by Cortland Henderson
Several studies of bird range shifts have found northward shifts in wintering distributions. However, these studies have been limited by studying the limits of bird migrations, rather than distributions of whole ranges or populations. Research on European butterfly species has found southern boundary retraction in addition to northern boundary expansion, which calls for further examination of whole bird ranges in European flyways. Using survey data from nine countries over a three-decade period, Lehikoinen et al. (2013) track the centers of gravity of three common waterbird species in Europe to determine spatial shifts of entire ranges. They found that a 3.8 °C increase in early winter temperatures in northeastern sites of the European flyway has been linked to north-eastwards shifts in the entire wintering range of all three waterbird species. In addition to overall shifts, they detected that northern boundaries of bird ranges experienced higher rates of expansion than the rates of retraction of southern boundaries. For the first time at the larger flyway level, wintering distribution changes have been linked to rising temperatures.
Lehikoinen et al. (2013) expanded on previous models of bird wintering range expansion by observing entire populations at the migration flyway level. No research encompassing entire flyways has been conducted prior this study because estimating population sizes and trends of widespread species over large geographical areas is difficult. Despite the challenge, it is important to study population shifts at the largest scale possible to reduce error from smaller distributional shifts that occur due to weather conditions. Climate and bird frequency data from 1980–2010 were compiled from nine countries over a climatic gradient about 3000 km long. Two-month temperature averages were obtained from the Finnish Meteorological Institute for the months directly before wintering seasons. To reduce gaps in bird frequency data, data were consolidated from the International Waterbird Census (IWC) and the Finnish winter bird count conducted every January. Only three waterbirds were studied in the North-West European flyway because all other species either wintered offshore or did not have enough survey data available. The three species observed were the goosander (Mergus merganser), the common goldeneye (Bucephala clangula) and the tufted duck (Aythya fuligula).
Over the 30-year study period, the average early winter temperature of the northeastern part of the flyway increased by 3.8 °C. During this period, wintering distributions of waterbirds increased at the northeastern edge of the flyway and simultaneously decreased in their southwestern edge distributions. Most changes did not become pronounced until the mid-1990s in which the northern boundaries experienced exponential increases in populations and the southern boundaries underwent smaller, but significant, decreases in abundance. Population sizes for all three waterbird species remained relatively stable during the time period, suggesting that populations have shifted, rather than increased, as a result of climate change.
Although climate change has been shown to affect species migrations, this study shows, for the first time at the flyway level, that wintering distributions of waterbirds have shifted rapidly northeastward as a result of changes in temperature. At the edges of the duck wintering ranges, the numbers were strongly positively linked to the wintering numbers the year before in the same region, suggesting that large parts of the waterbird populations likely tend to return to previous wintering locations on large spatial scales. It will become important in upcoming decades to reassess conservation locations frequently to protect water populations from hunters in northern sites.
Lehikoinen, A., Jaatinen, K., Vähätalo, A.V., Clausen, P., Crowe, O., Deceuninck, B., Hearn, R., Holt, C.A., Hornman, M., Keller, V., Nilsson, L., Langendoen, T., Tománková, I., Wahl, J., Fox, A.D. 2013. Rapid climate driven shifts in wintering distributions of three common waterbird species. Global Change Biology DOI 10.1111/gcb.12200. http://www.researchgate.net/publication/236061430_Rapid_climate_driven_shifts_in_wintering_distributions_of_three_common_waterbird_species/file/9c96051b72eff8c183.pdf