HWI veterans pioneer the use of stable isotopes
to learn the geographic origins of migrating Hawks in the Florida Keys.

By Casey Lott, HWI Florida Keys Project Director

Over the past several years a very strange thing has happened to Tim Meehan and I. If you know either one of us at all, you’ll most likely be shocked. Yes, against all odds, it’s true: we learned some chemistry. It started on a river trip in Grand Canyon, where a biologist named Larry Stevens mentioned in passing, "maybe you could use stable isotopes to figure out where the hawks that you see during migration are coming from." He could just have easily said, "dogs may be circumspect, but I prefer jocularity to Szechwan." It made just about as much sense at the time.

Just over two years later, with the help of a huge list of friends and collaborators, I am writing to let you know that we are now putting the finishing touches on an exciting project using stable isotopes to learn where the immature Cooper’s Hawk’s that we catch during migration in the Florida Keys are coming from. This method will dramatically improve our ability to learn the geographic origins of the migrants that we catch at our count sites, and be of great value as we interpret the trends that we see in count totals- improving our ability to connect observed trends with specific regions.

Tim presented the results of this study at the annual meetings of the Cooper Ornithological Society in April of 2000, and the second draft of a scientific paper is being reviewed by co-authors for submission to a peer-reviewed journal. The paper is currently titled: "Using hydrogen isotope geochemistry to estimate the natal latitudes of immature Cooper’s Hawks migrating through the Florida Keys."

By now you are probably asking yourself, "Just what the heck is a ‘stable’ isotope?" Stable isotopes are all around us; in fact, they are inside us too. But what exactly are they? Nearly all of the elements in nature have at least two different forms, or "isotopes". At the risk of sounding like a textbook… the difference between two isotopes of the same element is the number of neutrons in the nucleus. For example, the omnipresent element, hydrogen, has two stable isotopes: protium (abbreviated as ¹H) and deuterium (²H). Don’t worry. You will not be quizzed. What’s the difference between protium and deuterium? They are both forms of hydrogen, but protium atoms have no neutron while deuterium atoms have one.

Hydrogen is everywhere in nature, and anywhere you find it; you can measure the ratio of protium to deuterium. This ratio is often referred to as a hydrogen isotope signature. The peanuts that you are munching on have hydrogen in them. Therefore, they have a hydrogen isotope signature. The water in your faucet, the milk in your refrigerator, and getting back to our study, the feather of a Cooper’s Hawk, all have hydrogen isotope signatures.

The hydrogen isotope signature of a substance is measured with incredible precision by a very expensive machine called a mass spectrometer. This past winter Tim and long-time HWI bander and educator Ruth Smith pulled all-nighters, greeting the sunrise from the stable isotopes lab at the University of New Mexico, processing feathers from Cooper’s Hawks with the help of geochemistry wiz, Dr. Zach Sharp. The process goes something like this: first the feather sample from a migrant Cooper’s Hawk is cleaned, then it is dropped into a very hot chamber where it is vaporized. The resulting product is a mixture of gasses. A gas chromatograph separates out the hydrogen gas, which is then sent to a mass spectrometer. Along the way, a series of bending tubes, mirrors, and magnets separate the atoms of the two isotopes of hydrogen. Many boring mathematical equations later, a computer spits out something called a "delta" value for the sample. This delta value is the official way to express a hydrogen isotope signature.

O.K., here’s the cool part (FINALLY! I can hear you screaming!)... Atmospheric scientists have known for years that delta values for hydrogen in precipitation show a strong geographic pattern, with values becoming increasingly negative as you move further north in latitude. Recently, several researchers working with birds have discovered that the local hydrogen signature for rainfall ends up being recorded in the feathers of birds while they are being grown. The feather is analogous to a tape recorder, recording the isotopic signature of the rainfall where the hawk was born. Young birds grow all of their feathers on their natal territories, before they migrate. After the feather is grown, it is cut off from the bird’s blood supply, becoming metabolically inert. Even though the hawk may migrate thousands of miles to a wintering area and back, the hydrogen isotopic signature in the feather does not change until it is replaced during the next summer’s molt. As you all know, as the hawks migrate to and fro, some of them end up in our mist nets. So, if we take a feather from a migrating hawk, and analyze the feather for hydrogen at a stable isotopes lab, we have a delta value for the rainfall where the bird was born! Thankfully for us, the International Atomic Energy Agency operates a network of stations that measure hydrogen isotopes in precipitation all across North America. So, by comparing the delta value for hydrogen in the feather of the hawk that we catch during migration to a continental map of delta values of hydrogen in precipitation, we are able to tell, roughly, where the bird was born. Phew…you can all say "Wow!" now.

In a nutshell, this is exactly what we did with feathers from 48 immature Cooper’s Hawks that we caught during migration in the Florida Keys. We found that a majority of these birds did not come from the entire eastern range of the Cooper’s Hawk. Instead they originated primarily from the mid-Atlantic and north Southeastern states. In general, Cooper’s Hawk populations are not systematically monitored in this region. Thus, long-term data from our new count site in the Florida Keys will provide important data about population trends for Cooper’s Hawks in these regions. This data will be useful to land and wildlife managers when they evaluate the status to Cooper’s Hawk populations, or assess the impact of conservation strategies for habitats in these regions upon which Cooper’s Hawks depend.

We look forward to seeing the results of this study in the literature and hope that this study opens doors to wider future applications of stable isotope analysis to learn the geographic origins of migrating hawks. If you have any questions or comments about this research please feel free to email either of us at caseylott@hotmail.com or tdmeehan@hotmail.com.

Many people have contributed to the successful execution of this project. In particular, Tim and I would like to thank our coauthors: Ruth Smith, Zach Sharp, Bob Rosenfield, Brian Millsap, and Andy Stewart. We also thank Alex Kropp, Bob Dickerman, Cindy Ramotnik, Paul Cryan, Tomasz Durakiewicz, Jeff Kelly, Len Wassenaar, Kathy Klimkiwiecz, Viorel Atudorei, Jim Scheffel, Chris Menge, Jeff Acuff, Julie Heath, Nat Seavy, Dave Leonard, John DeLong, and Jessie Jewell for their involvement with the project. We thank the Florida Fish and Wildlife Conservation Commission, the Elizabeth Ordway Dunn Foundation, and the Florida Keys Audubon Society for financial support of the Florida Keys Raptor Migration Project during the execution of this work. As always, none of this would be possible without the support and good humor of the outstanding staff of HWI.