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Energy budgets were constructed for the Cliff Swallow during the nest building, incubation, and nestling periods using time budgets and aerodynamic theory. Mean daily energy expenditures during these periods were 1.55, 1.23, and 1.28 watts respectively, with required food harvest rates of at least 3.95, 4.42, and 4.07 watts. The cost of constructing an average size nest (600 g) was approximately 122 kilojoules expended over about 7 days, but the multiple use of the nest makes the cost per brood considerably less than this. The ecological advantages accruing from the nest apparently are related primarily to physical protection from predators and reduction of intraspecific aggression, rather than microclimatic conditions established within the nest.