Oikos 10.1111/j.1600-0706.2009.18349.x (2010)

Credit: ROBERT S. MULVIHILL/POWDERMILL NATURE RESERVE

Birds migrating throughout the United States are shrinking in size as temperatures rise, say scientists.

Between 1961 and 2006, Josh van Buskirk of the University of Zurich in Switzerland and colleagues collected data on body mass, body fat and wing chord — the distance from a bird's wrist to the tip of its longest primary feather — from almost half a million birds of over 100 species captured at a field station in western Pennsylvania. Over the 46-year study period, birds arriving at the banding station steadily became lighter in weight and had shorter wings. Buskirk and colleagues found widespread declines in the body mass of birds during all four seasons and in wing length during spring and autumn. Sixty of 83 spring migrant species shrunk in size, having smaller wings and weighing less. Of 75 autumn migrant species, 66 became leaner over time and 52 developed shorter wings.

Although the changes in body size were statistically significant, they were relatively small: on average, mass declined 3.6 per cent over 46 years. The birds are therefore unlikely to be harmed by the drop in their body sizes, say the authors.