Avian Conservation and Ecology (Dec 2023)
Redistribution of wintering American Common Eiders ( Somateria mollisima dresseri )
Abstract
The American Common Eider ( Somateria mollisima dresseri ) winters along the eastern coasts of the northern U.S. and southern Canada. Various lines of evidence indicate recent localized changes in eider abundance, particularly in the Gulf of Maine, where the most dramatic oceanic changes in the Northwest Atlantic are also underway. A range-wide overview of shifts in local abundance is needed to understand the geographic extent of changes in winter distribution, however, only the Canadian portion of their wintering range has been thoroughly and repeatedly surveyed by air in the last two decades (since 2003). We used these Canadian aerial survey data in combination with data from the Christmas Bird Count (CBC), a citizen science general bird census, to generate estimates of localized changes in abundance over a multi-decadal time series across the dresseri wintering range. CBC trends from the periods 1980–1999 and 2000–2020 were derived from negative binomial regressions and indicated an overall shift from mostly stable (λ = 1) or positive (λ > 1) localized trends in earlier years in the central region of the wintering range toward negative trends (λ < 1) in the most recent 20 years. Wintering numbers in Nova Scotia and the Bay of Fundy south through Maine showed the steepest shifts, where accelerating negative trends of λ = 0.85 - 0.95 have occurred since 2000. Many CBC sites in Massachusetts also shifted to negative trends, although a small number of sites from Cape Cod south have reported high counts in recent years resulting in locally high λ. Numbers in the far southern extent of the range also grew substantially, albeit with low numbers overall. CBCs at higher latitudes, in southeast Newfoundland and the Gulf of St. Lawrence, showed mostly stable or increasing trends. CBC coverage in the Canadian portion of the range is relatively poor, however, data from multiple years of aerial surveys in Canada corroborated CBC trends. The consistent and widespread nature of the declines in local abundance throughout the entire Gulf of Maine and surrounding ecosystems, from Nova Scotia to Massachusetts, suggests a large-scale redistribution away from the center of the overwinter range. Our confirmation of suspected spatial patterns in trends of American Common Eiders further supports the need for cross-jurisdictional coordination on the prioritization of harvest management and habitat conservation efforts among regions for this species.
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