Features

It may have felt like summer last week, but the birds know otherwise. The southbound migrants are on the move. 

Last week in Tilden Park, I spotted a few of the legion of travelers: western tanagers, warbling vireos, Wilson’s, orange-crowned, Townsend’s, yellow, and black-throated gray warblers. And I heard, but just missed seeing, a northern waterthrush, which should have been on another flyway altogether. More exotic strays are being reported from outer Point Reyes and from the Farallon Islands, landing field of last resort. 

Viewers of the recent documentary “Winged Migration”—for all its bombastic score and intrusive narration, an impressive film—may have come out with the sense that most of the traffic consists of big showy birds like geese and cranes. (I’m told that when the movie opened in Nevada City, a resident brought two ducks to see it. They were rowdy and raucous, though, and had to be removed from the theater. And I can see how the subject matter might have been disturbing to them.) 

In fact, smaller migrants—songbirds like those warblers, vireos, and tanagers—vastly outnumber the big guys. But since most of them migrate at night, they pose a challenge for the documentarian. 

The journeys they undertake are staggering to think about. The distances alone: from the northern boreal forests to the Amazon, in some cases. Western migrants don’t face the trans-Gulf of Mexico passage that eastern species do, but they still have long and risky routes to travel. 

They run a gantlet of dangers: bad weather, predators (hungry sharp-shinned and Cooper’s hawks are also moving south), human obstructions. In San Francisco’s Financial District, I’ve found thrushes and warblers dead on the pavement below high-rises, killed by collision with those glass cliffs. The buildings’ lights seem to draw birds, as do those of broadcasting towers where researchers have documented heavy mortality. And our transformation of the landscape has made the stopovers the birds need to rest and refuel fewer and farther between. 

Larger birds like geese learn migration routes from their elders. (They can also learn not to migrate, and hang around parks and golf courses all year). But most songbirds fly alone. They’re born with a genetically hardwired itinerary; some scientists think this involves a a time-distance program, a predisposition to fly along a specific compass bearing for a specific period of time. This has to be supplemented by careful reading of navigational cues: the positions of the stars and the setting sun, landmarks like mountains and rivers, even the Earth’s magnetic field. 

Some birds are dyslexic. That’s likely how the northern waterthrush wound up in Tilden Park. By the same token, every migration season brings a flurry of western strays to the East Coast. In spring and fall, birders in search of rarities haunt “vagrant traps,” like the cypress groves at Point Reyes ranches, where stragglers come to rest. 

If they stay on course, most western migrants reach wintering grounds in western Mexico and Central America. This is where their ancestors evolved before they began commuting north to take advantage of more favorable conditions for nesting. Some are drawn to habitats similar to their spring and summer quarters: black-throated gray warblers to oaks, hermit warblers to pines. Others are generalists. Richard Hutto, who has studied wintering songbirds in Mexico for over 20 years, has found that northern migrants are 

more tolerant of second-growth habitat than native birds. 

Biologists are still learning about the winter ecology of neotropical migrant birds. Their food preferences may shift: insect-eaters up north may become fruit or nectar-feeders down south. Black-headed grosbeaks wintering in Mexico feed on wintering monarch butterflies, being one of the few birds able to stomach the insects’ bitter taste. 

Some migrants stake out territories; in some species, males grab the best real estate, relegating females to less favorable spots. Quality matters: recent studies show that birds with better winter territories produce more chicks the following summer. Others join mixed flocks that may contain over 20 different species. By ganging up, they get access to the turfs of territorial birds, and there are more eyes to watch for predators. Hutto says that within these flocks, black-throated-gray warblers hang out with dusky-capped flycatchers; the flycatcher may benefit by snagging insects that the warbler flushes but misses. 

Although less publicized than the destruction of the Amazon rainforest, the area where western migrants winter is at grave risk. Mexico lost 3 million acres of forest per year over the last decade, a total area about the size of Ireland. Biodiversity advocate Edward O. Wilson says 70% of tropical dry forest, the predominant type in western Mexico, has been lost. Whether the forest is replaced by cornfields or marijuana plantations, the result is a net loss of viable wintering habitat. Hutto is now using satellite images to 

measure changes in habitat types and their impact on wintering migrants. 

Are North America’s neotropical migrants in trouble? Alarming population declines in the late 70’s and 80’s spurred the formation of international bird conservation groups like Partners in Flight. John Faaborg, author of the recent book Saving Migrant Birds, thinks things might not be all that bad, with losses for some species offset by gains for others. He points out that we still don’t have a lot of hard data on migrant numbers, let alone the fates of individual birds. 

Tools that promise to provide better data are being developed, though. Banding has its limitations: of over 140,000 Wilson’s warblers banded in the US and Canada, only three were ever recovered in Mexico and Central America. And with birds weighing only a third of an ounce, radiotelemetry is impractical. 

However, it may be possible to identify a wintering bird’s point of origin from its genetic signature. Analysis of feather chemistry is another promising technique: the ratio of hydrogen isotopes in a bird’s feathers can show how far north it was when those feathers grew in, just before it started its southward journey.