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| 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.
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| Seth (left) and Gary Graves (Right) 2013 |
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.
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| 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.
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| 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.
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| Santeetlah Creek Watershed, Pipevine in right center |
