Once a pair bond is formed, both sexes defend the nest site intensely. Purple Martins nest in colonies, but members of the breeding colony are not related. As with many native cavity-nesting species, Purple Martins compete with House Sparrows and European Starlings for nest sites. In many regions, such as the eastern United States, martins are extremely dependent on human-provided nest boxes. In the West, however, Purple Martins nest more frequently in natural sites, such as abandoned woodpecker holes, cliff crevices, and in the cavities of oak, sycamore, ponderosa pine, and spruce trees.
Nest building starts about a month before the first egg is laid.
Initially, pairs may begin to build in more than one cavity. But eventually one cavity is chosen, and the male and female build a nest made of grass, stems, twigs, straw, bark, leaves, and mud. The nest cup is lined with fine grasses and green leaves. Fresh green leaves are brought regularly until the eggs hatch. The nest also has a mud or dirt rim that may prevent the eggs from rolling out.