P>1. Avian embryos depend on the incubating parent to provide a thermal environment suitable for embryogenesis, but as the maintenance of optimal incubation temperatures is energetically costly, an incubating bird often must trade off embryonic investment against self-maintenance. 2. We manipulated the energetic cost of incubation in female zebra finches (Taeniopygia guttata Vieillot) by varying ambient temperature and clutch size during nocturnal incubation and recorded the corresponding effects on incubation metabolic rate and clutch temperature. 3. Females increased their night-time incubation metabolic rate more than twofold when incubating at 10 degrees C compared to when incubating close to thermoneutrality (28 degrees C). Furthermore, clutch enlargement caused females to elevate their metabolic rate with 2 center dot 8% per additional egg added to the clutch. 4. However, despite spending more energy, females did not fully cover the increased costs of incubation, because clutch temperature decreased with decreasing ambient temperature and increasing clutch size. 5. These findings suggest that parental investment in incubation can be energetically constrained and sometimes result in clutch temperatures below the optimal level for embryonic development, at least during nocturnal incubation.