Because of complex interactions among genes and the environment, individual organisms can display remarkable phenotypic variation throughout their lives. Environmental change, ontogeny, and patterns of trait covariation all contribute to within-individual phenotypic variation; taken together, these factors have been conceptualized as the "developmental norm of reaction" (DRN), a multivariate function describing phenotypic change across environments (i.e., phenotypic plasticity), through ontogeny, and as covarying traits interact. When considering the development and evolution of behavior, I argue that it is useful to consider learning as a fourth component of the DRN frame work. Not only does learning contribute to within-individual phenotypic variation, it also can interact in complex ways with the other three axes of the DRN. I investigated the role learning plays in the development of a complex and variable behavior; bat echolocation. In a field study, I demonstated that individual little brown bats, Myotis lucifugus, regularly hunted in a wide variety of habitats. Nightly, individuals foraged over a lake (open space) and within a moderately dense forest (cluttered space), and they changed the acoustical properties of their echolocation to match the physical properties of each environment. Using behavioral experiments, I found that flexible echolocation develops independent of direct experience with different habitats, and juvenile bats subsequently learn to improve calls as they gain experience hunting in various settings. The importance of learning differed across habitat treatments, indicating an interaction between learning and the environment. To investigate a possible interaction between learning and ontogeny, I studied the learning ability of adults in similar experiments. Adult bats foraged considerably less than juveniles in the experiments, making it difficult to record any echolocation calls at all. While this result may indicate an inability of adults to respond as well as juveniles to novel conditions, I argue that it represents an artifact of adults’ reduced motivation to forage following extended captivity. Nevertheless, this work demonstrated that interactions between learning and the other axes of the DRN can lead to complex patterns of phenotypic development.