Blog #367 Wings over Water—Navigation

How do animals like Ospreys navigate during long migrations? Studies suggest that most, if not all, rely on redundant location systems and orient using one or more of the following: the sun, celestial patterns, Earth’s magnetic field, olfactory maps, weather patterns, surface topography, ocean wave patterns, and low-frequency acoustics. Although advances in animal tracking, meteorological monitoring, magnetic modeling,  and computer processing have helped us understand, conflicting interpretations and the complexity of studying animal movements have left the mechanisms of animal navigation an enduring mystery of biology. 

Young Osprey on spring migration, courtesy of Dana Nesiti. 

Imagine how grueling and improbable it is for a juvenile Osprey to cross the Caribbean on their way to South America—a continuous 1,800 mile overwater flight during both  day and night in the absence of visual landmarks, stop-over locations, and foraging  habitats. Ospreys are diurnal birds but must fly at night while migrating. The young birds  get an initiation by fire when they run out of land at the tip of Florida; they have no  choice but to keep flying. During their first southward migrations, juvenile Ospreys flew 

2–10 times as fast, as far, and as long as they ever had before. Eighteen months later,  when these young adults make their return migration in the spring, they race back to  their natal area to look for nesting sites and possible mates. 

An Osprey equipped with a GPS tracker before migration, courtesy of USDA. 

Perhaps the most remarkable aspect of these transoceanic crossings is the straightness of the Ospreys’ flightpaths. Not only do they cross huge expanses of unfamiliar ocean, but they must constantly correct for drift: they must be able to navigate. Using high resolution satellite-monitored GPS track data, researchers have found that juvenile  Ospreys constantly course-correct over open ocean, spanning distances over 900 miles  despite the perturbing effects of winds and the lack of landmarks.  

GPS trackers mounted on Ospreys relay position to satellites, courtesy of OspreyTrax.

In a study following 25 migrating Ospreys, every bird demonstrated an impressive  navigational capacity to be no more than 6 miles off course for every 620 miles traveled.  Interestingly, juvenile Ospreys flying over land do not have the same ability to fully  compensate for the effects of perpendicular wind drift as those flying over water. A few  other species of raptor, songbird, and shorebird are thought to compensate for the  effects of drift, just as many marine mammals, reptiles, fish, and insects can also  compensate. Ospreys migrating over the open ocean also compensate for headwinds to  maintain a constant forward movement despite highly variable wind speeds and  directions. 

How do Ospreys achieve such precise orientation? Scientists don’t know for sure, but  redundancy is at the heart of it. No one navigational method works at all times of the  day and night, in different areas on the globe, and in all types of weather.  

Many visual cues are ineffectual when they fly over large expanses of ocean or fly at night. Clues from the stars and sun are in constant flux. Olfactory cues are unlikely to  provide positional information required for wind drift compensation over the open ocean  at the spatial and temporal scales observed. The surf, the only relatively continuous  source of infra-sonic cues, is probably not responsible for the constant course corrections. The remaining navigational cues would be magnetic and gravitational.  

Earth is a powerful magnet with magnetic lines running south to north, courtesy of techexplorist.com. 

Think of the planet as a powerful compass formed by the earth’s powerful magnetic  lines or vectors that run longitudinally from the south pole to the north, pointing upwards  in the Southern Hemisphere and downward into the Northern Hemisphere. The vectors  lose their intensity as they move from pole to equator. Ospreys and other animals use  the inclination and intensity of magnetic vectors when locating their position on the  earth. Bio-magnetite crystals, thought to be the compass mechanism, are found in the  trigeminal nerves of pigeons and in the smelling organs of migratory fish, both of which 

travel long distances to return to their natal sites. Magnetic navigational abilities have  been detected in twenty species of birds, and the Osprey is likely to utilize this  advantageous method of orientation. 

Another aspect of Ospreys migration is the bird’s ability to arrive at its destination at the  correct time. The spatiotemporal navigation—the “chord and clock” model—or  navigation helps predict movement in changing environments. The chord is “the scalar  distance or gradient between two locations in a coordinate space,” whereas the clock is “a natural mechanism for gauging the passage of time that is calibrated against  exogenous time-dependent cues” such as sunrise. The advantage of the chord-and clock model is that it accounts for the animal's basic need to arrive at its breeding  ground at precisely the correct time. A young Osprey returning north to the Finger Lakes for the first time must reach suitable feeding grounds as soon as the ice melts on the  lakes. If the bird arrives too early before the spring melt, it will starve. If it arrives too  late, the good nest sites will be taken. 

Ospreys are exquisitely adapted to navigating during their monumental migration  journeys and returning to their natal areas and precise overwintering sites. Ospreys  clearly sense the world differently than we do, using their extra abilities to accomplish feats for which we have to rely on technology.  

Eyes to the sky! 

Candace 

Candace E. Cornell 

Friends ofSaltPoint 

Lansing, NY 

Cayuga Lake Osprey Network 

NY cec222@gmail.com

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