Nest-site reduction has played a significant role in the decline of Barn Owl Tyto alba populations throughout Europe and North America. Techniques of nest-site augmentation, involving the provision of nest-boxes, have been widely used in a range of species of conservation concern, including falcons, eagles, parrots, owls and cavity-nesting ducks. A common method of Barn Owl conservation is the placement of nest-boxes on church towers. Despite the usefulness of nest-boxes, several studies have shown that there may be associated disadvantages and that nest-boxes may even act as ‘ecological traps’. The purpose of this research was to compare the survival rate of owlets hatched in nest-boxes with those hatched in the more “natural” environment of church towers. Survival time analysis elucidated that owlets developing in nest-boxes had significantly lower survival than those hatched in church towers. This difference was most obvious after the parent-dependent period of the life history. Surprisingly, the length of time from hatching to the onset of winter had no effect on the survival of the owlets, even though the accumulation of sufficient body reserves and acquisition of hunting experience are thought to be important in determining survival during the critical first winter of life. We propose possible causes for the negative effects of nest-boxes and recommend some modifications to the priorities of Barn Owl Action Plans, e.g. partial reopening of buildings instead of nest-box installation. This paper emphasizes the importance of considering revision of Species Action Plans in the case of other endangered species where conservation is based on nest-site supplement (e.g. hornbills, cavity-nesting seabirds, European Roller Coracias garrulus, Little Owl Athene noctua, Saker Falcon Falco cherrug, White Stork Ciconia ciconia).