Using Super-science to Track the Rusty Blackbird
January 1, 2011 by Tina Gheen
The rusty blackbird (Euphagus carolinus) is disappearing, and we don't know why. Scientists believe that the birds may be declining because of changes in their habitat. Specifically, the rusty blackbird is tied very closely to forested wetlands. They depend on them for food and habitat.
These wetlands are disappearing due to man-made changes in the environment, such as:
- Land development
- Acid rain
- Drying of the wetlands from global warming and climate change
However, scientists won't know why the birds are disappearing until they understand more about them.
Part of the problem with managing habitats for migratory birds is that we don't know exactly where the birds go when they migrate or what flight path they use. Generally, we know the rusty blackbird spends its summers in the boreal forests of North America across most of Canada and Alaska. That's a pretty big area for a team of scientists to cover.
In the winter, the blackbirds leave their northern breeding grounds and fly south to a much smaller area in the Southeastern U.S.—the Mississippi Alluvial Valley and the Atlantic Coastal Plain. But with the rusty blackbird, things are even more complicated because they respond differently to changes in the weather every year, and they are very difficult to capture. It's nearly impossible to capture the same bird twice.
A group of researchers from the Smithsonian Migratory Bird Center and Environment Canada had an idea about how to solve this problem. If they could match up the wintering populations in the south with the breeding populations in the north, they'd have a much clearer picture of how the birds live and what they face during and after migration. Once they connected the populations with their habitats, they could examine the condition and quality of the wetlands for each set of birds and discover the reasons for the birds' decline.
Unfortunately, the small team didn't have eyes watching the skies at every moment and nets set up across the country, so how could they track these elusive, wide-ranging birds? The answer in this case is to use a little chemistry, a little biology, a little geography, and a lot of math.
The scientists decided to use analysis of stable hydrogen isotopes found in rusty blackbird feathers to try to discover where the birds had been. Scientists can look at variations of elements, like hydrogen, in bird feathers using mass spectrometry, a technique that analyzes the composition and properties of molecules in the samples. They can then match the samples with similar geographic and ecologic data collected from different regions.
Depending on environmental conditions, the composition of elements can vary slightly from area to area. By analyzing these variations in the elements, or isotopes, scientists can track the movement of elements through the environment and the food chain. This technique can help researchers trace all kinds of things like food sources, pollution, and changes in the soil.
Between 2005 and 2009, the team of scientists measured ratios of stable hydrogen isotopes taken from over 500 rusty blackbirds captured in the wintering grounds in the Mississippi Alluvial Valley and the coastal plain of South Carolina and Virginia. The team compared the hydrogen isotopes found in the birds' feathers with the hydrogen isotopes recorded in precipitation in the breeding areas. These calculations allowed the scientists to estimate the location of the wintering birds' summer breeding grounds—east or west of the Appalachian Mountains.
Map showing likely migration paths of rusty blackbirds. Birds follow paths on either side of the Appalachian Mountains.
But the Smithsonian researchers wanted to see the historical picture as well. So not only did they analyze feathers from the live birds in the wild, they also took samples from 190 rusty blackbird specimens located in museum collections, including some that were over 130 years old!
The final analysis showed something very interesting. Based upon the comparison of isotope ratios, scientists could see clear population differences. There were two distinct subpopulations separated by the Appalachian Mountains. Birds wintering in the Mississippi Alluvial Valley spent the breeding season on the western side of the Appalachians from Alaska to Labrador. The birds found on the Eastern Coastal Plain in winter were only found in the eastern parts of the breeding range. They didn't cross the mountains during their migration.
The overall results also showed the change in population density and the extent of the birds' range over time. The scientists were able to map the probable origins of the birds using a GIS-based model to get a better idea of the distribution and density of the breeding bird populations through the years.
Why is this important? Now researchers have established a strong connection between the birds and their respective territories without recapturing the birds. They also have a better idea of the flight path the birds are using to get from the breeding grounds in the north to the wintering grounds in the south.
This makes it easier for wildlife managers to tailor their efforts to the eastern and western subpopulations. Habitats critical to the rusty blackbird's survival can be identified and steps can be taken to preserve them. Perhaps most importantly, the techniques used in this study can be applied to other migratory bird species studies.
How you can help
Rusty blackbird research has come a long way, but there is still a lot to learn about the rusty blackbird. And you can help! From January 29 through February 13, 2011, you can help scientists learn more about rusty blackbirds and where they spend their winters by participating in the Rusty Blackbird Blitz. Volunteers are asked to search for rusty blackbirds in any potentially suitable habitats and submit their observations via e-Bird.
This article summarizes the information in this scientific paper:
Hobson, K. A., R. Greenberg, S. L. Van Wilgenburg, and C. Mettke-Hofmann. 2010. Migratory connectivity in the Rusty Blackbird: isotopic evidence from feathers of historical and contemporary specimens. Condor 112:778-788.