Volume 2: Issue 4
Pam Narney pnarney@gmail.com
Many people think of migration like this. V
They do not normally see this.
Old beliefs
In ancient times when birds disappeared, Aristotle thought they hibernated.
In 1803 a 19-year-old John James Audubon was the first to band birds. He tied silver threads (some say silver metal rings) around the legs of Eastern Phoebe chicks to see if they would return to the same location the next spring. They did.
Seeing is believing. Ornithologists concluded that ospreys never traveled in flocks because no one had ever seen osprey flocks.
New observations and technologies
Osprey used to be considered solitary migrators. However, in Cuba, Freddy Rodriguez Santana observed flocks of fifty or more ospreys flying over Le Gran Piedras. He saw over six hundred ospreys in one day! Over seven thousand a year pass through Cuba’s Sierra Maestra mountains. What a fantastic sight!
Osprey tracking technology using long range electronic tracking devices, miniaturized radio transmitters, computer chips, satellite towers, GPS, radar, and thousands more citizen observers have expanded migration knowledge from observations to massive banks of electronic data-based knowledge.
Where do they go?
In August or September local Chesapeake Bay ospreys migrate to Central and South America and return to North America to the same nests every spring. Osprey fly four to five and a half thousand miles in three to four weeks.
They should stay in South America where the living is easy! In southern climes osprey spend their time catching easily available fish, and 90% of their time loafing and not doing much else.
Osprey mates and families do not winter with their families. They have few intruders to fight off and no mate or chicks to feed. They only have to take care of themselves.
Why do they migrate?
Why do they come back to the Chesapeake Bay area each spring?
Researchers have some theories, ideas, and facts, but the complete story evolves.
1. Instinct
The Chesapeake Bay Foundation cites instinct, but instinct covers a highly complicated process.
Ospreys experience a general restlessness (instinct), an urge to move triggered perhaps by change in day length and light, by hormones, or by other yet unknown stimuli. Poole calls this the “migration urge”, the part of their brain that tells them to move.
2. Ospreys are built to move
Ospreys might migrate because they move so well. Osprey can pick the best places to breed and the best places to rest and store fat for breeding.
Ospreys are built for movement. They can move from one food-rich environment to another and have the best of two worlds. Ospreys can go fifteen to twenty days while continuously flying without food and water. When the birds reach Cuba, the end of the land line and a concentration point, some fly over 300 miles across the Caribbean Sea, a two-three-day trip.
Most diurnal, active during the day, birds avoid flying over water longer than 15-20 kilometers. Ospreys are an exception. Osprey soar using wind thermals known as uplift, and then glide. They do not use up energy continuously flapping their wings. Night or day makes no difference to them during migration.
3. For food
In “Soaring with Fidel”, Gessner cites Keith Bildstein’s point that osprey follow food, although Poole and others say that food is not the primary motivator for migration. Osprey winter in warmer climates simply because there is enough easily caught food there.
Osprey probably left North America that first fall because the planet cooled, winter temperatures dropped, and the osprey’s main source of food, cold-blooded fish, moved deeper in the water to stay warm. Osprey can only catch fish that are within three feet of the waters’ surface, and, unless they are starving, they usually only eat fish.
They can’t find fish in below freezing temperatures.
The Atlantic seaboard is a key highway, a fish rich ribbon of coastline, sometimes called the osprey highway, seen in the following map showing GPS tracking of local osprey. Under the auspices of The Chesapeake Bay Foundation (CBF), Rob Bierregarrd and Brian Watts PhD studied migration patterns.
This CBF map shows the routes of six Bay osprey returning north from their wintering grounds. Notice how close the tracks are. Each track is an individual bird’s flight path.
Check Bierregarrd’s website for a fuller look at satellite transmitter tracking.
Also note the long distance over water from Venezuela to Cuba and points north. Crabby, one Bay osprey, was tracked flying from the Bay to Venezuela in three weeks. The southern flight takes a bit longer than the northern due to storms and winds.
The northern trip might also be faster due to the osprey’s mating instinct and stored supplies of fat from lazing around all winter. They do not need to catch many fish as they fly north. They also catch spring high pressure systems to aid in soaring.
Flying long distances over water is rather rare for birds. It is costly for any bird to fly over water due to the energy debt. Osprey fly for long periods over water but tend to stick to land.
The CBF map shows osprey hugging the coastline when possible. Their paths vary little from other ospreys’ and individual paths from year to year.
The northern flight plan is South America to Central America, through the Caribbean Islands into the Florida Keys. Over 1,600 ospreys were counted passing through one spring. They come back to us in the Chesapeake Bay and north into Canada.
How do they do that?
Poole’s research offers contributing factors to understanding the mechanics of osprey migration.
5. Magnetic fields
One idea for their precise navigation and straight-line course is that osprey use the earth’s magnetic fields. The course is locked into their physiology. They sense their course through magnetic chords: local magnetic signatures.
Songbirds’ beaks contain biomagnate which aids their migrations. It is possible that ospreys may also have the mineral crystal biomagnetite in their bodies.
6. Sun and stars
Another theory is that osprey navigate by the sun and stars, especially the sun’s position related to the time of day. Fall’s lessening daylight may start their migration even though food is still plentiful.
7. Learning and experience
Poole says osprey gain experience when they travel and have keen memories.
They seem to have an uncanny memory for landscapes and cue on them. They return to the same nests every spring and winter in the same areas too.
This redundancy of methods helps them navigate, especially over long stretches of water.
Young osprey, who do not migrate in their first year, often roam around during their first migrations, stay in one place for a few days, fly in circles, and generally try to figure out their route. They lack experience.
Why return to the Bay area?
The Chesapeake Bay area has the largest population of breeding ospreys in the world because ospreys originated here. The Chesapeake Bay area especially is a popular nesting area because the Bay supplies all of the osprey’s needs for habitat and food.
They return to their birthplaces, their natal area, to nest.
Poole explains, “Osprey are widespread in North America, the center of origin for this species.” The earliest known fossils of ospreys, from 13 million years ago, were found in Florida and California.
All of the preceding factors may play a part in migration, but migration is still somewhat of a mystery.
Osprey lovers in the Chesapeake Bay area look forward to their arrival every spring and mourn their loss every fall.
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Other websites to visit
Rob Bierregarrd’s satellite tracking studies
Tracking of Chesapeake Bay Ospreys from 2017.
Rob Bierregarrd’s website
Live osprey cam
Timed sequence showing fall migration
Sources
Chesapeake Bay Foundation.
Gessner, D. 2001. Return of the Osprey: A season of flight and wonder. Chapel Hill: Random House.
A lyrical look into osprey life. A beautiful and hopeful book.
Gessner, D. 2008. Soaring with Fidel: An osprey odyssey from Cape Cod to Bermuda and beyond. Boston: Beacon Press.
Excellent information on migration.
Poole, A.F. 1989. OSPREYS: A natural and unnatural history. Cambridge: Cambridge University Press.
THE seminal scientific authority on osprey. Whatever you want to know, Poole gives you information about that aspect of osprey life. Easy to understand and well organized. My go to source for facts.
Poole, A.F. 2019. OSPREYS: The revival of a global raptor. Baltimore: Johns Hopkins University Press.
A more accessible look at osprey for the general readers with individual sections on migration and different countries. Excellent pictures. A current and detailed pattern of the osprey migration story. This 2019 book includes the newest information on osprey migration and trends not available in 1989: satellite tracking and GPS, bird banding (ringing), plus thousands of reports of visual sightings from Citizen Scientists, the local osprey watchers.
Researchers have some theories, ideas, and facts, but the complete story is evo.ing..ing.in.i..one had ever seen osprey flocks. Seeing is believing.i
Pre 1989
Banding (ringing)
Personal observations
Post 1989
Satellite telemetry-tagging studies
Miniaturized radio transmitters
Project Osprey Watch-worldwide osprey nest monitoring program