New Harvard Forest Publication: Impact of Climate Change on Terrestrial Over-Wintering Birds

This study investigates the impact of the increasing average winter temperatures and habitat modification on winter populations of terrestrial birds in Massachusetts, based on Christmas Bird Count (CBC) data recorded annually by volunteers for the National Audubon Society. The large archival database of records for birds' species in their winter range was used to examine whether bird species are extending their winter ranges into more northerly regions. The ratio of southern to northern for bird populations in eight CBC across within four ecologically diverse regions of Massachusetts were shown to have increased significantly from the winter of 1980/1 to 2004/5, but there was a weak correlation when the ratios were compared to average winter temperatures. Examination of the changes in land-use in the CBC areas, over the same time period showed a correlation with the area of residential use, and the length of edge between forested and developed areas, which is increasing as a result of forest fragmentation. Separation of the bird species into habitat preferences of edge, woods, and grassland, showed a preferential distribution of birds in the edge habitat. Additionally, analysis of the feeding preferences of the bird species showed a predominance of seed-eating birds. Examination of individual species that use bird feeders as a supplementary winter food resource showed they are increasing in abundance and/or expanding their winter ranges. The increasing popularity of feeding wild birds may be improving the winter survival of some species at the expense of species diversity. The results suggest that the increasing numbers of winter populations of southern species in Massachusetts are occurring in response to a complex interaction of factors that include climate change, habitat modification, and supplementary winter food resources.

Rosemary Balfour completed her Master of Liberal Arts at the Harvard Extension School with the thesis "The Impact of Changes in Average Winter Temperatures and Habitat Modification on Populations of Terrestrial Birds Over-wintering in Inland Areas of Massachusetts." David Foster, Director of Harvard Forest, served as a member on her thesis committee.