July 6, 2005
DNA from feathers tells tale of eagle fidelity
WEST LAFAYETTE, Ind. - A trail of feathers led a team of Purdue University scientists to confirm that eagles from central Asia are quite possibly the most faithful of birds.
By performing DNA analysis on the feathers left behind at nesting sites, the researchers were able to identify individual Eastern imperial eagles in a nature reserve in Kazakhstan. Their analysis showed that not one adult strayed from its mate - a degree of fidelity highly unusual among birds, the vast majority of which mate with and raise offspring from multiple partners.
The study is the first to confirm monogamy in eagles. More importantly, it also is the first study to rely on feathers collected "noninvasively," or without trapping and handling.
"That we were able to use feathers we collected noninvasively as a source of DNA is the number one thing scientists will be interested in," said Andrew DeWoody, associate professor of genetics and senior author of the study, which was published online Friday (July 1) in the journal Molecular Ecology.
"People have been doing studies like this for several years with mammals, but this is a first for birds."
By developing a protocol for extracting DNA from feathers, the researchers also have added another tool to help conservation biologists study rare and elusive birds of prey.
The researchers used a technique called DNA fingerprinting to help them genetically "tag" individuals in the population without capture, said Jamie Rudnick, the paper's first author and a graduate student who conducted the study as part of her doctoral thesis.
"Collecting feathers at the nest site helped us determine population parameters we wouldn't have been able to otherwise," she said.
Those parameters, such as yearly survival rates and ratios of males to females, help conservation biologists monitor populations of rare or endangered species. Noninvasive sampling allows biologists to track changes in populations over time without the risks associated with handling live animals, she said.
"Eastern imperial eagles are hard to catch, and individuals are difficult to tell apart," Rudnick said. "By performing genetic analysis on feathers collected at the site, we were able to track the presence or absence of individual birds over a six-year period."
This kind of monitoring is essential to conserving rare or endangered species, said Todd Katzner, director of conservation and field research at the National Aviary in Pittsburgh and a study co-author.
"You can't conserve a population unless you know how big it is and whether it's growing or shrinking," Katzner said. "This approach allows us to monitor wild populations in a way that's less invasive and more cost-effective than other methods."
Observational studies of imperial eagles suggest these birds, like most large birds of prey, are at least socially monogamous. This means a male-female pair stays together throughout the breeding season and shares responsibility in raising young, DeWoody said.
But just because two individuals act like a pair doesn't guarantee they're not having trysts on the sly. In fact, studies over the last decade have shown most broods of socially monogamous birds include offspring from at least two genetic fathers.
"Our study actually stands out as a relatively rare instance in which DNA fingerprinting uncovers genetic monogamy in a bird population," DeWoody said.
Unlike their smaller songbird cousins, most raptors are believed to be truly monogamous, and this study provides the first genetic confirmation of monogamy in at least one species, DeWoody said.
"It's very likely that other raptor biologists will follow this with similar studies on other species of interest like bald eagles," he said.
This project is one part of a larger research program on the eagles of Naurzumski Gosudarstvennyi Zapovednik (Naurzum State Nature Reserve), a large nature reserve in northern Kazakhstan. This reserve supports an unusual diversity of raptors and is home to no less than 24 different species of hawks, eagles, falcons, owls and harriers.
Also participating in this study were Evgeny A. Bragin of the Naurzum State Nature reserve, Karamendy, Kazakhstan; and O. Eugene Rhodes Jr., of the Purdue Department of Forestry and Natural Resources. The Wildlife Conservation Society provided funding for this project.
Writer: Jennifer Cutraro, (765) 496-2050, firstname.lastname@example.org
Sources: Andrew DeWoody, (765) 496-6109, email@example.com
Jamie Rudnick, (765) 494-3997, firstname.lastname@example.org
Todd Katzner, (412) 323-7235 x210, email@example.com
A publication-quality photo is available at https://news.uns.purdue.edu/images/+2005/eagle-chicks.jpg
Using naturally shed feathers for individual identification, genetic parentage analyses, and population monitoring in an endangered Eastern imperial eagle (Aquila heliaca) population from Kazakhstan
Genetic analyses on noninvasively collected samples have revolutionized how populations are monitored. Most noninvasive monitoring studies have used hair or scat for individual identification of elusive mammals, but here we utilize naturally shed feathers. The Eastern imperial eagle (EIE) is a species of conservation concern throughout Central Asia and, like most raptors, EIEs are inherently challenging to study because adults are difficult to capture and band using conventional techniques. Over six years, we noninvasively collected hundreds of adult feathers and directly sampled EIE chicks at a national nature reserve in Kazakhstan. All samples were genetically sexed and genotyped at a suite of microsatellite loci. Genetically profiled adult feathers identified and monitored the presence of individual eagles over time, enabling us to address a variety of issues related to the biology, demography and conservation of EIEs. Specifically, we characterized: 1) the genetic mating system, 2) relatedness among mated pairs, 3) chick sex ratios, and 4) annual turnover in an adult breeding population. We show that EIEs are genetically monogamous and, furthermore, there is no apparent relatedness-based system of mate choice (e.g., inbreeding avoidance). Results indicate that annual adult EIE survivorship (84%) is lower than expected for a long-lived raptor, but initial analyses suggest the current reproductive rate at our study site is sufficient to maintain a stable breeding population. The pristine habitat at our study site supports an EIE population that is probably the most demographically robust in the world; thus, our results caution that populations in marginal habitats may not be self-sustaining.
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