The resting metabolic cost of egg laying and nestling feeding in great tits

To estimate the metabolic costs of regrowth of reproductive organs and formation of eggs, we compared the resting metabolic rate (RMR) of female great tits (Parus major) during the periods of ovarian recrudescence, egg laying, nestling feeding and during winter. We found RMR of individual females, as measured in an open circuit respirometer during night, to be significantly higher during all the breeding phases when compared to measurements during the winter. Females had a 12% increase in RMR during the nest-building phase, an increase of 27% during egg production and an increase of 20% during the chick feeding phase compared to RMR during winter. However, we found no significant difference in energy expenditure during the night between females producing eggs and females feeding chicks. A causal link between RMR and egg production was further confirmed by females producing large eggs having a higher RMR than females producing small eggs. Mass-specific RMR increased steadily from the winter throughout the breeding season, being highest when females were feeding their nestlings. Thus, even though females did not produce ovary-oviduct tissue or eggs during chick feeding, they had a very high RMR. We conclude that the biosynthetic cost of egg formation will probably not limit clutch size but may well, together with the cost of ovary-oviduct recrudescence, influence the timing of reproduction. We suggest that the high RMR of females feeding nestlings, probably is due to an increase in size and efficiency of the alimentary tract, needed to sustain a high rate of energy turnover during this period.