Although dense conifer growth can provide shelter for roosting at night, losses in hard winters can be heavy. In a Finnish study, only one-tenth of the wintering goldcrest population, which mainly fed on spiders, survived to spring. Each group roamed within a defined winter territory, and their winter survival depended on the density of the food supply. For these northern birds there is a trade off between staying put and risking starvation, or facing the perils of migration. Even in somewhat milder regions, where over-wintering is normal, exceptionally cold winters can cause such heavy losses that breeding populations take several years to recover. In 1930, the English ornithologist Thomas Coward wrote: