PRBO Conservation Science
Quarterly Journal of PRBO Conservation Science, Number 137, Summer 2004: 2004 Notes from the Field

Focus 66


Hour by hour, day and night, birds are rushing through, over, and by--in huge numbers....

Fall Landbird Migration

Lasting Legacy
Sonoran Desert Saga
Central Valley Wetland Life
Bering Sea Bonanza
Urban Least Terns
Eastern Sierra Education
Bird Bio
Measuring Ocean-Climate Change
Focus on Fall Migration
Estate Gift to PRBO

Swainson's Thrush. © Ian Tait
Leaving Home. A young Swainson's Thrush, recently fledged from its nest in root tangles on the flood-cut wall of Muddy Hollow Creek in Point Reyes National Seashore, is learning to forage and dodge predators. It is dark and cool in the elderberry-fern understory below a closed riparian canopy. Calls from her siblings and songs from her father ring through the stillness. All is well.

In mid-August, though, just nine weeks out of the egg, she is beginning to yearn in unfamiliar ways. There is a feeling of anxious wildness, and her body is changing. This young bird is now foraging not just for strength but also for travel. She is eating more and gaining weight--more than is needed to stay at home: she is piling on fuel.

One afternoon in early September she is quivering with nervous excitement. Not long after dark she rockets from a tree in her natal home into black and unknown skies, on the first leg of a 4,000-mile trip. Over the next six weeks she will fly during 16 nights, covering about 250 miles each, and in between flights will rest, hide, and refuel on the ground.

This young and small bird is eagerly hurtling itself through space to reach a place she has never been before. She will know when the long journey is finished; she will know where to feed, where to hide, and how to shelter. She will know how long to stay, as well. In the heavy understory below a closed rainforest canopy it is dark and cool. Calls from other Swainson's Thrushes ring through the humid stillness. All is well.

Mass Exodus. Every fall, many millions of landbirds depart North America north of Mexico for tropical haunts where forage is abundant and there is little chance of a freeze. More than half are youngsters making their first journey. Most will arrive to spend the northern winter in a place their species has been hereditarily assigned. There is much magnificent order to bird migration: there has to be!

Vagrant Season. A small fraction of each species' population is genetically deviant: they are afflicted with mirror-image misorientation relative to the night sky, causing navigational errors and far-flung vagrancy.

For birders, this is great. Many "rare" birds are found scattered about, sometimes in dazzling variety. For many of the birds, it's not so great, and their faulty compass will eventually guide them out to sea. Sad, it seems, but flawed navigational traits must not return to the gene pool.

In California, fall vagrant season for landbirds is from mid-August into mid-November, peaking in the last half of September. Often the first eastern birds hit the coast about September 1st (the Labor Day wave), and then there are weather-dependent pulses into October. Most of these fall vagrants pile up at river mouths and isolated clumps of trees along the coast or at desert oases. Others, fewer, found scattered throughout the forested interior, are headed for the coast.

Winter Arrivals. Just as landbirds that nest here move south, so do some that nest further north winter here. While such movement occurs throughout the fall, the third week of September is when representatives of many wintering species appear. American Pipits; Ruby and Golden-crowned Kinglets; Hermit and Varied thrushes, Audubon's Myrtle and Townsend's Warblers; Fox, Lincoln's, Golden- and White-crowned Sparrows (of the pugetensis subspecies): all may suddenly be present on a September 25th when there were none just two days earlier. By early to mid-October, all are common along the coast.

The Fall Rush. For anyone paying attention, fall bird migration is a powerful rush from summer into winter. For sheer bulk and variety of bird life, no other season comes close. Hour by hour, day and night, birds are rushing through, over, and by--in huge numbers and astonishing species aggregations. It is the single most energized pageant of movement held by native animals on the planet. For some of us, a year made up of 12 Septembers would be nice.

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