Colourful traits in females are suggested to have evolved and be maintained by sexual selection. Although several studies have evaluated this idea, support is still equivocal. Evidence has been compiled in reviews, and a handful of quantitative syntheses has explored cumulative support for the link between condition and specific colour traits in males and females. However, understanding the potential function of females' colourful traits in sexual communication has not been the primary focus of any of those previous studies. Here, using a meta-analytic approach, we find that evidence from empirical studies in birds supports the idea that colourful female ornaments are positively associated with residual mass and immune response, clutch size and male-mate preferences. Hence, colourful traits in female birds likely evolved and are maintained by sexual selection as condition-dependent signals.

High baseline glucose levels are associated with pathologies and shorter lifespan in humans, but little is known about causes and consequences of individual variation in glucose levels in other species. We tested to what extent baseline blood glucose level is a repeatable trait in adult zebra finches, and whether glucose levels were associated with age, manipulated environmental conditions during development (rearing brood size) and adulthood (foraging cost), and lifespan. We found that: (1) repeatability of glucose levels was 30%, both within and between years. (2) Having been reared in a large brood and living with higher foraging costs as adult were independently associated with higher glucose levels. Furthermore, the finding that baseline glucose was low when ambient temperature was high, and foraging costs were low, indicates that glucose is regulated at a lower level when energy turnover is low. (3) Survival probability decreased with increasing baseline glucose. We conclude that baseline glucose is an individual trait negatively associated with survival, and increases due to adverse environmental conditions during development (rearing brood size) and adulthood (foraging cost). Blood glucose may be, therefore, part of the physiological processes linking environmental conditions to lifespan.