By Rex Graham
A team of avian crime-scene investigators has solved the mysterious case of the vanishing Wilson’s Warblers.
The species is named after the father of American ornithology, Alexander Wilson. The technology invented to identify a potent new threat to the warblers could help protect hundreds of other species.
Wilson’s Warbler decline
Birders in northern coastal California this summer saw fewer of the 3.75-inch birds with olive backs, yellow undersides and dark, uncolored tails (males sport a black yarmulke). The chryseola sub-species that nests there to southwestern coastal Canada, sports a distinctive orange-tinged yellow forehead.
What’s the reason so few were seen the summer of 2013? The chryseola sub-species may be headed for extinction.
Oddly, numbers are normal for the slightly larger pileolata sub-species (without the orange tinged forehead), which nests in east-central California, north to the northern two-thirds of Canada and most of Alaska.
How could two closely related sub-species of Wilson’s Warblers face such radically different fates? Ornithologists had no idea. Surely Wilson would have been desperate to know if a sub-species of his namesake warbler had become a doomed canary in an ecological coal mine.
A multidisciplinary team of researchers described in the August 2013 issue of Molecular Ecology a new analytical technique that enabled them to determine the “migratory connectivity” of the chryseola and pileolata subspecies. You could call them the avian CSI team.
They tested warbler feathers as if the chemical and genetic markers in each one was the encrypted text of a birth certificate; de-encrypting the markers revealed the parentage and geographical home of each bird.
“No other group had tried to develop a statistically sound approach of combining genetic and chemical markers into a single model,” said Thomas Smith, director of UCLA’s Center for Tropical Research and co-author of the Molecular Ecology paper. “Hydrogen isotopes in each feather show its latitudinal origin, and genetic markers narrow down the longitudinal source.”
Funneling into disaster
On a map of North America, Smith’s group marked a colored-pencil dot in the geographic position of each bird captured in winter, then drew a line to its birth-certificate nesting home in the north. As more lines were added, the migratory-connectivity map of both sub-species emerged as if by magic.
Instead of migrating to multiple locations in Mexico and Central America, the chryseola sub-species funnels to the southern tip of Mexico’s Baja Peninsula.
Massive development there may explain why that sub-species was in such steep decline while the pileolata sub-species, which doesn’t winter there, is not declining.
A formerly lush spot for warblers, the Baja Peninsula has been transformed into an ecological trap. Golf courses, beach clubs, boating marinas and other resort and housing developments from Cabo San Lucas to Loreto have replaced leafy habitat. A few nature preserves remain, but these token remnants of green are an additional threat: they are used as marketing tools to attract even more American and Canadian home buyers and vacationers.
Smith’s team didn’t set out to focus on Baja or any particular wintering location. Since the combined chemical and genetic analysis technique had never been tried before, they didn’t know what it would generate.
More effective conservation
“This approach is a foundation for combining other kinds of tracking data into a single model of migratory connectivity for many species,” Smith said in a telephone interview. “The sky’s the limit, and we’re very excited that we may be able to create migratory connectivity maps for other birds, and possibly identify many key areas that should be preserved.”
Unfortunately, it may be too late for the chryseola sub-species. In winter the birds feed amid shreds of landscaping between plush resorts with names like Esperanza, Starwood, and Hacienda del Mar.
Ornithologists who for more than a century had relied on bird-banding studies to study Wilson’s Warbler migration had no idea the Baja Peninsula is so critically important. The new analytical approach proved it.
“It’s a breakthrough type of analysis,” said David DeSante, an old-school ornithologist with a sharp eye for innovative new technologies. “It’s way more informative than bird banding.”
The co-authors of the Molecular Ecology paper, including DeSante, said the analysis tool could “identify patterns of migratory connectivity on a broad scale.” The study has international political implications because many of the roughly 2,000 species of migratory birds worldwide often breed in one or more countries, but winter in others.
In addition, many species use stop-over locations to rest and refuel during migratory flights. Potential threats there could be avoided with human help.
Other newly developed tracking methods could not be used with Wilson’s Warblers. Satellite tracking devices costing up to $4,000 each are used on raptors, shorebirds and other larger birds to electronically follow them on migration. However, at four grams each, a satellite tracker would be half a Wilson’s Warbler’s weight.
Even one-gram geo-locators costing $100 each are too heavy for an eight-ounce bird to lug on its migration. (An additional problem with geo-locators is they must be recovered in the field to download the carrier bird’s travel data.)
Unorthodox research team
In the case of the Wilson’s Warbler study, the unconventional makeup of the team itself proved to be a winning combination:
• Bird banders in Canada, the U.S., Mexico, El Salvador and Costa Rica plucked a feather from each Wilson’s Warbler they captured. Many of the participants are part of the Institute for Bird Population‘s network of hundreds of mist netting volunteers across the U.S. and Canada.
• A team of researchers at UCLA’s Center for Tropical Research set up a “freezer-farm” that now contains more than 150,000 feathers, accumulated over many years.
• A geochemist, Oklahoma University Professor Jeffrey Kelly and a geneticist, UCLA Assistant Adjunct professor Kristen Ruegg. They combined their intersecting areas of research with statisticians to create a model of migratory connectivity for Wilson’s Warblers.
• Margery Nicholson, the major contributor funding the investigation, who “got it” when the research team described their unorthodox approach.
GPS in feathers
Several years ago, geochemist realized there were small variations in the ratio of non-radioactive hydrogen isotopes in the keratin protein of feathers. About 99.98 percent of the hydrogen atoms have a nucleus of one proton; and about 0.0026 – 0.0184 percent, have a nucleus with one proton and one neutron, called deuterium.
The amount of deuterium in feather proteins varies, depending on the rainfall of the region where it was grown. That variation across North America and other continents amounts to encrypted geographical information. It is stamped into the molecular birth certificate of each feather of each migratory bird.
“Satellite trackers and geo-locators are complementary to our isotopic and genetic techniques, and we should leverage these more expensive approaches to make the science of migratory connectivity better,” said Kelly, an associate professor of biology at Oklahoma University and co-author of the Wilson’s Warbler study.
“However, with hundreds of species of migrants to study, we can’t afford to use expensive tracking devices on all of them,” Kelly said. “The inexpensive genetic and isotope markers can get you further down the line at a fraction of the cost of other tracking technologies.”
Ruegg, an assistant adjunct professor at UCLA’s Institute for the Environment and Sustainability, said the genetic tools used in the Wilson’s Warbler study were borrowed from salmon geneticists.
“The genetic tools are improving quickly and dropping in price,” Ruegg said. “With them we have recently discovered eight distinct groups of Wilson’s Warblers in their Western population, and this sub-population structure was not detected by banding more than 100,000 Wilson’s Warblers.”
Ruegg said the molecular tracking technology she and her collaborators have developed can be used to create genotype-frequency maps for hundreds of species. “We could then sample feathers of migrants anywhere in South America and quickly determine where each bird nested in North America the previous summer,” she said.
More research funding is required, Ruegg said. And the public database it would generate would help wildlife resource managers worldwide make more informed decisions on which conservation projects to pursue.
“Birds tend to distribute broadly on their breeding grounds, but funnel narrowly to their wintering grounds,” Ruegg said. “So, what happens on the wintering grounds can have broad impacts on migratory birds in North America, Asia and Europe.”