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NATURE HAPPENINGS - May 2001

Mysteries of Migration                   
by Damian Fagan          

 

The time is half past sunset. The crowd is restless, shifting from foot to foot. They want to be off, moving northward. Voices keep the group in communication, occasionally an individual bursts into song. El Norte brings the promise of rich verdant lands, endless opportunities and room to raise a family.  

Yes, these individuals are migrants, but not agricultural workers. They are songbirds - lazuli buntings, black-headed grosbeaks, plumbeous vireos and many others. Collectively they are known as “neotropical migrants,” birds that winter south of the United States/Mexico border and who breed north of this line in summer.

This migration to the north in spring is considered the norm, yet this annual spring ritual of northbound migration is but one of many types of migration. There is elevational migration, where wintering birds simply move upslope to higher elevations to breed. There are leapfrog migrants that play this childhood game whereas populations leapfrog over sedentary populations and seek out unoccupied habitat. There is trans-equatorial migration and irruptive migration, where flocks either migrate to areas of sufficient food resources and climatic conditions.

Scientists do not necessarily agree on one aspect of bird biology that determines how birds know where they are going. Birds utilize the sun, stars and earth’s magnetic field to determine direction. Some studies suggest that olfactory sense, the “follow your nose” principle, leads birds back to their breeding or wintering grounds. Others believe that birds can remember landmarks, but for many of these songbirds that migrate at night, perhaps this is not the best approach.

But to me what is truly amazing is how a six-ounce bird who can beef up its weight prior to this exhausting event makes this sojourn in a relatively short period. One time I figured that for me the equivalent weight-to-distance migration would require that I walk/run/fly roughly 78,000 miles in a few months time. And only snack along the way.

Not all birds are long distance migrants. Some, like great homed owls or ring-necked pheasants stay put during the winter in their breeding grounds. Dark-eyed juncos move short distances, as well. But others, like nighthawks, bam swallows or cliff swallows, travel south of the Equator, wintering in Brazil or northern Argentina. The Swainson’s hawk, a western North American nester, travels in huge flocks to Argentina, where they spend their winter eating grasshoppers and other insects. Their annual migration distance is between 11,000 and 17,000 miles round trip.

So one question that begs to be asked is, “Why migrate?” Why expend a considerable amount of energy, risk predation and the unknowns of the new territory. One theory is that the ancestors of today’s migratory birds, at one time in their geologic past, nested in the present day wintering grounds. As climatic changes occurred and the more northern lands became hospitable to these migrants, birds left their breeding grounds to seek out new territories in these northern latitudes. When severe weather late in the year created a shortage of food resources, these birds retreated southward to their historical grounds. Hence, the pattern of migration became a habit.

Another theory reverses this thinking. In the geologic past, way before the recent Ice Ages, the birds nested in the north, where a warm climate prevailed. As these climates changed, due to advancing and retreating glaciers, the birds left their breeding grounds to seek out new territories farther southward. Hence, the pattern of migration.

But whatever the reasons for the habit of migration, Springtime in the Canyon Country is punctuated by the sweet songs of these neotropical migrants (and those that don’t migrate) as they vie for breeding territories and mates. I guess I’d sing loudly too, if and when I finished my long trek.

photos by Damian Fagan

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