My research is dependent on migratory birds being present on their wintering grounds in the Appalachian Mountains in the month of March. But this year it was an unseasonably warm winter, and it was not possible to know when migrants would depart for their breeding grounds. Luckily, the temperatures dropped again and the migrants hung around these wintering grounds for another month.
It is becoming more common that organisms are changing where they live and when they migrate in response to climate change. Climate change affects ecosystems and climate zones by disrupting weather and temperature patterns and by producing catastrophic storms (IPCC 2013). Almost all plants and animals need stable environmental patterns in order to survive. Once unstable, species must find a new suitable habitat or will be unable to survive. This map (courtesy of Dan Majka) shows projected paths taken by organisms relocating to more suitable habitats. But what if some species are just not able to adjust? Plant and animal diversity may consequently decline at an alarming rate as organisms succumb to changing environments.
Since we often focus on a single habitat and species when thinking about climate change (think sea ice and polar bears, for example), it’s easy to overlook animals that migrate and rely on sometimes several habitats for survival. Migratory birds use many different habitats in the course of a single journey, which may provide them with a built-in ability to survive environmental change. On the other hand, if habitats along the migratory journey change too much from their normal conditions (e.g. food availability, suitable weather), this could greatly affect the birds’ survival. Scientists have already measured shifts in migration and breeding timing for some species. Yet not all migratory animals are shifting enough, which could render their populations vulnerable and susceptible to decline.
The Ketterson lab at Indiana University, where I am a graduate student, has been studying the dark-eyed junco, a small migratory sparrow, for over 30 years. The junco presents an interesting case study in climate change responses because not all individuals of this species migrate. Migratory and sedentary populations overlap in geographic range at different times of the year. What’s really interesting, is that my lab is studying how these migratory and sedentary populations of juncos are diverging in real time! These two populations differ not only in migratory behavior, but they also differ in size and coloration, and so they are considered two separate subspecies. The sedentary population starts its breeding season in early spring, in the presence of the migratory population, but the populations do not seem to interbreed, even though it is biologically possible.
I am very interested in what drives divergence within a species that varies in migratory behavior. Dark-eyed juncos are, fortunately, a bird of “least concern”, meaning they are very abundant across North America and not at risk of extinction. Their divergence in migratory behavior is characteristic of many other songbirds and other migratory organisms, making them an excellent example of both climate change response and evolution, which we can apply to other species.
One way divergence could continue between migratory and sedentary populations (even if populations shift) is if a female only breeds with familiar males from her population. Female behavior is difficult to measure and typically studied in captivity, and that approach doesn’t accurately represent how birds behave in the field. I began addressing this question at my field site, Mountain Lake Biological Station located in the Appalachian Mountains, this spring. I’ve developed a field model for use in the wild: with the help of my field technician, we placed 2 males (one each from the migratory and sedentary subspecies) in separate small cages in the area we knew a female junco lived; we played pre-recorded male songs to attract the female; and then sat quietly for long periods of time in the cold and damp, observing her preference responses, which we defined by how close she was to each male and whether she displayed for him. Sounds like fun, right?
For two weeks, we attempted to get females to respond and got little response. But as females started to reach the egg laying stage in the breeding season, we started to get responses and learned that females were more likely to respond under certain circumstances. By the end of the season, we were able to start addressing how mate preference may enable divergence to continue even as separate populations come back into contact.
This research may help answer some big questions: What does climate change mean for the survival and adaptability of migratory species in general? Are migratory populations more at risk of extinction in a changing climate, or are sedentary ones? By studying how migratory animals behave and select mates, function physiologically and respond to climate change, we will hopefully be able to contribute to protecting biodiversity.