Faced with the advancement of spring due to climate change, avian species must adjust their phenological timings in response. However, as the mechanisms underlying organisms’ phenology widely vary, responses to altered ecological conditions may differ, which can cause a phenological mismatch resulting in population declines in migratory species. To better understand the mechanisms and effects of these mismatches, I studied the timing of breeding in a long-distance migratory songbird, the purple martin (Progne subis), and compared it to the timing of its key resources. I first investigated whether mistiming between purple martin breeding stages and insect emergence negatively affected reproductive success. I found direct evidence for the impact of phenological mismatch on martins’ reproductive success, where greater misalignment between peak energetic demand of the nest and peak prey availability resulted in lower fledge success. Next, I investigated whether there were fine-scale differences in the environmental phenology of local breeding sites, and if so, were birds able to align the timing of nesting with this variation. I found that while peak insect variability varied widely (0-49 days) at the micro-habitat scale, nest timing did not, suggesting that birds were misaligned with available resources at this scale. Overall, my results show that phenological mismatches negatively affect migratory birds’ reproductive success, and that the lack of synchronization with microhabitat variation may indicate that temperature—shown to influence the timing of egg laying—does not necessarily translate to birds' synchronization with resources at a micro-habitat scale. This rare evidence of direct effects of a mismatch of migratory songbird timing on fitness can help us better understand causes of population declines in purple martin and other aerial insectivores. Future studies should further investigate the mechanisms driving the timing of breeding, such as individual quality and carry-over effects of migration, as well as the population-level consequences of phenological mismatch in martins and other aerial insectivores.