Thursday, August 6, 2009 - 3:20 PM

COS 101-6: Changes in bird arrival times and cohort sizes in eastern Massachusetts (1970-2008)

Libby Ellwood and Richard Primack. Boston University


Bird species have served as effective model organisms to study the impacts of climate change. These studies indicate that bird species vary greatly in their response to climate change. What is largely unknown is how responsiveness to climate change affects the ability of species to maintain their population sizes over time. Our research presents an analysis of bird banding data that sheds lights on changes to populations of migratory birds. The Manomet Center for Conservation Sciences, in Manomet, Massachusetts, has been consistently mist netting and tagging birds since 1970. From these thirty-nine years of records we can determine dates of first, mean and last arrival, as well as population cohort size, for thirty-two species.  Initial analysis of these data examined how arrival dates have changed over time and relative to temperature.  This was then expanded in order to test if evolutionary relatedness helps to further explain variation seen in arrival dates.  A phylogenetic analysis was done utilizing these migration data in combination with mitochondrial DNA sequences, natural history traits such as breeding habitat, nesting location, feeding guild and winter range, and morphological traits including body mass and wing length. 

Mean arrival date is the most effective indicator of climate response because it is not affected by population size. Of thirty-two species, thirteen species have mean arrival dates correlated with temperature and six species are arriving significantly earlier than they were in 1970.  These results are similar to those of a previous study that made use of data from Manomet spanning a shorter time period. However, with additional years, the patterns are evident with more species and/or a greater degree of statistical confidence.  The population cohort size of twenty-one species has significantly declined since 1970.  This trend is so strong in several species that in future years they will likely not be caught at all or in numbers so reduced as to exclude them from analysis.  Contrasting a similar European study, we found no relationship between changes in how responsive species are to climate change in their arrival times and changes in cohort size. These disparities are likely due to different methodologies and different factors between the regions affecting bird populations.  Phylogenetic analysis provides further explanatory value to patterns seen in arrival dates and future analysis will elucidate relationships among species, especially changes in cohort size, based on migratory behavior.