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Researchers Untangle Secrets of Bird Migration

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Wilson's warbler along the California coast | Photo: David A. Hofmann/Flickr/Creative Commons License

It's no secret that some birds travel incredible distances during their annual migrations. Finding out which birds go where, though? That's always been a little harder. It's easy enough to determine that a widespread bird species spends its summers in Canada and the northern U.S., and winters in Mexico and Central America.

It's difficult, however, to get more precise detail about specific bird populations. If a migratory bird has summer populations in California, Alaska, and Montana, and winter populations in Baja, the Yucatan and Costa Rica, scientists have had little way to figure whether, say, all the Alaskan birds go to Baja and the Californians to Costa Rica.

And that's important, because migratory birds are in trouble across the continent. If a summer bird population in California is declining, knowing where those birds go in winter might offer hints as to the reason. The good news: a new study of a small migratory bird that passes through California might offer a way to fine-tune our understanding of bird migration.

There are a few ways of getting a bit of data about where individual migrating birds go. Birds banding, in which a light band of metal or plastic, imprinted with a tracking number is clamped loosely around a bird's leg, is one of the most common. But banding only works if someone finding a banded bird knows to report the band and location, and re-finding a banded bird is incredibly uncommon: only about one band in 10,000 is ever seen again.

Radio telemetry can work on birds that weigh a pound or more, but isn't so useful for anything smaller. Study of regional differences in birds' mitochondrial DNA has shed a little light on migration patterns. So has study of the differing ratios of isotopes of elements the birds ingest, which can be matched with the equivalent ratios in food available in varying summer or winter territories.

But for the most part, long-distance migration has been a bit of a black box: we might know the summer populations and winter populations, but we haven't been able to determine which summer population is what winter population.

The study, now being considered for publication in the journal Molecular Ecology, focused on the Wilson's warbler (Cardellina pusilla). The research team, led by UCLA's Kristen Ruegg and predominantly made up of scientists from UC campuses and other California institutions, studied variations in about 1,650 samples of Wilson's warbler DNA. They compared 96 genetic markers in that DNA called "single nucleotide polymorphisms" or SNPs -- in slightly plainer English, spots on the organisms' DNA that differed from their companions' by one base pair, or one "letter" in the genetic alphabet.

SNPs, usually pronounced "snips," are very simple mutations. It doesn't take much to make a SNP: a stray cosmic ray or bit of chemical damage to a cell's DNA, or even just a glitch in the cell's DNA copying and error correcting mechanisms can do it.

If the SNP is created in a section of DNA that isn't involved in building proteins, it may have no effect on the fitness of the organism in which it resides. That means natural selection won't weed it out. SNPs in that "non-coding" DNA can thus accumulate and be passed on to succeeding generations.

And since SNPs are relatively easy to generate, separated populations within a species that don't usually interbreed, and thus don't share genetic material, can rapidly develop their own "trademark" sets of SNPs.

Enough with the genetics. What this all means for purposes of the Wilson's warbler study is that researchers were trying to find out whether regional populations of the warbler could be associated with a characteristic set of SNPs their members tended to share. If so, then genetic testing of warblers at each end of the annual migration could finally link summer and winter-range populations.

What's more, scientists capturing a bird in mid-migration would be able to determine where that bird was probably headed. And that would allow a better sense of difference in migration timing among different populations.

The results bore out the premise. The team first examined DNA from 22 Wilson's warbler feathers taken from birds in well-documented locations. They then compared those feathers' SNPs with those from 1,626 feathers from a collection maintained at UCLA.

The scientists determined that what had been two recognized populations of Wilson's warbler, on the East and West Coasts, were actually at least six distinct populations. Eastern North America's population remained essentially unchanged. The western warblers, though, turned out to consist of five populations. Two summer in California: one in the Sierra Nevada and one along the coast. Another population summers in Alaska, a fourth in the Pacific Northwest, and the fifth in the Southern Rockies and Colorado Plateau.

The researchers determined that coastal California Wilson's warblers head for southern Baja and western Mexico in the winter, as do birds from the Sierra Nevada and Pacific Northwest populations. Birds from the Colorado Plateau and Souhern Rockies head for El Salvador, while East Coast birds headed for the east coast of Central America, from the Yucatan to Costa Rica. Alaskan warblers fly past Baja and Western Mexico to get to Central America.

Perhaps most excitingly, it turns out that the western populations seem to migrate northward on a staggered timetable. Co-author Kristina Paxton of the University of Hawai'i, Hilo spent two successive northward migration seasons in Cibola, AZ, along the Lower Colorado River, capturing and sampling warblers. The material Paxton collected provided an insight into the warblers' transit schedules. In 2008, Wilson's warblers headed for the California coast came through Cibola the week of March 22. Pacific Northwest birds followed the week of March 29, Sierra Nevada birds the week of April 15, and Alaska birds the week of April 26.

The researchers hope to extend this study to members of other migratory species, potentially including Endangered migratory birds.

Why is this important? Because different populations of a species can thrive to different degrees.

Imagine that the coastal California population of Wilson's warblers was doing okay but that the Sierra Nevada population was in trouble. (In reality, the species is declining across its western range due to destruction of its preferred riparian willow habitat, and Californian coastal birds are especially suffering due to resort development in their Baja wintering grounds. But let's go with the hypothetical for now.)

Let's also imagine that California was building, say, large solar installations on the migration path for Wilson's warblers that pose a risk of harming migrating birds. (Which it is. Wilson's warblers have been found dead at the Ivanpah Solar Electric Generating System during both fall and spring migration, and are quite likely at risk at other facilities as well.)

If we have a sense of when birds from each population are most likely to be within range of that solar plant, that may well make it easier to schedule plant shutdowns or other ways to lower the risk to that population as it passes through the area. And that doesn't just hold for solar plants, but any other potentially damaging activity we can reschedule around migration of an at-risk population: tree trimming, crop dusting, what have you.

It's also just cool news from the perspective of those of us who find the natural world interesting. It seems like wherever you look at nature, detail reveals itself. What had seemed a big amorphous mob of tiny yellow birds heading north and south becomes a carefully choreographed, neatly scheduled annual campaign.

Sometimes knowledge it just beautiful on its own, even if it has no practical application.

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