Parent birds may adjust the onset of incubation to minimize periods of high risk of nest failure due to predation (the Nest Failure Hypothesis) or of mortality to adult birds (the Adult Predation Hypothesis). We examine temporal patterns of risk of nest failure and predation on adult females in a population of Green-rumped Parrotlets (Forpus passerinus) to determine whether those patterns explain observed patterns of incubation. Over one-third of nests (38.6%) failed completely, and an additional 2.2% failed after fledging one or more young. Most nests failed due to predation and infanticide. Because parrotlets begin incubation on the first egg, we examined a range of hypothetical failure rates for the period prior to the onset of incubation. Daily survival probabilities for nests were higher during the nestling stage than during incubation or fledging. Survival of adult females varied little through the nesting cycle, but was highest while attending nestlings. Model predictions were highly dependent on assumptions made about survival rates during the pre-incubation period. When empirically based values were used for this period, maximum productivity was achieved with first-egg incubation, consistent with observed patterns. Models were most sensitive to those parameters most difficult to estimate. This study represents the first test of the Nest Failure model with a nonpasserine or tropical species, and the first assessment of the Adult Predation model using field data.