Global Change Biol. 14, 1959–1972 (2008)

The first robin sighting of spring may not be the best indicator of climate warming.

Abraham Miller-Rushing at Boston University and his colleagues analysed 33 years' worth of data on 32 bird species in Massachusetts. They found that the first sighting of a migratory bird, although one of the most commonly used statistics, can be confounded by population size. As bird numbers decline, there are fewer early outliers, making the first birds harder to spot. By extension, growing populations are expected to distort results in the opposite direction.

Instead, the team used the mean arrival date, which did not change with population size. Overall, the birds' mean arrival was two and a half days earlier by the end of the period covered by the data set than at the start.