Thermal cameras are being used increasingly in the bird-ringing community and preliminary reports suggest they have a wide range of relevant applications. However, there has been little quantitative assessment of these methods. In this study we tested the use of a thermal imager to assess the occupancy of enclosed nests, using nest-box populations of small passerines. Based on a thermal image of each nest box, observers were able to predict nest-box occupancy with a combined success rate of 85.9%. The observers were able to identify when nest boxes contained a heat source relating to an active nest but unable to differentiate between warm clutches and live broods. The age of the brood significantly influenced the predictions, reflecting the limited ability of young broods to maintain their body temperatures independently. There were no significant effects from potential confounding variables relating to solar exposure, nest-box design or ambient temperature, although these may have accounted for a small number of incorrect predictions. Overall, it was found that thermal imagers provide an effective, non-invasive and efficient method for monitoring nest-box occupancy, with inferences about suitability for other types of enclosed nest.