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

To estimate the metabolic costs of regrowth of reproductive organs and form ation 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, a s measured in an open circuit respirometer during night, to be significantl y higher during all the breeding phases when compared to measurements durin g the winter. Females had a 12% increase in RMR during the nest-building ph ase, 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 n o significant difference in energy expenditure during the night between fem ales producing eggs and females feeding chicks. A causal link between RMR a nd egg production was further confirmed by females producing large eggs hav ing a higher RMR than females producing small eggs. Mass-specific RMR incre ased steadily from the winter throughout the breeding season, being highest when females were feeding their nestlings. Thus, even though females did n ot produce ovary-oviduct tissue or eggs during chick feeding, they had a ve ry high RMR. We conclude that the biosynthetic cost of egg formation will p robably not limit clutch size but may well, together with the cost of ovary -oviduct recrudescence, influence the timing of reproduction. We suggest th at the high RMR of females feeding nestlings, probably is due to an increas e in size and efficiency of the alimentary tract, needed to sustain a high rate of energy turnover during this period.