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How and Why Do Birds Get Lost?
Fog Plays a Role in Migration Errors
by Dan Guthrie

This is one of two times of the year, when, for a few weeks, we have many migrants passing through and a chance to see something really rare—a vagrant—a bird not usually seen in our local area.  Vagrants are migrants that are “lost.”
 
We don’t know nearly as much about bird migration as we would like.  For a few big species, such as Snow Geese and Sandhill Cranes, some individuals have been fitted with transmitters which allow tracking by satellite.  This technique has shown us something about rates and routes of migration for these big species, but we still know relatively little about how small songbirds migrate.  Bearing that in mind, here are a few observations and comments.
 
Bird migration is determined partly by weather and geography.  For instance, Galileo Hill, near California City, is an excellent place to find migrants in September as it is straight south of the Sierras, and there are no barriers along the route.  Galileo, the oasis, is a stopping place for many birds heading south out of the mountains for the winter. However, these birds tend not to fly over Mt. Baldy,  Instead, they funnel around the mountains and through the passes, heading more toward Descanso Gardens, below Route 14, to the west of us and toward Glen Helen, below Cajon Pass, to the east of us rather than, for example, toward Rancho Santa Ana Botanic Gardens.
 
Birds don’t like to fly in fog.  The result is that in October when our area is often socked in with coastal fog, birds that started out from the north in clear weather, arrive in our foggy basin and will not leave. Only when the fog lifts do all the migrants disappear.  Along the coast, migrants may linger a while longer.  Farther inland, where it is clear, they tend to keep moving.
 
The direction birds fly during migration seems to be determined by a combination of following routes based on past experience and, amazingly, on directions predetermined genetically.  It has been shown that most songbirds, which migrate at night, use the North Star and the movement of constellations around it as a navigation tool.  A bird may be programmed to fly southeast a certain number of degrees east of a north-south axis.  Genes can mutate, however, and errors can occur.
 
It seems that some birds suffer from “mirror image misorientation.”  Instead of flying a certain number of degrees east of a north-south axis, they fly the same number of degrees west of the axis.  This explains why many eastern birds, flying south, end up on the California coast.  Their flight path takes them over the Pacific Ocean.  They usually fly at about 1000 feet and would not ordinarily fly out over the ocean if they could see it.  However, when it is foggy,  they cannot see the ocean and mistakenly fly out over it, coming down to the surface to land only at daybreak.  Unexpectedly encountering a liquid landing strip, these birds struggle to find a place to land,  ending up on offshore islands and even boats, or, exhausted, they make it back to land.
 
This helps explain the good numbers of vagrants found along coastal areas such as Huntington Central Park, the tamarisk rows in Oxnard, Oceano Campground near Pismo Beach, and in places such as Pt. Reyes and Pt. Loma in San Diego.
.
. . .The Blackpoll is one of several species that fly out over the Atlantic.  This seems like a bad idea, until you realize that the trade winds deflect their flight to the south, and the birds end up in Venezuela!  They appear to congregate along the Atlantic Coast waiting for the right weather.  Studies have shown that only birds with good fat supplies attempt to fly over the Atlantic in this way.  Weaker birds fly along the coast, refusing to go out over the ocean.

Think what would happen if these birds picked the wrong day to fly out over the Atlantic.  If they even happened to encounter a hurricane, it could be disastrous for the species.
 
A second type of mistake may be reverse misorientation.  Instead of flying south, they fly north.  This may account for California autumn sightings of what are normally Arizona birds, such as Grace’s Warbler, Painting Bunting, and Painted Redstart.
 
It is still not clear how much weather patterns play a role in migration in our western states.  Storms in the north Pacific may shift birds that would normally fly south from Siberia into China and Malaysia, into Alaska, where they follow the coast south.  We also know that hurricanes off Baja can send tropical birds north into our area, either carried by the winds or flying intentionally off course to avoid the weather.
 
Birds migrate until the “urge” is over.  If normal migration from Canada to Central America takes four weeks, they will continue trying to  migrate for four weeks, even if they are  going in the wrong direction.  If, after four weeks, they are still on land, they will settle down and try to spend the winter.  Unfortunately, they usually arrive here before the allotted time for migration has passed, and they usually move on after a few days.
 
A Grace’s Warbler in Santa Barbara was an exception.  A Grace’s Warbler spent one winter in Montecito and returned to the same tree several subsequent winters.  It is thought that once a bird has successfully migrated and wintered, it remembers and follows the same route the next year and continues to return to the same place.
 
Next time you are out during migration, marvel at the energy requirements and dangers these birds face twice a year.  All the way from northern Canada to Claremont, only  to be grabbed by a cat!

NOTE:  This article by Dan Guthrie was reprinted from the “Chaparral Naturalist”, a publication of the Pomona Valley Audubon Society,
with permission from the author.


Sea & Sage Audubon Society
PO Box 5447 • Irvine, CA 92616 • 949-261-7963

http://www.seaandsageaudubon.org