Understanding statistical power of monitoring to detect changes in wildlife populations is essential for allocating monitoring effort and scarce conservation resources. We evaluated the statistical power of monitoring to detect declines in occupancy of forest birds on Christmas Island over the next 10 years. We fitted dynamic occupancy-detection models to repeat detection data collected over three seasons to estimate detection probabilities for four species of concern: Imperial pigeon (Ducula whartoni), Christmas Island white eye (Zosterops natalis); Christmas Island thrush (Turdus poliocephalus erythropleurus); and Christmas Island emerald dove (Chalcophaps indica natalis). Our study demonstrates how data collected during the early stages of monitoring can be used to fine-tune design decisions so that monitoring has the greatest chance at meeting its objectives.