Phenotypic development is shaped by the interaction between an organism's genetics and its environment, including indirect and delayed environmental effects on its parents. Environmental conditions can constrain development or prepare offspring for future environmental challenges. Understanding whether these responses are adaptive requires clear hypotheses and experimental testing. This review explores how different paradigms in this field relate, their predictions, and the data needed to distinguish them. It focuses on birds, though the theories are not taxon-specific. Environmental effects on phenotypic development are likely mediated by endocrine systems, with evidence from avian studies highlighting areas where key information is lacking. Environmental effects can act directly or indirectly, through maternal influences. For example, poor maternal condition can lead to low-quality offspring, while environmental cues during development may shape phenotypes to better match future conditions. The 'thrifty phenotype' hypothesis suggests that early poor nutrition leads to physiological adaptations that are beneficial in resource-poor environments but detrimental in resource-rich ones. Conversely, the 'silver spoon' effect indicates that individuals born in good conditions have higher fitness in good environments. These models differ in their predictions about how developmental and adult environments interact to influence fitness. Studies in birds show that early developmental conditions can have long-term effects on adult fitness, including breeding success and survival. Manipulations of early nutrition and environment in birds have revealed effects on adult size, immune function, and cognitive abilities. However, distinguishing between direct and indirect environmental effects remains challenging. Hormonal and endocrine mechanisms are likely key to understanding these effects, though research is limited in birds compared to other species. The review emphasizes the need for more experimental studies to clarify the interactions between early and adult environments, the role of hormones, and the mechanisms underlying phenotypic plasticity. It also highlights the importance of considering sex-specific differences and the need for interdisciplinary collaboration to advance understanding of how environmental changes affect phenotypic development and fitness.Phenotypic development is shaped by the interaction between an organism's genetics and its environment, including indirect and delayed environmental effects on its parents. Environmental conditions can constrain development or prepare offspring for future environmental challenges. Understanding whether these responses are adaptive requires clear hypotheses and experimental testing. This review explores how different paradigms in this field relate, their predictions, and the data needed to distinguish them. It focuses on birds, though the theories are not taxon-specific. Environmental effects on phenotypic development are likely mediated by endocrine systems, with evidence from avian studies highlighting areas where key information is lacking. Environmental effects can act directly or indirectly, through maternal influences. For example, poor maternal condition can lead to low-quality offspring, while environmental cues during development may shape phenotypes to better match future conditions. The 'thrifty phenotype' hypothesis suggests that early poor nutrition leads to physiological adaptations that are beneficial in resource-poor environments but detrimental in resource-rich ones. Conversely, the 'silver spoon' effect indicates that individuals born in good conditions have higher fitness in good environments. These models differ in their predictions about how developmental and adult environments interact to influence fitness. Studies in birds show that early developmental conditions can have long-term effects on adult fitness, including breeding success and survival. Manipulations of early nutrition and environment in birds have revealed effects on adult size, immune function, and cognitive abilities. However, distinguishing between direct and indirect environmental effects remains challenging. Hormonal and endocrine mechanisms are likely key to understanding these effects, though research is limited in birds compared to other species. The review emphasizes the need for more experimental studies to clarify the interactions between early and adult environments, the role of hormones, and the mechanisms underlying phenotypic plasticity. It also highlights the importance of considering sex-specific differences and the need for interdisciplinary collaboration to advance understanding of how environmental changes affect phenotypic development and fitness.