This image comes from a remarkable new paper about the migration of wood thrushes. A team of ornithologists led by Calandra Stanley and Maggie MacPherson from Bridget Stutchbury’s lab at York University in Toronto have used tiny “light-level geolocators” to track the individual migration routes of wood thrushes. Geolocators use the daily cycle of sunlight and darkness, referenced to the time of day and date, to calculate where in the world they are. Sunrise and sunset vary consistently according to longitude (sun rises earlier in North Carolina than it does in Tennessee) and latitude (day length is longer in New York than in Belize in summer, but shorter in winter — except on the equinox when the machines get confused). Geolocators are not as accurate as GPS but they have the great advantage of being very small and lightweight. GPS needs a big, heavy antenna — there is no way that a songbird could carry even the smallest GPS unit.
The image above shows the path of one wood thrush over two years as it moves between its wintering area in Belize and its breeding grounds in Pennsylvania. The particularities of the route taken bring the map alive. The details change each year. In the fall of 2009, the bird came south over Florida and Cuba, but took the direct route across the Gulf of Mexico the next year. The map makes clear that migration is not an abstraction, but a yearly marvel.
This bird winged across Tennessee twice. That makes my heart leap — I may have heard this bird in Sewanee’s woods — but it also gives me chills. I have a freezer full of thrushes that hit windows and cars. (The dead birds are for use in the anatomy labs in my ornithology class.) We’ve thrown so many hurdles in the way of these migrating birds. To see the migration path is therefore not just to marvel, but to imagine the dangers.
A composite of the maps of multiple individuals shows the diversity of migratory paths within the species. Some birds hug the Mexican coast, some come through the Florida peninsula, and others take the dare-devil ocean crossing.
Joanna Foster’s article at the NYTimes Green blog does a great job of putting this study into the larger context of climate change and habitat loss. The important finding of this study was that the date of departure for spring migration barely varied from year to year. Individual birds, in other words, were consistent in when they left Belize. This is surprising — you’d expect them to be more sensitive to local conditions like the weather or their body condition. In the words of the authors, this lack of variability “may limit the ability of individuals to adjust migration schedules in response to climate change.” As my previous post of thrushes described, these birds have been declining for decades, so this is not good news.
Images here are from the paper: Stanley CQ, MacPherson M, Fraser KC, McKinnon EA, Stutchbury BJM (2012) Repeat Tracking of Individual Songbirds Reveals Consistent Migration Timing but Flexibility in Route. PLoS ONE 7(7): e40688. doi:10.1371/journal.pone.0040688 [Creative Commons]
For another mind-blowing set of maps from geolocators, see the Arctic Tern Migration Project website and click on “maps.”