Individually variable energy management during egg production is repeatable across breeding attempts.

It is axiomatic that whole-animal metabolism, measured for example as daily energy expenditure (DEE), plays a central role in determining reproductive success and survival (fitness) in all organisms. Nevertheless, strong evidence for consistent systematic relationships between DEE and either individual traits (age, sex, body size), environmental factors (e.g. food availability, temperature) or 'fitness' traits (e.g. number of offspring, survival) remains far from compelling in birds and mammals. Recently, we suggested that female birds might utilise complex, individually variable energy management strategies to meet the metabolic demands of reproduction, generating a wide spectrum of effects on reproductive DEE, from overcompensation (net decrease in DEE) to additive effects (net increase in DEE). Here we show that this individually variable adjustment or 'plasticity' in energy expenditure associated with egg production is repeatable among individuals between successive breeding attempts in female zebra finches (Taeniopygia guttata). Our study highlights the importance (a) of measuring 'plasticity' or change associated with transitions of physiological state (e.g. non-breeding to breeding) based on multiple measurements of the same individual, and (b) of extending consideration of how selection might drive the evolution of phenotypic plasticity per se to include physiological and metabolic traits.