Off peak

Abstract

Migratory birds failing to exploit changes in climate may be losing out, writes Nigel Williams. Migratory birds failing to exploit changes in climate may be losing out, writes Nigel Williams. The latest compilation by Birdlife International on the status of the world's bird populations made for much grim reading: many more species are in decline than even some pessimists had imagined. Habitat loss appears to be a major feature as agriculture intensifies and spreads. And many migrating birds also appear to be in decline, often for unknown reasons. But a new study suggests one reason why some migratory species are faring badly; they may not have made adjustments to their migration timings in the face of climate change which is seeing plant and invertebrate activity advancing in northern latitudes. These events are vital to the breeding success of many migrant species and they need to time their breeding just right: arrive late and they may miss out on the peak of food for their offspring. Anders Moller in Paris and his European colleagues Diego Rubolini and Esa Lehikoinen report in the Proceedings of the National Academy of Sciences (published online) a study of the migration timings of 100 species that head north to Europe each spring to breed. Researchers working with birds are fortunate to have a vast array of data from researchers who have monitored details of their behaviour, timings and numbers for decades. So Moller and his colleagues were able to examine the timing of spring migration in these species from data stretching back to 1960, looking specifically at data from 1970–1990 and also 1990–2000. “We predicted that migratory bird species that did not anticipate timing of spring migration should demonstrate declining populations,” they wrote, “while taking into account several variables previously shown to be associated with population trends or response to climate change.” Among variables potentially affecting the timing of response to climate change, the researchers considered migration distance, intensity of sexual selection, and total population size. They were concerned that a small population size, and thus lack of genetic variation, potentially could explain the lack of response to climate change. The researchers found, however, that European migratory species with declining breeding populations in the last decade responded the least to recent climate change. “Our findings have important implications for future evaluations of conservation status and for attempts to manage populations of declining species. If migratory bird species do not advance their timing of migration, they may arrive later relative to the phenologically optimal timing of reproduction,” they conclude. They are particularly hampered by the apparent greater ability of plants and invertebrates to advance their growth and reproduction in the face of rising temperatures. Mis-timing of reproduction results in reduced reproductive output, the researchers argue. “Therefore, we predict that, under current climate change scenarios, species with a threatened population status and declining breeding populations will suffer further losses.” “Overall, our findings indicate that factors affecting population declines of European bird species varied temporally in the second half of the twentieth century. Specifically, farmland breeding habitat, wintering in Africa, and northern breeding latitudes were associated with population declines during 1970–1990.” But the researchers found climate-driven change in spring migration timing as the only significant predictor of population trends in more recent decades. “Therefore, an important message of this study is that factors affecting population trends and extinction risk of birds appear to be dynamic over time, and this fact should be taken into account in evaluations of conservation priorities for declining species.”