foraging success while in a flock was about twice that for solitary birds. A consequence of feeding in a flock is that foraging sites may be restricted to avoid competition with other species. In a Swedish study, coal tits and goldcrests foraged in the outer foliage, while the larger willow and crested tits used the inner canopy. In sites where the numbers of willow and crested tits was artificially reduced, goldcrests and coal tits extended their foraging to include the inner canopy, but did not do so where the larger tits were retained. In some areas, wintering birds have developed the habit of coming to feeding stations and bird tables to take fat, sometimes with warblers such as the common chiffchaff and blackcap. Winter survival Several small passerine species survive freezing winter nights by inducing a lower metabolic rate and hypothermia, of a maximum of below normal body temperature, in order to reduce energy consumption overnight. However, in freezing conditions, it may be that for very small birds, including the tiny goldcrest, the energy economies of induced hypothermia may be insufficient to counterbalance the negative effects of hypothermia including the energy required to raise body temperature back to normal at dawn. Observations of five well-fed birds suggest that they maintain normal body temperatures during cold nights by metabolising fat laid down during the day, and that they actually use behavioural thermoregulation strategies, such as collective roosting in dense foliage or snow holes to survive winter nights. Two birds roosting together reduce their heat loss by a quarter, and three birds by a third. During an 18‑hour winter night, with temperatures as low as in the north of its range, goldcrests huddled together can each burn off fat equivalent to 20% of body weight to keep warm. Migrating birds rely largely on stored fat and they also metabolise protein as a supplementary source of energy. Those with a relatively large amount of fat, may make stops during migration of only 1–2 days; although they have lost weight since commencing their journey, they have enough energy reserves to reach the wintering areas. The proportion of migrating males increases as they travel south through Europe. There is competition within the species even during migration, and the larger and more aggressive males may get more food. Their death rate is therefore lower than that of the females both on the southward migration, and in