Special energetic adaptations are of great evolutionary significance for birds that encounter transient problems in finding food during the breeding season. House martins, as aerial insectivores, encounter such problems during spells of bad weather, when they must survive on body reserves. This species employs the following behavioural and physiological adaptations to save energy: Low basal metabolic rate (only 43% of the values predicted by allometric equations); low thermal conductance 51% (day) and 67% (night) of the predicted values; clustering behaviour; high tolerance of the young to periods of low food supply; and the ability to become torpid, found in adults and young from the age of 11 days on. House martins are the first passerine birds in which torpor has been found. These adaptations might have played a role in the great success of the house martin, one of the 10-15 most abundant bird species in Europe.