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How does migration work?


Migration Strategies-Hop, Skip, or Jump



 GETTING FROM POINT "A" TO POINT "B"



    
Scientists who study bird migration generally agree that there are three primary ways birds find their way between point "A" to point "B." 

     Piloting
is probably the most simple. It involves following a series of familiar landmarks to a predetermined destination. Without knowing it, we pilot to and from work, the supermarket, or school by using familiar landmarks like streets, signs, or buildings. For birds flying several thousand feet above the ground, landmarks are often on a grander scale, like mountains, rivers, or coastlines. New Jersey has some great landmarks for migrating animals. Birds, like airplanes follow the coastline. The Delaware River valley directs birds and fish north and south. Even the Garden State Parkway may be a landmark for a migrating bird - this long, gray; north/south running road could viewed as a river! However, landmarks are not always visual, so their use is limited to those animals that migrate during the day.

     Orientation
is the second method birds use to find their way. During the last 50 years, scientists have discovered that migrating birds use the sun, the stars, and the earth's magnetic field as compasses for orientation. Because these features vary little from year to year, they are reliable directional cues. 

     True navigation
, the last mechanism of direction finding, requires that an animal determines its position relative to the position of its destination and then travels a course to reach it. This is especially important when birds are blown off course by strong winds or during bad weather. Birds displaced during migration are likely to encounter unfamiliar landmarks. Orienting on a fixed heading relative to the position of an external cue may result in a bird flying farther off course. How birds accomplish true navigation is still a matter of conjecture. One possibility is that birds possess an internal map. With its ability to orient based on using external cues like star patterns, its internal clock, and the map, a bird could plot its position and steer a course home. Another possibility is that birds create mental maps as they travel the migration route. If displaced or disoriented, they could retrace their flight path and get back on course. Unfortunately, little evidence is available to support either hypothesis of true navigation in birds.




MIGRATION STRATEGIES - HOP, SKIP, OR JUMP


    
Migrating birds employ several strategies to get them from Point "A" to Point "B" and it is difficult to pigeonhole each species to the type of strategy used. Birds may utilize several strategies depending upon the ecological barriers (Gulf of Mexico, Atlantic Ocean, mountain ranges, etc.) they encounter along the way. Also, a specific bird within a species may use different strategies depending on where they live within the geographic range of the species. For some bird species these geographic ranges are quite extensive and if the bird lives in the northern part of the range it may use a different migratory strategy than one that lives in the southern part of the range.

    
Generally Neotropical migrants fly short distances every night - perhaps 150 to 200 miles. These "hops" take the birds from habitat to habitat along its migration route. When it descends into a habitat in the morning, the birds eat to refuel, and then travel again the next night or several nights after that. Because of this behavior, the birds require local or regional closely interspersed habitats. 

     As some birds migrate northward from the tropics they congregate along the northern coast of the Yucatan Peninsula in Mexico and encounter the Gulf of Mexico. Or they may move up through Appalachian Mountains and encounter the Great Lakes. These ecological barriers force them to "skip" or fly without stopping for a great distance - perhaps up to 800 miles in one flight. In this scenario the habitats at each end of this migration are extremely important - before leaving the birds have to feed heavily in order to give them the reserves and strength to make the trip. Upon arriving at the other side, they once again, have to rest and also eat non-stop to replenish their reserves.

     Occasionally, some Neotropical migrants make an incredible flight that is truly a "jump" from one part of the hemisphere to another. In the autumn, after breeding, the Blackpoll feeds as it works its way from west to east in North America until it arrives at the Atlantic coastline somewhere in Maine or Canada. Here it spends time feeding until it more than doubles its weight from 9.7 grams to about 20 grams. When the bird determines it has enough weight to take on the next step in its journey, it leaves and with a good tail wind, flies non-stop over sea until it reaches its final destination in South America up to forty-eight hours later. Imagine flapping continually for that period of time!