A main cause of nestling loss of the ‘Critically Endangered’ Giant Ibis Thaumatibis gigantea appeared to be mammalian predation. Predator-exclusion devices were added to a randomly selected half of 52 nesting trees, with the other half acting as controls. The number of young fledged per nest was 50% higher for protected nests than control nests. Each protected nest produced almost two-thirds (0.63) of an extra chick. During the nestling period, the daily nestling survival rate was (99.9%) (equivalent to 90% survival over the nestling period) for protected nests and 99.3% (61.3% survival over the whole period) for control nests. Nest protection devices cost US$5 per tree. Each extra nestling fledged as a result of this intervention therefore cost $8. If it was also necessary to pay to locate the nests, then the cost of protecting each nest would be $140 and the cost of producing each extra chick $224. Trees with protectors added were significantly more likely to be reused in the next year than unprotected trees.