Migratory birds

 Migratory  birds  

 Have you ever asked yourself how  migratory  birds prepare for such long  flights when it can take some of us days just to prepare one suitcase for a weekend trip?

     Before departure, migratory birds become gluttons. They overeat and store body fat. They eat almost without restraint. A bird that is about to migrate alters its diet so drastically that it eats about 30% more than normal in calories, with a special appetite for fatty foods. This change in diet makes the bird’s digestion and metabolism more efficient, increasing the bird’s fat stores by almost half its body weight. The human equivalent would be a 150lb person gaining 75lbs by the end of a month. Can you imagine such a transformation? It is almost too incredible! Still harder to believe is the fact that in the time it takes the bird to migrate, which is one or two weeks, the bird will lose almost all the fat they gained.

      But why is so much fat important to a bird about to migrate? One might think that such an increase in weight might negatively impact a bird’s flight ability, like an airplane that has taken on too much luggage. The difference is that the birds have overloaded themselves with what is practically pure energy and water—resources that are imperative for the thousands of miles of flight between winter homes and breeding grounds that some species undertake.  Our migratory birds will have become little balls of fat and water before they leave their wintering sites.

             Since the exertion of flying produces so much heat, birds can become dehydrated on long flights, the way your levels of thirst will differ between running around your neighborhood block and completing a half-marathon. Normally birds do their migratory flying at night, because one of the simplest ways to control dehydrationالجفاف is by flying in currents of cold air, which are easily found at great heights, or when the sun is down. Another reason for flying at night is to use the stars as guides and avoid predatorsالمفترسين. I find that the former motive provides a more beautiful image: birds still follow the stars the way our greatest navigators circled the globe centuries ago—think Marco Polo or Ferdinand Magellan. But these explorers also used compasses to get around . It is likely that some birds have something similar to an internal compass that allows them to detect surrounding magnetic fields and use them as pointers on migratory flights.

 Of course, it seems like it’d still be easier to fly by day.   Sure Magellan and Polo enjoyed having the information the sun can provide ,so  some migratory birds

prefer to fly by day. These birds tend to use the position of the sun (many birds might be able to see polarized light, making this easier even on cloudy days), cuesالعظة from surrounding terrainالتضاريس , the magnetic fields, and, though you may be skeptical المتشككينagain, smells and sounds. There is reason to believe that homing pigeons can hear infrasoundتحت الصوتية , allowing them to detect extremely low frequency sounds the way elephants do, and some scientists think birds can differentiate large-scale odor رائحةchanges over long distances (say, thousands of miles). I should point out that this latter claim is a bit controversialمثير للجدل —although some scientists have a little evidence supporting the hypothesisالفرضية , many have yet to concurيتفق. So as you can see, there are still many questions about bird migration.

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