Abstract
Using an 18-year dataset of arrival dates of 65 species of Maine migratory breeding birds, I take a deeper view of the data to ask questions about the shapes of the distribution. For each year, most species show a consistent right-skewed pattern of distribution, suggesting that selection is stronger against individuals that arrive too early compared to those that arrive later. Distributions are consistently leptokurtic, indicating a narrow window of optimal arrival dates. Species that arrive earlier in the spring show higher skewness and kurtosis values. Nectarivorous species showed more pronounced skewness. Wintering area did not explain patterns of skewness or kurtosis. Deviations from average temperatures and the North Atlantic Oscillation index explained little variation in skewness and kurtosis. When arrival date distributions are broken down into different medians (e.g., 5% median and 75% median), stronger correlations emerge for portions of the distribution that are adjacent, suggesting species fine-tune the progress of their migration. Interspecific correlations for birds arriving around the same time are stronger for earliest migrants (the 25% median) compared to the true median and the 75% median.
Highlights
Abundant evidence for global climate change exists in monotonic increases in carbon dioxide concentrations, melting of polar ice caps with concomitant sea level rise, and record hot temperatures around the world [1±4]
Over the eighteen-year period of the study, arrival dates are associated with fluctuations in mean springtime temperature as well as by the strength of the North Atlantic Oscillation (NAO) [19,20]
I have data on 145 species of migratory breeding birds, I restrict this study to an analysis of each of the 65 species that have a minimum of 20 arrival dates reported for each year of the study
Summary
Abundant evidence for global climate change exists in monotonic increases in carbon dioxide concentrations, melting of polar ice caps with concomitant sea level rise, and record hot temperatures around the world [1±4]. Birds are unique barometers of climate change because of the rich database on migration contributed in large part by lay citizens [5,6,12,13]. Recent reviews of the large quantity of data leave little doubt of a fundamental impact of climate change on bird migration [14±16]. Over the past 18 years, I have been coordinating a citizen-science effort to document the first arrival dates of Maine migratory breeding birds across the state [18±20]. Previous comparisons of these modern data to data taken over a century ago from the Journal of the Maine Ornithological Society (published from 1899 until 1911) provide mixed evidence of earlier contemporary arrivals for some but not all species [21]. Over the eighteen-year period of the study, arrival dates are associated with fluctuations in mean springtime temperature as well as by the strength of the North Atlantic Oscillation (NAO) [19,20]
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