Within a population there exists variation among individuals in physiological traits. There is increasing recognition that the rearing environment, including that experienced prenatally, can affect the adult phenotype and contribute to this variation. Using captive Japanese quail, we have been exploring the role that temperature, experienced either pre- or post-natally, has on an individual’s growth rates and thermal physiology. To our surprise, we have found that effects can be long lasting, and influence adult energy expenditure. Using wild birds we have been testing whether maternally-derived antibodies, transferred to offspring via egg yolk, influence nestling metabolic rate and capacity to mount an immune response. Effects are detectable at the fledgling stage, although we cannot track effects in adults. These experimental systems highlight the role that early environmental effects have on an individual’s energy expenditure however direct links with fitness remain to be shown. [more]