Migration Plasticity as an Adaptation to Climate Change: The Spatial Distribution and Abundance of a Subset of Neotropical Migrant Landbirds Wintering in the Northeastern United States

There is a need for accurate predictions of the effects of climate change on wildlife populations. Bioclimatic relationships however are potentially complicated by various environmental factors operating at multiple spatial and temporal scales. Therefore, in order to test the hypothesis that climate constraints of winter bird distributions are modified by species-specific responses to weather and climate, I relied on Christmas Bird Count data (CBC). With nearly 100 years of data, the CBC is a valuable source of information on historic and recent changes in the status and distribution of birds during the early winter period in the United States and Canada. The first chapter is a regional study that asked whether seasonal weather fluctuations and/or a warming climate indicative of the supposed increase in December presence of seven neotropical migrant landbirds near the northern edge of their winter ranges. CBC data and historical weather data from the U.S. Historical Climatology Network (USHCN) were analyzed to confirm the northward shift in average abundance over time among species near the northern limits of their winter ranges. I determined whether there was a strong temporal autocorrelation and/or linear relationship of December bird abundance consistent across species near the northern limits of their ranges and found strong evidence in support of the hypothesis that the migratory behavior of many bird populations has recently changed in response to climate warming. In addition, the effects of localized, short-term weather variations due to climate change proved to be just as, if not more important as the long-term global warming trend on the distribution of these wintering bird populations. In the second chapter, Christmas Bird Count-style counts conducted in November, December and February on Block Island, Rhode Island across eighteen winters (1995 - 2013) were used to describe long-term trends in the species abundance and diversity of 103 species of landbirds on Block Island, as well as subsets of species defined by migratory status and expected geographic distribution. The objectives were to determine what the eighteen-year trends in composition and occurrence of landbirds on Block Island across the winter season were; establish whether there was any evidence of changes in phenology and migration strategy in the migratory species as witnessed by increasing numbers in abundance and diversity in November and December; ascertain if the data supported evidence of multiple species shifting their non-breeding ranges to higher latitudes; and figure out what the long-term trends in the actual landbird winter populations on Block Island can tell us about fluctuations in local weather patterns leading to range shifts and demographic changes in a number of migratory species. The third chapter, focused on the observed range shifts and demographic changes in a subset of migratory landbirds referred to as Half-Hardies. This group of species increased on Block Island between November and December, although southern New England is at, or near, the northern limit of their normal winter distributions and stereotyped southbound migration is typically expected to have been completed well before mid-November. In addition, the body measurements of, and hydrogen isotope concentrations of the feathers of two half-hardy species wintering within Sandy Hook National Park in New Jersey, provided evidence of facultative, post-migratory movements among this subset of landbird species. I confidently concluded that many of the Half-Hardies present in the Northeast during the winter months are not "passive victims", i.e. simply injured, diseased, or otherwise unfit individuals but are in fact adaptively exploiting a new resource and are benefitting by avoiding the cost of long distance migration while gaining an advantage by remaining closer to the breeding grounds. In addition, my results support the distinction between late migrants and half-hardy wintering species in northeastern North America while disproving the notion that the early-winter timing of CBCs exposes them to the presence of lingering southbound migrants. The fourth and last chapter took a look at some of the other criticisms of the Christmas Bird Count data. I described the correlations and linear relationships between warmer temperatures and lighter winds with the number of observers and foot miles covered; and the total number of landbirds observed. Although there was a strong correlation between wind speed on count day and the number of birds observed on that count, the loss of detections, i.e. the inability to detect birds at longer distances as the wind increases, was not significantly relevant, and the difference in the variations tended to even out. My results support that statistical analyses of CBC data are a powerful tool for the study of birds on a local level, and concludes that the CBC is a valuable source of information on historic and recent changes in the status and distribution of birds during the early winter period in the United States and Canada.

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