Hylaeus anthracinus is an endemic Hawaiian yellow-faced bee (Colletidae) that is federally protected under the Endangered Species Act of 1973. On O‘ahu, H. anthracinus populations are restricted to a few isolated areas of intact coastal strand habitat. A detailed understanding of H. anthracinus nesting ecology is required to protect and expand the remaining fragmented and isolated populations. Here, we explore how nest parameters such as tunnel inside diameter, cell length, cell partition material, and plant species utilization compare among H. anthracinus and three non-native coastal bee species. Solitary bee nests from coastal habitats on O‘ahu, Hawai‘i were dissected and nest parameters were measured, compared, and described. We found overlap in most nest parameters among H. anthracinus and the introduced species: Hylaeus strenuus, Ceratina smaragdula, and Ceratina dentipes. No significant differences for inside diameter of nest entrance, length of tunnel nest, or # of cells/nest were found among the four bee species that were utilizing cavity nests. Thus, competition for nesting resources could be occurring. This increased understanding of H. anthracinus nest ecology will inform future conservation actions that could include the development of captive rearing programs, translocations, use of artificial nest sites, and the enhancement of natural habitat to increase nesting resources to support the existing populations.