The timing of the chick-rearing phase is known to have a profound effect on the reproductive success of birds. However, little is known about the energetic costs faced by the parents during different periods of the breeding season. These costs may have vital consequences for both their survival and future reproduction. In most studies, daily energy expenditure (DEE) of breeding and non-breeding birds has been compared, without controlling for the effect of season. In the present study, we examined the energy demands of breeding compared to non-breeding Palestine sunbirds Nectarinia osea and whether there were sex-specific differences in DEE within and between different seasons. We predicted that DEE would be elevated when birds rear chicks, especially at cooler ambient temperatures. Time-energy budgets were constructed for pairs of sunbirds, rearing chicks, or not breeding, in spring and summer. There were significant seasonal differences in estimates of DEE in non-breeders that were 21% higher in spring than in summer. We attributed these to increases in non-flight metabolic rate rather than changes in time spent on different activities. Our estimates of DEE for the birds that were rearing chicks were higher than non-breeding adults. In females the increase in DEE when breeding, compared to when not breeding, was similar in both spring and summer, while males increased their DEE much less when breeding in spring. The differences in estimated DEE, however, were not significant between male and female birds in any season. Between seasons, female breeders had 17.1% higher DEE in spring than in summer, while male breeders showed no difference in DEE when rearing chicks in different seasons. Accordingly, our initial prediction was supported, as DEE in chick-rearing adults was higher than in non-breeding adults. In addition, although temperatures are lower in spring, breeding in the spring is only more costly than breeding in summer for females. Apparently, males are more flexible in reallocating their time and energy spent on different activities.