The causes and consequences of multi-brooding were assessed in an intensive nest monitoring study of the Eurasian reed warbler Acrocephalus scirpaceus, a species for which there is both evidence for earlier breeding and an extended season duration. Greater invertebrate availability was found to predict an increase in the probability of double brooding and to reduce the interval between broods. Given that invertebrate availability is likely to have increased as a result of warming temperatures, the relationship between invertebrate availability and double brooding supports the idea of an increased propensity to multi-brood driving extended breeding seasons. Weather conditions also influenced both the incidence of double brooding and inter-brood intervals. The relative value of nesting attempts throughout the whole breeding season was assessed which illustrated that later nesting attempts are of lower reproductive value. Assessment of the potential costs of extending the breeding season with late nesting attempts, however, revealed no evidence for any costs of extending the season for individuals. These results suggest that extending the breeding season at the individual level is a low value, low cost strategy. Relative parental investment, in the provisioning of nestlings, was considered as an additional predictor of multi-brooding, but there was no evidence of an effect. Provisioning rates also did not appear to vary substantially across the breeding season or between sexes. The potential for counting singing birds as a method for monitoring breeding season length was assessed by testing the relationship between the number of singing birds and known neststhroughout the season and a positive relationship between the number of singing birds and the number of nesting attempts in the early stage of the nesting cycle was found. Season-long censuses of singing birds may therefore offer a low intensity method for estimating breeding season length; a demographic parameter which currently is not well monitored at large spatial scales.