Osprey Migration

The Osprey migration is one of the most intricate of all birds. Some populations are year-round residents, others perform short-distance migratory movements, and others perform some of the longest migrations among North American birds. In this article, we explore some of the more remarkable facts about Osprey migration.

Photo: Mike Maguire

Why do Ospreys Migrate?

Ospreys migrate because fish becomes inaccessible. They must migrate or else they will starve to death.

Ospreys eat 99% fish, which are cold-blooded and sensitive to changes in temperature. When the weather is warm during the Spring and Summer, ospreys can easily catch fish near the surface. As temperatures cool, fish will go deeper to warmer water. Likewise, fish that normally live in shallow waters seek refuge in deeper waters, making them inaccessible to Ospreys.

Type of Osprey migrations

Ospreys perform Fall and Spring migrations in an annual cycle. 

Osprey Fall migration

Fall migration is triggered by a dwindling number of daylight hours and cooler temperatures. Ospreys migrate primarily because of changes in food sources in the Fall. Essentially, cooler temperatures reduce the availability of fish, the Osprey’s main food, as fish move into deeper, warmer water where Ospreys can catch them. 

The seasonal changes that trigger Osprey migration in the Fall begin in early September through late November, although females begin their migration earlier. Even though fewer daylight hours occur along the latitudinal width of North America, the migration of Ospreys varies with region. 

Osprey Spring migration

Similar changes trigger ospreys’ Spring migration, this time occurring in the Southern Hemisphere. Although changes are not as dramatic and fish are still available in most of the wintering grounds, the main reason for migrating north is the abundance of food in the breeding grounds.

A virtual explosion of food resources coincides with the return of wintering Ospreys. As the northern hemisphere emerges from the cold winter months, fish begin to reproduce and become abundant. Food abundance is conducive to successful breeding. 

What month do Ospreys migrate? 

During the Spring migration, most Ospreys arrive and start breeding during the months of March, April, and May. Notice the difference in the time of arrival between Florida and Labrador in Canada. The Fall migration occurs in a shorter period of time and occurs mostly in the month of September. Ospreys become significantly quiet in Semptember.

Ospreys migrate south after the breeding season. Females begin their migration first, followed by birds that failed to breed that year (males and females). Males and the young of the year are the last to leave the breeding grounds.

Studies that involved fitting birds with satellite tags to track their migratory movements revealed that:

  • September was the month when most Ospreys initiated migration,
  • Females begin the movement south as early as August 19 (Atlantic Coast), late July (Florida), August 1 (Minnesota), August 28 (Oregon and Washington),
  •  Males initiated the Fall migration approximately a month later, in September. The reason for the delays in males is that they stay longer to continue feeding the young Ospreys until these become nutritionally independent. 
  •  Young birds of the year begin to migrate in September or later, depending on how early they fledged the nest. 

Spring Migration

Ospreys return to the wintering grounds over a long period of time. The time of arrival is highly variable and depends on the distance where the birds spent the winter and their breeding destinations. Ospreys arrive in northern latitudes as temperatures rise and fish are more readily available.

Birds tracked with satellite tags, and direct field observations indicate that:

  • Spring migrants arrive in South Florida as early as early January,
  •  In the Chesapeake Bay, most nests are occupied by late March,
  •  In San Francisco – California, most nests are occupied by February, \
  •  Ospreys begin to arrive in Northern Idaho from late March to early April,
  •  Ospreys start to arrive in Kentucky from early March to late April, 
  • Birds arrive in northern Idaho, from late March to early April, 
  •  Birds return to nest sites along southeastern Lake Huron around the second week of April, 
  • Returns in Wisconsin peak from mid-April to mid-May, 
  •  Birds begin to arrive in central Alaska by mid-April, 
  • Birds come in southeastern Massachusetts from the second week of March to late April,
  • Birds arrive in the Labrador region of Canada by late June,

Do all ospreys migrate in the winter?

Some Ospreys migrate while others are year-round residents. Those that migrate breed in northern latitudes where seasonal temperature changes result in dramatic changes in food availability. Hence, the need to relocate.

Ospreys are year-round residents in tropical and subtropical latitudes (further south) where seasons are not well marked, and fish is available throughout the year. 

Being a year-round resident does not mean that they don’t move. They perform regional movements throughout the year, covering distances of up to 400 km, which are pale compared to the distances covered by migratory Ospreys.

Most birds breeding in North America are migratory. 

Most Ospreys breeding in the Western U.S. migrate and winter in Mexico and Central America. Most Ospreys breeding in the Eastern U.S. migrate to Central and South America and to a lesser extent to the Caribbean Islands.

Where do Ospreys go in the winter?

Most ospreys breeding in North America migrate south to lower and warmer latitudes during the winter. The destinations they choose to spend the winter vary from a few hundred miles to thousands of miles from the breeding areas. 

Ospreys exhibit a high site fidelity to their breeding and wintering grounds. They migrate from the same breeding grounds to the same wintering grounds year after year. 

A study that fitted bands on the legs of young Ospreys determined that birds breeding in separate regions of North America tend to winter in separate areas in the breeding grounds as follows:

  • Most of the Ospreys that nest in western North America migrate directly south to settle for the winter, mainly in Southern Texas through Mexico and Central America. Western birds rarely overwinter in the Caribbean basin and South America. 
  • Ospreys that nest in the Midwestern U.S. winter predominantly in Mexico, with only a few individuals in the Caribbean basin.
  • The vast majority (78%) of Ospreys nesting in the Eastern U.S. migrate south to overwinter in South America, with only 22% staying in the Caribbean basin for the winter.

The study also found that young Ospreys establish their wintering sites during the first migration after leaving the nest. 

Young Ospreys spend approximately 18 months in the wintering grounds after leaving the nest. Following that, they return to their natal area to breed. Most birds banded as nestlings returned and nested within 15 to 40 kilometers of where they were born. 

Three birds that completed three migration cycles returned to the same wintering area, alternating sites about 140 km apart.

How far south do ospreys migrate? 

Osprey photographed in Villa Marshes in Lima-Peru in the month of February. Photo: Kevin Jimenez.

The distance between the breeding and wintering grounds is variable. Some Ospreys are short-distance migrants that travel a few hundred miles and settle in an area where they spend the winter.

Other Ospreys are considered medium-distance migrants. These birds fly further south to find better conditions but generally stay in Mexico, Central America, and the Caribbean Basin.

Most Ospreys are considered long-distance migrants. These are birds that spend the winter in the Southern Hemisphere, where it is Summertime. These Ospreys settle in the entire length of South America down to Chiloe Islands, over 7,000 miles from the nest sites of Osprey breeding in Maine, Canada, and Alaska. 

Approximate migration distances between selected breeding and wintering sites. Distances were obtained by connecting known breeding and wintering sites along Osprey migratory flyways.

Breeding area to Wintering AreaApproximate distance in miles
The Chesapeake Bay to Florida818
Washington State to Southern California950
Oregon to Southern California1,012 
Maine to Central Florida1,521 
Alaska to South Texas3,614 
Maine to Northern Venezuela3052 
Alaska to Central Mexico3,992 
Maine to Central Amazon4,139
Maine to Chiloe Island, Chile7,001 

How fast do Ospreys fly during migration?

Ospreys take longer to get to the wintering ground in the Fall. Fall migration takes place after the breeding season, and adults and the young of the year do not have any reason to rush to the wintering sites fast. 

Conversely, adult breeders need to get to the breeding sites soon to reclaim the nest they used the year before or find an optimal location for a new one. 

During the Spring and Fall migrations, Satellite-tracked Ospreys took approximately 7 to 68 days to reach their wintering destination. The wide range of time is because Ospreys breed in a vast area in North America and winter in an even bigger area from Mexico to Southern Chile. 

Interestingly, birds covered longer distances per day during the Spring migration than during the Fall migration. 

During the Fall migration, Ospreys covered an average of 237.5 km/day. 

During the Spring migration, birds covered an average of 321 km/day, 83 km per day more than in the Fall.

During the Fall and Spring migrations, birds flew at similar speeds. The difference is that birds take longer rests and stopover during the Fall migration while they make shorter rests stops and spend more time traveling during the Spring migration. 

Do Ospreys migrate alone or in flocks?

Osprey migrating alone above Central Florida.

Ospreys migrate alone. Males, females, and juveniles migrate independently, unlike most migratory North American raptors. While they may temporarily join flocks of migratory raptors, if they are all flying in the same direction, Ospreys will continue on their own. 

Observations on the ground found that most (80%) of the Ospreys migrating south flew alone. Just 11% were observed accompanied by other migrating raptors. In reality, this association may only be because the other raptors happened to be migrating in the same direction as the Ospreys.

Concentrations of migratory Ospreys of about 90 individuals have been observed flying over Cuba. Rather than forming migratory flocks, these birds were following their traditional migration routes. Birds from the Caribbean Basin and Florida funnel on a narrow passage as they fly over Cuba to other Caribbean islands or Central and South America.

Not even mated pairs migrate together

Even mated pairs do not migrate together. The male and female Ospreys leave the breeding grounds at different times. 

Females always leave up to a month earlier than males. One or more weeks after the young Ospreys leave the nest, the male continues providing food for the young while the female begins to migrate south. 

Gradually, the young Ospreys and their father begin to diverge. Some birds stay in the area for some time, while others migrate south at different times. 

During the non-breeding season (winter), paired Ospreys do not spend time together

After the young of the year leave the nest, the female begins to migrate south. As long as a month after she leaves, the male starts migrating south, and each bird settles in separate regions in the wintering grounds. 

A mated pair will not see each other until the following breeding season

Young ospreys migrate separately as well. The Young of the year may remain together for a few weeks and begin migrating south at different times, covering different distances, and settling in other places.

As birds migrate south, they settle in a wide variety of areas, ranging from South Florida and Mexico to the extreme southern tip of South America.

When it is time to return to the breeding grounds, the male departs and arrives first. About a week later, the female arrives from where she spent the winter. 

Juveniles that migrate south for the first time stay the first year in the wintering areas and return the second spring, about 18 months later. Then, they are ready to breed for the first time. On rare occasions, juveniles return during the next spring after hatching, but they do not breed.

The dangers that Ospreys face during migration

During migration, Ospreys face multiple dangers. Many birds depart the breeding grounds in North America but never return. Although poorly understood, some individuals migrate very long and dangerous distances, while others choose to stay closer to breeding areas during the winter.

Osprey fly over large expanses of open ocean

Unlike other migratory raptors, Ospreys cross large expanses of open oceans. Other migratory raptors migrate overland to benefit from updraft currents generated by land. As ocean water does not create updrafts or thermals conducive to efficient soaring flight, birds flying over the ocean must use active wing beats for flight.

Biologists have found that many Ospreys disappear as they cross the Caribbean Ocean on their return from South America. During spring migration, birds fly between 400 and 700 miles between the Gulf of Venezuela and one of the Caribbean Islands. This stretch of the route takes 30 to 40 hours of non-stop day and night flight. 

In large expanses of the sea, ospreys can be caught up in strong winds and thunderstorms, which can blow them off their course so far that they cannot reach land before they drown. Ospreys are excellent anglers, diving into the water to catch fish, but cannot land and rest on the water like ducks or gulls.

Ospreys are often shot in the wintering grounds

Osprey with large fish caught at a fish farm. Photo: Shirley Freyre.

Ospreys come in conflict with fish farm owners that see Ospreys take one fish after another. 

Once an Osprey finds a fish farm with thousands of fish at the surface, it returns for more, invariably. Small operations put nets and deterrents over the farming ponds, but many do not and proceed to eliminate the problem bird. 

According to a US Fish and Wildlife Service report, approximately 14,000 Ospreys are shot and killed by fish farmers every year. In light of the number of countries for which no data was collected, this number may underestimate the real amount of Ospreys shot every year.

Other sources of osprey mortality in the wintering grounds include electrocution and collision with powerlines. But this is difficult to quantify.

References:

  • Kerlinger, P. and F. R. Moore. (1989). “Atmospheric structure and avian migration.” In Current Ornithology, edited by D. M. Power, 109-142. Springer.
  • Martell, M. S., C. J. Henny, P. E. Nye and M. J. Solensky. (2001a). Fall migration routes, timing, and wintering sites of North American Ospreys as determined by satellite telemetry. Condor 103:715-724.
  • Mestre, L. A. M. and R. O. Bierregaard. (2009). The role of Amazonian rivers for wintering ospreys (Pandion haliaetus): Clues from North American band recoveries in Brazil between 1937 and 2006. Studies on Neotropical Fauna and Environment 44 (3):141-147.
  • Poole, A. (1989a). Ospreys: a natural and unnatural history. Cambridge, U.K: Cambridge Univ. Press.
  • Poole, A. and B. Agler. (1987). Recoveries of Ospreys banded in the United States, 1914-84. Journal of Wildlife Management 51:148-155.
  • Postupalsky, S. (1989a). Inbreeding in Ospreys. Wilson Bulletin 101:124-126.