Measurement of survival in nestling ciconiiforms is difficult due to dense vegetation, cryptically plumaged young, and unpredictable movements of mobile nestlings in treetops. Thus, we used posturesensing radio transmitters to measure survival of nestling tricolored herons (Egretta tricolor) during the 40- to 50-day period when young are mobile in the colony but dependent upon parental feedings; we used the known survival of birds marked with radio transmitters to assess the use of other techniques to estimate survival. We found nestling mortality rates nearly 3 times as high in the post-mobile stage as during the pre-mobile stage. Nestling censuses that used unique color-band combinations along a marked nest transect became unreliable after 21 days of age. Similarly, censuses of young in congregation and loafing areas within the colony resulted in large underestimates of nestling survival. Poor detectability of carcasses on transect walks (33-50%) precluded use of carcass counts to estimate chick mortality directly. We conclude that telemetry is probably the only reliable method for the accurate measurement of nestling survival in most colony situations.