Over the past 50 years, a range of environmental stressors has resulted in aerial insectivores throughout North America experiencing sharp population declines. Vaux's swifts (Chaetura vauxi) are small, long-distance migratory birds that are currently experiencing range-wide population declines, especially at the their northern range limit. Many studies attribute the Vaux’s swift’s population decline to a loss in nesting habitat; however, other potentially exacerbating factors, such as changes in insect composition and diet, remain unstudied. Here, we examine a ∼26 year dietary archive of Vaux's swift guano to ask whether diet composition has changed over time. Vaux's swifts roost communally at the same roost sites each year when migrating, often within decommissioned brick chimneys. As a result, guano accumulates at the base of these roost sites, providing a chronostratified snapshot of the bird's historic diet. We obtained a vertical core sample from a Vaux's swift guano deposit in a chimney on Vancouver Island, BC, Canada. We symmetrically stratified the guano and assessed diet composition by visually analyzing egested arthropod exoskeletons, identifying them to order, and measuring δ¹⁵N signatures at each layer. Our 26-year data set revealed an increase in the ratio of Hemiptera to Coleoptera corresponding with an increase in δ¹⁵N, suggesting a possible decline in diet quality through time because Hemipterans tend towards higher trophic status than Coleopterans, but are of less caloric value per capita. In addition, δ^l5N was significantly negatively correlated with an annual population index (Breeding Bird Survey). We suggest a reduction in diet quality may be contributing to the decline of Vaux’s swift populations, and could be contributing to more declines in other aerial insectivore species.