When banding, Leppold sometimes takes a tail feather or small amount of blood from the birds. From those biological materials, the UMaine researchers have been able to determine where birds were born or bred by looking at the stable isotope signatures of those samples.
An isotope is a form of a chemical element that contains a different number of neutrons than its normally occurring form. A stable isotope is one that doesn’t give off radiation over time. Scientists can track the presence of a stable isotope, known as a signature, in a bird that has over time been a part of the food chain. Take, for example, a hydrogen stable isotope known as deuterium that naturally occurs in the environment. Its levels rise in the environment where there is a lot of water, including geographic areas that have received a lot of rain.
Plants take in the deuterium, which is incorporated into their tissues through photosynthesis. Plants are eaten by insects, which incorporate the deuterium into their bodies. Birds eat the insects and therefore the stable isotope is transferred through the food chain and gets incorporated into the growing tissues in the bird.
Stable isotopes can be measured in something like a feather, which incorporates the proteins with the stable isotope signature. Even if the bird moves to a different habitat and the feather grows out, the stable isotope signature of the original habitat will be found at the end of the feather, giving an indication of the kind of environmental conditions into which the bird was born. The carbon signatures in the feather closer to the bird’s body can give researchers an idea of what kind of environmental conditions a bird has been more recently for breeding purposes.
If the range of stable isotopes is available for a large region, such as the Americas, researchers can then map the geographic areas that have specific levels of those stable isotopes.
If Holberton finds the deuterium signature in the feather or a bird, she can therefore assume the bird has at some point been in an area that has had a lot of water. That might seem too broad a geographic area to be of help to researchers, since there are rainy seasons all over the world, but scientists can look deeper to gain specifics. If Holberton has a feather from a bird known to be in Alaska and Newfoundland, for example, she might check the stable isotope signatures against those commonly noted in those areas to determine where a bird has been.
“You can find different habitats anywhere in the globe, but the rain patterns vary north and south,” Holberton says. “By also measuring stable carbon isotopes, which can vary with habitat type, in the same tissues, you can start putting another coordinate on where that bird was. So we’re taking these different tissues that give us a different time stamp, and we’re putting together a composite of stable isotope signatures that help give us some information.”
The information again informs the Network’s research by revealing more about birds’ travels.
Blood samples are tested for indications of the birds’ immune health. Holberton checks for ratios of hetereophils, which are ever-present immune cells that fight bacterial infections, and lymphocytes, which are cells that provide a longer-term line of defense yet have a high cost to a bird in terms of the energy it takes to produce them. If researchers find elevated levels of heterophils and normal levels of lymphocytes, the assumption is that the bird is fighting an infection but hasn’t yet suffered any long-term consequences. However, if lymphocyte levels are low, the testing may indicate the bird is having trouble maintaining its immune response critical to survival.
“A lot of times we see a situation where birds are not fighting an infection yet, but they’re under some kind of stress, whether the stress is from bad weather or crowded conditions, or they can’t find enough food,” Holberton says. “The immune system is a luxury that’s only going to be good if the animal stays alive, so what animals do is redirect energy away from the production of lymphocytes in order to deal with the currently stressful situation. They can become immunosuppressed and more susceptible to infection and disease. Things that you might normally have thrown off, like a cold, can kill you.”
Energy metabolite levels also can indicate changes in the behavior and physiology of a bird. If those levels are elevated, combined with indications of changes in the immune system, it could indicate that the birds isn’t doing well. That means it’s more likely to make a lot of stops during migration, which means taking a longer journey around the gulf. Birds in good shape will cut across large bodies of water, because they don’t need to stop as often.
Another important element of research for the Northeast Regional Migration Monitoring Network is data picked up by both radar and passive acoustic surveys. The acoustic data, gained through recorder arrays throughout the Gulf of Maine coastline and inland, give researchers an idea of which species of birds are flying overhead, especially at night.
As birds move through the sky, they call to each other via a series of short, usually high frequency noises, many lasting a mere hundreds of a second, which sound to us like clicks or chirps. An interactive sound analysis software system called Raven, which was developed at Cornell University, translates each call into a spectrogram or visual picture of the sound.
A group of UMaine undergraduates working in Holberton’s lab compare each bird’s song picture to standard, confirmed images of a species’ spectrogram to determine which species of bird has made the call.
“It puts a face to the radar profile, and allows us to identify different species flying overhead,” Holberton says. “Some species may not call, so this technique doesn’t tell you what’s not flying overhead. But this does help give us an idea of what is overhead in a certain time period and the corridor of the birds’ movements.”
Acadia University biologist Phil Taylor’s work also adds to the network’s diverse research techniques, especially related to individual songbirds. In addition to operating the Network radar sites, a graduate student in Taylor’s lab is deploying tiny transmitters on small songbirds as they are captured at banding stations in southwestern Nova Scotia. As the birds move through the Gulf of Maine, automated receivers set up along the coast from Nova Scotia to Scarborough Marsh can detect the flight and stopover movements of the individual bird.
Citizen research is also expected to be part of the project, with bird-watching fans in the populace recruited to help identify bird species and enter their observations into a database.
“No one research method by itself will tell you what you want to know,” Holberton says. “The value of the Network really is the integration of the different methods and the extensive expertise of the collective group of participants. That’s the beauty of it.”