Isotopes in Bird Feathers reveal their diet and geo-location

Seth Newsome and Gary Graves, Santeetlah Creek Watershed, 2013

         One of my favorite projects with Seth Newsome that sparked my interest in geoecology was a collaboration with Gary Graves, Curator of Birds, at the Natural History Museum of the Smithsonian. Gary is known as a “hard-ass” dude in ornithology. I’ve seen his prickly nature when he reviewed a paper we coauthored and he found the statistical treatment of the analyses insufficient. With Seth and me, I’ve seen the other side of him. In the field when we collected warblers, plants, insects, and water, Gary is completely switched on to the ecology of the region. As a native of the state of Arkansas, on the drives to the field areas, he likes to imitate Bill Clinton and tell jokes about all manner of things. In the rural North Carolina town where we stayed, we enjoyed breakfast and sweet tea at the local McDonalds with the senior citizens and chicken fried steaks in local restaurants for dinner. We had a lot of laughs throughout the collaboration gossiping about colleagues, reviews, and science. This collaboration is a good example of something that has held great importance in my career—the enjoyment of meeting and working with new, smart people.

         Seth and I had read papers by Hobson et al. (1997) and Rubenstein (2002) and had become intrigued by their results on bird migration. By measuring the hydrogen isotopes in a bird’s feather, one can ostensibly figure out the geographical region where that bird grew the feather. For birds that migrate from North America to the Caribbean and back each year, scientists are curious if the birds return to the exact place year after year. Bird-banding, placing a small band around a bird’s leg with an identifying number, allows scientists to determine that indeed some birds return to within 100 meters of where they had been the year before.

         Graves had collected (i.e. killed and stuffed) thousands of Black-throated Blue warblers in a decade long study of their breeding habits and ecology. Our collaboration with Gary has lasted over 10 years. We began with a series of feathers collected from warblers that were sampled along a latitudinal gradient from northern Georgia into southern Canada. Collaborating with Matthew Betts (Oregon State University), we analyzed hundreds of hydrogen isotope values from 22 sites ranging from 34 to 46 degrees North latitude. There was a clear influence of latitude on the hydrogen isotope composition of feather keratin with more positive values measured in the south and more negative values from the north.

Seth (left) and Gary Graves (Right) 2013

    The samples included adult birds and juveniles in their second year of life.  Black-throated Blue warblers migrate each fall from continental North America to tropical islands in the Caribbean, where they spend the winter.  They fly north again in spring to breed and molt their feathers. Our initial research question was the following: did Black-throated Blue warblers juveniles return to where they had been born or did they strike out for new territory?  We found that, with very few exceptions, juvenile Black-throated Blue warblers returned to the same general region where they were born. This work prompted further studies.

         Graves had colleagues at Chicago’s Field Museum with freezers full of Great Gray Owls, one of the largest birds in North America. These birds leave their territories in Canada and Alaska and invade the lower 48 states periodically, probably in response to a lack of prey. Once here, they still face dangers. During the winter of 2004-2005, 265 owls were killed in Minnesota alone by vehicular collisions. We sampled tissues from this unprecedented collection of avian specimens and published our work on carbon and nitrogen values of muscle, liver, and feathers (Graves et al., 2012). Muscle tissue analyses showed greater nutritional stress and a poor body condition. Nitrogen isotopes of muscle showed that the owls potentially spanned three trophic levels.

         The carbon and nitrogen data were interesting, but the hydrogen isotope data on feathers revealed much more. Hydrogen isotopes of animal tissue are determined from the hydrogen coming from an animal’s diet (i.e., organically bonded hydrogen) as well as drinking water (Estep and Dabrowski, 1980; Hobson et al. 1999, Newsome et al., 2017). Drinking water hydrogen isotopes are generally considered to be similar to that of precipitation and are dependent on latitude, altitude, and distance from the ocean. Hydrogen isotopes in plants are also primarily influenced by precipitation. Herbivores show an enrichment of the heavy hydrogen (²H) relative to plants. The hydrogen isotopes in diet are a combination of lipids (isotopically more negative), carbohydrates (isotopically more positive), and proteins (variable). Carnivorous birds, two to three trophic steps above plants, have much more positive hydrogen isotope values in their feathers than do herbivorous birds. 

Hydrogen isotope data from Great Gray Owls

         Great Gray Owls eat small mammals, placing them at least two and perhaps three trophic levels, above plants. These owls invaded northern tier of states south of Canada in late October, were found on roadsides, and were collected throughout the winter and spring until mid-May. We questioned whether owls arriving earlier were from nearby regions of Canada vs. those coming from colder, more mountainous western regions. We measured the greatest range in hydrogen isotope compositions from owls collected in February.

         We estimated that in February, owls arrived from parts of Canada extending from maritime Canada in the east to the Canadian Rocky Mountains and Northwest Territories to the west. Wow! Owl invasions happen only periodically. It remains unknown as to why Great Gray Owls from across the entire continent pull up stakes and fly south.

         Much of this work remains unpublished, both for the warblers and the owls. Why? Although we submitted an article on the warblers—twice—it was rejected both times because reviewers wanted us to assign a more exact geographic location for each specimen based on its isotope composition. I don’t feel this is a valid reason for not accepting a manuscript. My co-authors were disheartened, as people often are, when a paper is rejected. I continue to hope the data will be published in its entirety soon. We countered these reviews by measuring hydrogen isotopes in even more feathers.

         Graves undertook special collections in 2012 and 2013 taking 20 specimens in each year. From each bird, he sub-sampled 25 different feathers—primaries, secondaries, tail feathers, and body feathers. We found that the variation in hydrogen isotopes within one bird could be almost as great as the variation expected from birds living from Georgia to Pennsylvania. Based on these data and on data from 12 years of sampling birds at Santeetlah watershed, it’s evident to me that there is inherently more complexity built into the isotope signal than just geo-location. The diet of the bird and the weather that year are significant drivers of isotopic compositions of bird feathers, providing a much more interesting glimpse into their ecology.

Butterfly collecting Seth Newsome, 2012

         Seth, Gary and I continued to work on Black-Throated Blue warblers specifically in the Santeetlah Creek watershed of rural, mountainous North Carolina. We wanted to document all of the steps of the bird’s food chain from water to plants to caterpillars finally to birds. During our fieldwork, we noticed a specific species of butterfly, the Pipevine Swallowtail that could be found throughout the watershed. Hence, I am proud to say that we were “butterfly collecting”, a derogatory term people use for scientists who don’t have a serious enough research agenda. It felt good to collect some of these butterflies, the caterpillars that metamorphose into them, and the host plants. The analyses continue to this day.

Santeetlah Creek Watershed, Pipevine in right center