OOS 20-2: Are songbirds arriving earlier in Thoreau's Concord as the climate warms?
Libby Bacon, Boston University and Richard B. Primack, Boston University.
Background/Question/Methods Studies of migrating birds show that birds are arriving earlier now than they did in the past. This information has provided crucial evidence that climate change and global warming are already affecting biological communities in the United States. Widespread changes in the timing of bird migrations could also impact the success of other species within the system. Surprisingly, the observations of the first arrival times of birds to Concord, Massachusetts as recorded in the 1850s by the well known philosopher and naturalist Henry David Thoreau, have not previously been analyzed. Such an analysis could extend the investigation of bird arrival times further back in time than any previous studies published to date in the United States. In addition to Thoreau's journals, the journals of the ornithologists William Brewster (covering 1886-1919) and Ludlow Griscom (covering 1928-1954) provide further information. Observations of arrival times for the past 30 years were obtained from a bird watcher local to Concord. Collectively, a series of bird arrivals was produced spanning over one hundred fifty years from 1851 to the present, with up to 63 observations of first arrival for each species.
Results/Conclusions Twenty-four species of songbirds were studied. Many species are arriving earlier now than at the time of Thoreau, but a few species are actually arriving later. This distinction is likely due to migration distances, with shorter distance migrants generally arriving earlier and birds with longer distance migrations arriving at the same time or later over the course of the study. Temperature appears to be an important factor in the changing arrival times of these birds. These results must be interpreted with care, as these four data sets were gathered in different ways for different purposes. Also, many species of songbirds are declining in abundance, and the landscape is changing due to forest succession and habitat fragmentation, Further, Thoreau's thoughtful observations of the biology of Concord differ from the modern birder's approach to actively seeking bird sightings in established or unusual locations. Despite these qualifications, the influence of climate change on bird arrival times in Concord can be detected. In addition to their scientific values, our results should be of wide public interest due to the connection with Thoreau.