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Nocturnal Migration

October 27, 2018

 

Bayou-Diversity (27 October 2018) If someone were to invite me to go birdwatching along the bayou in the middle of the night, I would suspect shenanigans of the mythical snipe hunt.  However, such might not be the case as there are opportunities to birdwatch at night in Louisiana.  September and October are some of the best times. 

 

During this period, fall migration is in full swing, and it is common to see flocks of hawks, vultures and pelicans riding the afternoon thermals.  These birds that soar and glide on fixed wings usually migrate during the day to take advantage of vertical updrafts generated when the sun heats the earth.  On the other hand, birds that employ wing-flapping flight generally migrate at night.  Most songbirds, shorebirds, and many waterfowl travel under the cover of darkness.  

 

For years, nocturnal migration was thought to be an adaptation to allow birds to avoid predators or to effectively navigate using the stars.  Neither of these theories explains the behavior adequately for all species.  For instance, many large birds that often migrate at night such as herons, ducks and geese are rarely at risk from avian predators, and flights of various species on cloudy, starless nights are common.

 

Recent theories suggest that nocturnal migration is based on atmospheric conditions.  During the day, the atmosphere is less stable as the sun heats the earth.  Airplane passengers frequently experience the bumps and jolts of turbulence.  Imagine the effect on a two-ounce songbird.  After dark, thermal activity ceases, the air smoothes, and small birds can travel more efficiently.  Air temperature also affects flight.  Flapping wings generate body heat, and birds must be careful not to overheat.  Flying at night when the air temperature is cooler is an advantage.  Radar studies along the coast convey the scope of nocturnal migration in Louisiana.  During peak migration, up to 50,000 songbirds per hour pass through each linear mile scanned by radar. 

 

To experience this timeless phenomenon, choose a bright moonlit fall night.  A half to full moon is best.  Focus a pair of high-powered binoculars, or better yet a telescope, on the disc of the moon, and watch for dots that move across the moon's face.  Sometimes you can even hear faint birdcalls – messages from avian travelers responding to ancient urges. 

 

(Adapted from “Bayou-Diversity: Nature & People in the Louisiana Bayou Country;” LSU Press; photo credit Lee Harris)

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