1. Life-history decisions are strongly affected by environmental conditions. In birds, incubation is energetically expensive and affected significantly by ambient temperature. We reduced energetic constraints for female tree swallows (Tachycineta bicolor) by experimentally heating nests during incubation by an average of 6.9 degrees C to test for changes in incubation behaviour. 2. Females in heated boxes (hereafter 'heated females') increased time spent incubating and maintained higher on-bout and off-bout egg temperatures. This indicates that female energetic constraints, not maximizing developmental conditions of offspring, determine incubation investment. Furthermore, this result suggests that embryonic developmental conditions in unmanipulated nests are suboptimal. 3. We found individual variation in how females responded to experimental heating. Early-laying (i.e. higher phenotypic quality) females with heated nests increased egg temperatures and maintained incubation constancy, while later-laying (lower quality) heated females increased incubation constancy. Changes in egg temperature were due to changes in female behaviour and not due directly to increases in internal nest-box temperatures. 4. Behaviour during the incubation period affected hatching asynchrony. Decreased variation in egg temperature led to lower levels of hatching asynchrony, which was also generally lower in heated nests. 5. Our study finds strong support for the prediction that intermittent incubators set their incubation investment at levels dictated by energetic constraints. Furthermore, females incubating in heated boxes allocated conserved energy primarily to increased egg temperature and increased incubation attentiveness. These results indicate that studies investigating the role of energetics in driving reproductive investment in intermittent incubators should consider egg temperature and individual variation more explicitly.