The V formation, called a skein, is the symmetric V-shaped flight formation that osprey use to fly long distances. The V formation greatly boosts the efficiency and range of flying birds, particularly over long migratory routes. All the birds except the first fly in the up wash from the wingtip vortices of the bird ahead. The up wash assists each bird in supporting its own weight in flight. In a V formation of 25 members, each bird can achieve a reduction of induced drag by up to 65% and as a result increase their range by 71%. The birds flying at the tips and at the front are rotated in a timely cyclical fashion to spread flight fatigue equally among the flock members.
In March, 2012, the days shortened and a cool zephyr rolled over the north of South America. The dry season was coming to an end. Ozzie and Harriett looked north. They did it instinctively, subconsciously, involuntarily. An invisible force turned their heads and whispered, “Go home”.
Seasons in Venezuela are marked by rainfall rather than temperature. Venezuela has a rainy season from May through November and a dry season from December through April, which are referred to as winter and summer, respectively. The very young osprey, spend their first 18 months in South America maturing, courting and finding a mate for life. You may recall the love affair that brought Ozzie and Harriett to matrimony. At Lake Maracaibo, back in the winter of 2009, under the Catatumbo lightning and its love mist, they were bonded for life. Except for the chicks, there is no place in Venezuela for ospreys during the summer. They all take flight and migrate back to their natal and fledgling homes.
The primary motivation for migration appears to be food. Also, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. The timing is controlled by changes in day length. These changes are also related to hormonal changes in the birds and genetic influences.
Migrating birds navigate using celestial cues from the sun and stars, the earth’s magnetic field and perhaps some learning. Osprey migrations begin with the birds starting off in a broad front. This front narrows into one or more preferred routes termed flyways. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns, or avoid geographical barriers such as large stretches of open water. The advantages of migration offset the high stress, physical exertion costs, and other risks of the migration such as predation.
Flyways of the Americas
Ozzie and Harriett utilize the Atlantic Flyway. Good sources of water, food, and cover exist over its entire length.
Ozzie is a strong flier. He migrates in a flock to reduce energy cost. In a V-formation he can conserve 12 –20% of the energy he would need to fly alone. Osprey fly at varying altitudes during migration. Seabirds fly low over water but gain altitude when crossing land. Migration can consume 30 – 40% of their body weight, which is stored as fat, to fuel this uninterrupted journey.
When Ozzie returned from wintering in South America he expected to find his old nest on the Red Channel Marker in a bit of disrepair, but nothing he could not straighten up before Harriet arrived. Imagine his distress when he could not find the old Red Channel Marker, much less their old nest.
This is the image in Ozzie’s brain.
But The Red Channel marker is not there.
To be continued…………….