Hagan, J. M. and J. R. Walters (1990). Foraging behavior, reproductive success, and colonial nesting in Ospreys. Auk 107:506–521.
• Henny, C. J., R. A. Grove and J. L. Kaiser. (2008b). Osprey distribution, abundance, reproductive success and contaminant burdens along lower Columbia River, 1997/1998 versus 2004. Archives of Environmental Contamination and Toxicology 54 (3):525-534.
• Poole, A. (1985a). Courtship feeding and Osprey reproduction. Auk 102:479-492.
• Poole, A. F. (1981). The effects of human disturbance on Osprey reproductive success. Colonial Waterbirds 4:20-27.
• Poole, A. F. (1982b). Brood reduction in temperate and subtropical Ospreys. Oecologia 53:111-119.
• Reese, J. (1977). Reproductive success of Ospreys in central Chesapeake Bay. Auk 94:202-221.