Human exploitation and disturbance often threaten nesting wildlife. Nest guarding, a technique that employs local people to prevent such interference, is being applied to an increasing number of species and sites, particularly in South-East Asia. Although research has begun to assess the cost-effectiveness of nest guarding, case–control studies are rare and the circumstances in which the schemes are most useful remain unclear. We experimentally tested the effect of nest guarding for the critically endangered white-shouldered ibis (Pseudibis davisoni), a species exploited opportunistically for food and now largely confined to dry forests in Cambodia. We randomly applied guarded and unguarded (control) treatments to 24 and 25 nests, respectively, at a single site over 2 years. Nest guarding had no detectable effect on nest success, with an overall probability of nest success of 0.63–0.86 at guarded and 0.55–0.82 at unguarded nests. Nest monitoring across 4 study sites over 3 breeding seasons found a combination of natural predation, weather, and anthropogenic activities (robbery and vandalism) responsible for nest failure, although causes of failure remained unknown at 58% of nests. Nest guarding itself increased nest destruction at 1 site, indicating that this intervention needs cautious implementation if only a small proportion of the local community gains benefit. Comparison with other studies suggests that nest guarding effectiveness may be context-specific and differ between species that are exploited opportunistically, such as white-shouldered ibis, and those routinely targeted for trade. © 2013 The Wildlife Society.