Blog #460 The Cost of Migration
When the urge to migrate calls, overwhelming instincts steer each Osprey to fly south. This fall migration is primarily triggered by dwindling daylight hours and colder temperatures driving the Osprey’s prey— fresh and saltwater fish—to warmer, deeper, and inaccessible depths. Impending hunger drives the birds on these fantastic journeys.
Ospreys are able to perform their remarkable long-distance migrations by alternating between two types of flight methods—gliding flight which is ultimately fueled by solar energy and energy-intensive flapping flight. These diurnal raptors begin their southward flights in the morning as the sun warms the cool night air forming thermals—rising columns of buoyant air that transfers heat energy upwards.
Ospreys and other gliding birds catch thermal up-drafts close to the ground and ride the rising air currents hundreds of feet aloft. When they reach the peak, the birds soar forward and glide downward at high speeds, until they catch another thermal and repeat this process. The birds soar and glide on the thermals for as long as the thermals hold up and can cover long distances while consuming very little energy. When a power boost is needed, or the thermals die, Ospreys switch to flapping flight, a more energy-expensive form of flight.
Ospreys travel using air currents that create thermal avenues, by AerospaceWeb.
This system of alternating flight methods saves energy and minimizes the number of times an Osprey must flap its wings on long flights. The thermal air currents intensify as the sun moves higher in the sky, and the heating strengthens. As the sun descends during late afternoon, the convective currents lose their strength, and thermals break down. Ospreys use these down times for fishing and finding a safe place for the night.
Since there is no nesting imperative pulling the birds south, the fall migration is often more relaxed than its spring counterpart. Adult Ospreys know the way to their traditional winter homes and, therefore, have no reason to rush. Fall migrations can take 7–68 days depending on the destination. In the Fall, migrating Ospreys travel an average of 148 miles per day, about 50 miles fewer than on the spring migration.
Migrations cost Ospreys a great deal of energy and often their lives, but these flights are crucial for the species’ survival. These extraordinary autumnal migrations are considered the most dangerous part of the annual cycle. Such long journeys can turn in to harrowing flights if the birds are blown off-course, encounter tropical storms, succumb to predation or hunting, run out of energy, or collide with the human structures.
Fall migration is far more dangerous for the younger birds because of their inexperience fishing, flying, and navigating long distances, especially crossing open water. Instinct tells young Ospreys to fly south and so they do, but many potential hazards await. The young will not become proficient hunters until they migrate and they often make an extended stop-over at a favorable spot to hone their hunting skills before continuing their journey. Those juveniles that don’t perfect hunting early on in their fall voyage often die of starvation, especially if blown off course.
An Osprey diving for fish, courtesy of Cynthia Sedlacek.
I once saw 50+ juveniles fishing for menhaden or “bunkers” in a small cove at the mouth of the Connecticut River. A school of bluefish had trapped the “bunkers” in the cove for the past week and were picking them off one by one along with the hungry Osprey.
Once the Ospreys arrive in Central or South America, the adults fly to their winter homes and the younger birds set out to find theirs. It can take months for the young birds to find winter homes and they may switch to other locations during their 18-month
stay. Once the young decide on a home, they will return there every winter hence. Close to 60% of juveniles do not survive their first year and most are lost in transit. After that, the mortality rate drops precipitously as experience is the Osprey’s best protection.
Perhaps the biggest danger for the Ospreys once they arrive in the tropics is fish farmers. Some farmers cover their tanks with nets and use other deterrents to keep the Osprey and other birds away, but not all farms can afford these protections and resort to killing the birds. The U.S. Fish and Wildlife estimates that 14,000 Ospreys are shot annually by fish farmers in Central and South America and the Caribbean Islands, but this incomplete data is likely much higher.
Evolution would not tolerate such risky behavior as migration unless there was a considerable upside. Winters provide needed down time for the Osprey after their hectic breeding season. In winter, adults need only hunt 1–2 times a day depending on their hunger and no longer have to fish for their families as well. Ospreys are highly territorial around their nests in the summer, but relax their aggressions in the winter. Ospreys are seen roosting together along tropical river banks and coastlines. Recently, researchers tracked 70+ individual Ospreys sitting on a tropical beach together. The more we learn about Osprey winter habits from their tracked movements, the more thoroughly we will understand them.
Eyes to the sky!
Candace
Candace E. Cornell
Friends of Salt Point, Lansing, NY
Cayuga Lake Osprey Network
EYES ON OSPREYS
WATCH!
Salt Point Osprey Nest Cam 2022-23
READ!
On Osprey Time: Ospreys of the Finger Lakes
VISIT!
Cayuga Lake Osprey Trail Nests Driving Tour
Complete Cayuga Lake Osprey Trail
HELP PROTECT OSPREYS:
•Join the Cayuga Osprey Network: cec222@gmail.com.
•Stay 300 feet away from nests during the breeding season.
If the Osprey vocalizes, you are too close! BACK OFF IMMEDIATELY.
•Carry binoculars to view wildlife from afar.
•Recycle used fishing lines, twine, and nets, which can kill Osprey.
•Help keep local waters clean, healthy, and safe. Pick up trash and do not litter. Trash stays in the ecosystem a long time before it disintegrates. Plastic just breaks down into smaller pieces.
Aluminum Can-------------------80 to 100 years
Plastic 6-pack Holder----------100 years
Orange or Banana Peel-------Up to 2 years
Plastic Film Container---------20 to 30 years
Plastic Bags----------------------10 to 20 years
Glass Bottles---------------------1,000,000 years
Plastic Coated Paper----------5 years
Nylon Fabric---------------------30 to 40 years
Leather----------------------------1 to 5 years
Wool Socks----------------------1 to 5 years
Tin Cans--------------------------50 years
Source: NYSDEC
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