Blog #423 Juveniles on Migration

Migrating juvenile Osprey at Hawk Mountain, courtesy of Cynthia Sedlacek.

Earth’s magnetic field provides animals with a wealth of navigational information: the direction, intensity, and inclination decrease as you travel from the magnetic poles to the magnetic equator. It may also reveal the magnetic declination or the angle on the horizontal plane between magnetic north and true north. These components make up the animal’s navigational map.

The Route

Fall is a restless time of year for birds in the northeast as millions instinctually have an uncontrollable urge to migrate thousands of treacherous miles past hungry predators, strong winds, and stormy weather to get to a safe tropical winter home with plentiful fish. Mortality rates during and just after migration are high: 60% of juvenile Ospreys perish from starvation or accidents before their second birthday. But Ospreys have no choice; they must leave the cold northern states and dwindling food resources to overwinter in the tropics.

One of the most amazing aspects of the fall Osprey migration is that the Ospreys, young and old, make this 2,000-5,000-mile journey flying solo without help from other Ospreys. How do the young Ospreys know where to go? Over the last two decades, however, a wealth of knowledge about Osprey migration has been elucidated by outfitting Ospreys with GPS-tracking technology, but many questions remain such as how instinct urges birds to fly southward. Theories abound but many details of these migrations remain obscure.

Fall migration pathways of Ospreys and other raptors in North America,

courtesy of Hawk Mountain Sanctuary.

Northeastern Ospreys and other diurnal migrants navigate southward guided by the sun, stars, mountain ranges, and the coastlines of North America to the tip of Florida. They cross the Straits of Florida to Cuba before heading east to Haiti and the Dominican Republic. From there the birds fly a treacherous 500 miles over open water to the Gulf of Venezuela. After reaching Venezuela, most fly to Lake Maracaibo to feed and rest for a few weeks. Here, the juveniles recuperate from their arduous flight before heading onward. The Ospreys fan out to settle along the coast, or on the shores of Amazonian lakes, rivers, and estuaries as far south as Argentina. Adults return to their previous wintering ground while the young of the year explore the new tropical territory and eventually settle in a place with ample fish. Juveniles will stay there for a “gap year,” spending 18 months maturing before returning north to find a mate, claim nest territory, and raise a family.

How Do They Do It?

It is almost inconceivable that the 3.5 to 4-month-old Ospreys who probably have flown a total of 10 miles from their nests must fly thousands of miles from their homes in North America to the Southern Hemisphere where they have never been before. The young birds have no guidance from their parents, siblings, or friends. They have no previous knowledge of stopover spots, visual landmarks, or places to forage. Instincts tell them to fly and keep on flying until they reach a suitable place to overwinter: a few stay on the Caribbean Islands or in Central America, while two-thirds venture into the South American tropics. It is estimated that 60,000–7,000 Ospreys overwinter in the Amazon basin.

Many Osprey overwinter on the shores of the fish-filled Amazon River, courtesy of LiveScience.

A few young Osprey go off-track each year, flying to the east or west before heading south. Those that fly east may fly go out over the Atlantic Ocean. If they course-correct quickly for the south, they will probably survive, but those who don’t will perish from exhaustion. Sometimes a stranded Osprey may hitch a ride on a boat before turning around or continuing on its fated route.

Crossing the Caribbean to Venezuela is a particularly dangerous, do-or-die endeavor. On a cloudy night, it is especially easy to get lost on the trackless ocean with no reference points. Ospreys must use tiring, energy-costly flapping to stay aloft over water, as opposed to gliding on thermals on the overland route. If lost or exhausted, the Osprey cannot rest on the water and often drown. Unlike a gull’s plumage, Osprey plumage soaks through quickly, causing them to die of hypothermia.

An Osprey in the tropics on its winter territory, courtesy FireflyForest.

Feats of Navigation

Ospreys have amazingly precise navigation abilities and perceive the world much differently than us. Migrating juveniles crossing the Caribbean were tracked and found to fly farther and faster than they’d ever flown before. Instead of flying in a straight line over the water and being buffeted off-course by high winds, the Ospreys continually course-corrected as they flew, never straying more than 6–12 miles off a straight-line course. The migrating birds corrected for wind drift and adjusted their speed according to the changing headwinds and tailwinds. They flew fastest into headwinds to compensate for the drift and reduced their speed in tailwinds to save energy. This way the Osprey maintains an energy-saving, near-constant ground speed hour by hour keeping the birds on track. But not all Ospreys can maintain this consistency. Some of the young become lost and die while others get blown off-course by storms and perish.

The secret of the Ospreys’ super-navigational abilities is magnetism, but the mechanism is still being elucidated. Research shows there is a critical period a few weeks before to several weeks after fledging where the young imprint on the magnetic signature of their natal area. They use this signature as a basis for forming a mental magnetic map of the Earth to follow on their journeys.

The Ospreys’ key to super-navigational abilities lies in their beaks and retinas, courtesy of Andy Morffew.

How do Ospreys use the geomagnetic field for navigation? It has been known since the 1970s that certain chemical reactions respond to magnetic fields. Birds sense the magnetic field of the earth via their specialized ocular photoreceptors in their retinas containing magnetically sensitive chemical intermediates formed by the photoexcitation of cryptochrome proteins. This chemical transmits critical information via the optic nerve to the brain where the visual system processes the directional information.

Migrating adult Osprey on a Florida beach, courtesy of Andy Morffew.

Magnetic intensity appears to be perceived by magnetite-based receptors in the beak region: the information received here is transmitted by the ophthalmic branch of the trigeminal nerve to the trigeminal ganglion and the trigeminal brainstem nuclei. The information is received by a radical pair—short-lived intermediary chemicals formed in response to an applied magnetic field and transformed into a nervous signal, which is sent to the brain centers where magnetic information is combined with other navigational signals for the navigational processes.

Crystals of magnetite associated with the trigeminal nerve have been found in the beak of pigeons and 20 other migrating bird species and are suspected to be found in most migrants. A magnetic compass confers an enormous evolutionary advantage and is capable of orienting at night, in varied weather, and in all parts of the globe.

Ospreys also use their sun compass and their internal clock in unison as they track the position of the sun in relation to the time of day. Another navigational aid is the stars in the Northern Hemisphere which rotate counterclockwise around the polar star. Ospreys may use this north-bearing pole for orientation as many songbirds have been shown to do. Ospreys add roosting and foraging sites to their mental magnetic maps as they return to the same spots along the journey. They may even anticipate what lies ahead.

Rotating Earth Magnetic Field