Wunder Explores Boundaries of Migration Research and Discovers New Territories
Wunder in South Park
Living creatures store chemical signatures in tissue that create a record of where they have been and what they have been up to, and those who know how to read these signals can snoop into an individual's past and predict future behaviors. This isn't the machination of a National Security Agency plot, or a storyline for the next futuristic television crime drama; it's the research realm of Assistant Professor of Integrated Biology Michael Wunder. Wunder and students from his lab are tracking the migration patterns of animals all over North America by comparing these chemical signatures in isotopes of everyday elements like carbon, oxygen and hydrogen in migratory birds and animals. They collect and analyze samples of metabolically inert tissues made by animals (such as hair, nails, whiskers, feathers and scales) and use this data to reconstruct patterns of geography using complex models. Combining mathematical modeling, physical chemistry and variability patterns in biogeochemistry, Wunder is crossing boundaries of science, and heading into territories far from the traditional homeland of a population ecologist.
Wunder and his family migrated to Colorado when he was only ten days old. He grew up in Fort Collins, graduated from the University of Colorado Boulder, double majoring in Biology and Environmental Conservation, and earned his PhD from Colorado State University (CSU). He spent his first two years in college pursuing a degree in electrical engineering, but decided that the discipline was too centered on solving other people's problems. He says, "I don't see it that way anymore, and I have nothing but respect for engineers and their creativity, but at the time I saw engineering as this future where I would be given problems and asked to optimize solutions, and the more I learned about the world the more I realized I wanted to be the one framing the bigger questions." He recognized the study of biology as a venue for creativity, not to mention spending time outdoors, and after earning his BS he went to work for the Federal government, first as a volunteer. Wunder remembers, "It was my job to wander around in the woods all night long, all summer long, looking for owls. I thought that was a lot of fun, so I looked for a way to keep doing that."
For the next eight years, Wunder worked in wildlife management for government agencies and non-governmental organizations, including six years as a Research Associate at the Colorado Natural Heritage Program (CNHP), the science support arm of the Nature Conservancy, housed at CSU. The applied conservation projects he worked on were often short-lived, and Wunder grew antsy yearning to ask longer-term questions about populations and migration. Migration is a behavior that is energetically expensive for animals, and population biologists are still trying to discover what motivates migration. Wunder says, "There are lots of proximate explanations that can be provided for local examples, but generally we still don't understand how the evolution of migration comes to pass." In the case of the research going on at CNHP, Wunder says, "A concern with studying migratory species is that they are experiencing different parts of the world at different times, so it can be difficult to get your head around how those different geographies can be influencing survival and basic kinds of demographic functions that drive population dynamics. We were making lots of decisions on how to manage populations of migratory animals while they were here in Colorado, but they were only here for 3-4 months, and then somewhere else. So it was natural to wonder how population dynamics were affected by things that were happening far away from these summering grounds."
When Wunder decided to pursue his PhD in order to start researching the bigger picture, his existing connections at CSU through his work with CNHP made it natural to study there. Working with Fritz Knopf of the USGS and Barry Noon at CSU, his dissertation focused on patterns of migration for mountain plover, a species of bird that's been proposed for listing as endangered off and on since 1999. Plover populations are segregated in the summer at their various breeding grounds across western North America, but birds intermingle in their wintering grounds. Because much of plover survival is owed to winters spent thousands of miles away in California and Mexico, Wunder was stymied getting a complete picture of the challenges faced by the small number of plovers that come to northern Colorado in spring. In California the plovers depend on the same farm landscape that supplies fresh fruits and vegetables to people all over the US and the world, and changing agricultural practices could leave birds vulnerable. In order to get a clear picture of how plovers connect Colorado to California and Mexico, Wunder first had to determine where his particular flocks were spending their winters. The challenge was daunting, as plovers are too tiny to track with large GPS devices and too unpredictable to rely on traditional band-release-recapture methods.
He found the answer in biogeochemistry: the stable isotope chemistry of feathers, which reflect the environmental conditions where they were formed. As Wunder explains it, the amount of free energy in any given system helps to determine the stable isotope ratios in food and water available to the animals Wunder studies. He says, "Isotopes of Carbon and Nitrogen are subject to known discrimination mechanisms that create these nice gradients. Water-use strategies in plants determine when air exchange occurs, and therefore determine the isotopic composition of CO2 available for sugar production. That means carbon isotopes in plants vary predictably along water-use efficiency gradients, and because animals ultimately get their carbon from plants, the carbon isotopes in animal tissues reflect the habitat type where they fed. And because animals aren't capable of producing all the amino acids they require, patterns of nitrogen isotopes tend to reflect location on the trophic ladder."
Chemistry collides with the study of animal behavior, says Wunder, "We can use these chemical signals to understand something about what animals were eating, at what level they were feeding, the quality of the diet and so on. You really are what you eat. Then we can use geographic markers that are associated with the water to fix locations the animals have been. As it turns out, you are also where you eat." This is not always easy to do, because steep gradients don't exist everywhere, so the first order of business for Wunder was to uncover patterns of variability and figure out if he could use these mechanisms to tie animals back to specific locations. The answer, for hydrogen and water over most of the globe, appeared to be yes. "Isotopes of hydrogen and oxygen in precipitation show continent-level gradients that reflect temperature tendencies. Rain tends to be comprised of more 2H and 18O at lower latitudes, lower altitudes, and along coastlines."
The next order of business was to discover time signatures, or how long these clues remain in tissues. In the case of plovers, Wunder says, "If an animal is in a place for a couple of months it's going to be integrating the chemical signature of that environment into its feathers. Once grown, those feathers are fixed and the environmental signal is taken along with the bird as it migrates. Plovers grow new feathers at the end of summer before migrating to wintering grounds and again at the end of winter just before they migrate back to summer grounds. So we can pluck the feathers of the birds here in Colorado and assign them back to various locations where they spent the winter." Wunder has used these techniques to research many other species of birds and mammals, including populations of migratory bats and ducks from Canada, and shorebirds and Stellar sea lion from as far away as Alaska.
Once he knows a migration link between point A and B, the final piece of the puzzle for Wunder is determining the route that animals take between these two locations. Once again new technologies and methods are making this possible. Eli Bridge, a colleague of Wunder's in Oklahoma, developed a tag that records light intensities and time signatures every few minutes. The entire device weighs about half a gram, so it can be attached even to small birds. Because satellite communication requires heavy batteries, these tags don't send data remotely. The birds must be recaptured after they've made the journey and the data downloaded directly. Wunder and his team identify twilight events in the light data to estimate the angle of the sun. They then couple this with the timestamp and solar physics to calculate location estimates accurate to within 100 kilometers and map the journey from this data. Current Master's student Jessica Fish has already collected a migration data-set using these tags on bluebirds, which are simpler to collect because bluebirds return to nest year after year in the same birdhouses the team built in their range here in Colorado. These bluebirds are facultative migrants, which means they don't always migrate, and Wunder and Fish hope to use the data they're collecting to find out what drives them to migrate some years and not others.
Wunder's philosophy in mentoring students is simple, he says, "I allow my students a certain degree of independence, so if a student wants to come and own a project – working not just to solve a problem but also to understand why that question needs to be asked – that's the kind of training I can provide." Andrew Doll received his MS in Integrative Biology in May 2013 and studied shorebirds that migrate between northern Alaska and southeastern Asia. He says of his experience working with Wunder, "Collaborating with researchers at the US Fish and Wildlife Service and US Geological Survey, we developed a research project to investigate the migratory behaviors of shorebirds using stable isotope markers in their tissues. Mike's instruction has been vital to my understanding of the physiological and ecological processes that make stable isotopes a useful tool in wildlife studies. His extensive knowledge of ecology and his ability to make complex statistical concepts approachable have greatly benefited my development as a professional scientist." Rebecca Bryan graduated with an MS in Integrative Biology in May 2012, and says of her time in working with Wunder, "It is very easy to get caught up in the details and following the self-imposed 'rules' when working on a big project, and Mike tried to steer me toward having fun with the research while still collecting good, solid data." Bryan recently published her thesis research examining how migratory burrowing owls sharing habitat in prairie dog colonies might be eavesdropping on the prairie dog's predator alarm calls in the journal Ethology. She says "Mike has also been very supportive and helpful as I've worked to get my masters research published." Students in Wunder's lab work all over the country, and Wunder has set a goal of visiting each of their field sites this year.
Wunder continues to ask big questions, and sees his methods and research as potentially applicable to a whole host of future industries that are hardly imagined today. On top of gathering valuable conservation data for animal populations, the methods Wunder helped develop studying migration are applied in an astounding variety of ways. Approaches Wunder helped refine are used to source food products for origin and pedigree, especially in Europe where butter, expensive, certified wines and bottled water. Wunder's colleagues, Distinguished Professor Jim Ehleringer at the University of Utah, and Lesley Chesson run a cottage firm called IsoForensics that uses stable isotope tracers to solve all kinds of forensics problems, including the source of confiscated cocaine and geographic origins of counterfeit currency. These methods have even helped to identify unknown victims in cold cases. In a world of shifting climates, Wunder sees great potential in studying migratory animals as a way to model global patterns of movement for human populations, as well as cargo and transportation development. He believes nature and the patterns of migrating animals will lead the way, and says, "There are lessons that can be learned from looking at a system that's been around far longer than anything we've invented. I'm excited to keep discovering new ways we can use these findings in the future."