Some birds migrate and others don't. Why is this? How do some adapt to winter and why do others migrate exhausting miles to their wintering grounds and back again?
Ken Gansmann, in a letter to the editor, wondered where the osprey family that occupied the large nest in Silverthorne went for the winter. He mentioned in the letter that the parents left first, then the young seemed to realize that they weren't coming back and took off also.
But why do ospreys leave? Most birds migrate because of food. In most climates, ospreys could survive the winter temperatures, but the cold weather forces their food source to move to deeper waters, or to lurk beneath the ice and snow of the river and reservoir.
Ospreys need water that is not frozen and fish that are near the surface, so to survive they must migrate to a climate where the fish are more accessible. Most ospreys from North America, Europe and northern Asia will migrate south in the fall. Ospreys from North America generally go to Mexico, Central America and South America, and ospreys from Europe and northern Asia generally go to Africa, India and southeast Asia. A good guess is that our ospreys are enjoying the beaches of Mexico or the Caribbean.
Some people wonder why migrating birds don't stay in the south, where it's sunny and warm. Why do they expend so much energy to migrate north in the spring. The reason is because places like the tropics are too limited in space for breeding. Many bird species need a lot of space when raising young and northern breeding grounds offer more room, food and nesting habitats than the wintering grounds in the south.
When osprey families migrate, they leave as individuals and migrate south alone. A typical osprey departure schedule is that the mother leaves first, then the father, and then the once the parents have left, the chicks are on their own at the nest. The young stay a little longer to gain strength for their migration flight, and then they leave.
Ospreys are strong fliers capable of crossing large areas. Instead of building up large fat reserves, like some species, ospreys they fish along the way.
Satellite transmitters attached to the backs of Ospreys allow researchers to follow the birds as they head south. For many of the young birds it's their first flight away from their nest sites into uncharted areas.
On a website from the University of North Carolina, I found researchers tracked individual ospreys to their winter sites. Once on the webpage, you can choose a bird to follow from the list and click on the link. You'll be able to follow the bird through satellite imagery on its first trip south.
One young osprey they named Belle was followed from her birthplace on Martha's Vineyard, Mass., to a reservoir in Brazil. Belle glided down across the western Atlantic, spent a few days in Haiti then sailed across the Caribbean.
Once on the South American mainland, Belle cruised up and over various mountain ranges and across many rivers deep in the Amazonian rain forest, exploring rivers, lakes and other potential wintering areas as she sailed along on her odyssey.
Belle finally ended up fishing and resting in and around the Samuel Reservoir near Porto Velho, Brazil. Just amazing! Don't we wish we could soar along with Belle!
Large broad-winged birds like ospreys rely on thermal columns of rising hot air to enable them to soar. They migrate in the daytime and do not cross over large bodies of water, since thermals only form over land.
Juveniles must rely on their instincts for orientation. The young travel on their own when they head south. They do not follow their parents, but work on pure instinct. Something on their brain programs them to "Go south, and stay over land if at all possible."
The young remain in their wintering grounds for an extra year instead of heading north the following spring. They don't breed until they're 3 or 4 years old, so they may use the extra time to locate an ideal fishing spot for future migrations. It seems that they don't wander around that much during their time in the tropics, but rather find an ideal spot to call home.
The ability of birds to navigate during migrations cannot be fully explained even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be explained by the cognitive ability of the birds to recognize habitats and form mental maps. Satellite tracking of day migrating raptors such as ospreys has shown that older individuals are better able to make corrections for wind drift.
Osprey tend to mate for life, but they don't always spend winters with their mates but meet up on the way to, or at their breeding grounds in the north, like our much watched pair in Silverthorne. So after taking separate winter vacations, the osprey pair will return to the same large nest year after year, renovating it with fresh sticks each spring as part of their courtship, and making repairs to winter storm damage.
Ospreys have a typical lifespan of 25 years and they are found worldwide except in Antartica. They are truly amazing birds.
Breckenridge resident Dr. Joanne Stolen is retired from teaching microbiology at Rutgers University.