Our understanding of the linkages between developmental environment and emergent phenotypes in non-avian reptiles has grown immensely since the discovery of temperature-based sex determination half a century ago, yet little is known how developmental temperature affects avian phenotypes despite their shared evolutionary history. In birds, the sum of knowledge on this topic is restricted to just two precocial families, Megapodidae and Anatidae, where data suggests developmental temperature can have significant effects on avian phenotypes, with important implications for future survival and fitness. In these four chapters, I provide the theoretical justification for future studies on developmental plasticity based on previous experiments and provide an analytical framework for study. In addition, I use both experimental and long-term datasets to analyze the effects of costly developmental environments in a model Passerine bird, the Tree Swallow (Tachycineta bicolor). Emerging research is beginning to illustrate the developmental environment in birds has the potential to expand our understanding of what drives fitness, survival, and the evolution of life history traits in birds.