Pleistocene lake deposits in Ulupau Crater on Oahu contain the oldest fossil bird remains known from the Hawaiian Islands. These fossils are important for documenting evolutionary rates in a variety of lineages of land and water birds from the islands. Detailed stratigraphic analysis reveals distinct cycles of deposition involving interbedded lacustrine deposits, colluvium, and paleosols. The volcanic basin was breached by marine erosion prior to the peak of the last interglaciation (marine isotope stage or ‘MIS’ 5e) over 125 ky ago, constraining the age of the lake sediments to the middle Pleistocene. The three cycles of deposition (Units III, IV, and V) appear to be closely linked with intergiacial highstands with 100 ky cyclicity. Given the complexity of the succession of crater sediments and the time required for their formation, it is probable that the lake basin filled over several interglacials, reaching a maximum extent of over 50 ha during MIS 11, about 400 ky ago. This is also the likely maximum age of the richest fossil beds in Unit IV, but MIS 9 (300 ky) also remains a possibility. Probable habitat requirements of the various elements of the avifauna provide insights into the paleoecology of the site during the middle Pleistocene.