The abundance of Tachycineta bicolor (Tree Swallow) has fallen by half across North America over the past 50 years. This study presents 38 years of observations on their nesting success from coastal Maine. We document long-term declines in nest-box occupancy and fledging success. We show that nest-box occupancy was affected by proximity to other nest boxes and to buildings, and that it increased with time since mowing. The number of young fledged per nest, on the other hand, decreased in wet years and years with many cold days, and it increased with time since mowing. These local factors do not, however, explain the long-term decline in nesting success, which we tentatively attribute to anthropogenic effects on the wintering grounds or along the migration route.