Island sheep may help resolve a fierce debate in ecology about population crashes. British researchers are using sheep populations on St Kilda, a remote archipelago off the west coast of Scotland, to check the relative importance of climate and local factors such as the availability of food. Ecologists usually do this by comparing populations living near each other. But animals migrating from one population to the other can confound results. Now Brian Grenell of the University of Cambridge, UK, and his colleagues have analysed more than 40 years of sheep population data from the islands of Boreray and Hirta, between which migration is impossible. "Sheep don't swim," Grenell explains. The researchers have reported that the number of sheep on two islands change in synchrony, suggesting that the weather has been a very significant factor.