The magnitude of the impacts of human activities on global biodiversity has been documented at several organizational levels. However, although there have been numerous studies of the effects of local–scale changes in land use (e.g. logging) on the abundance of groups of organisms, broader continental or global–scale analyses addressing the same basic issues remain largely wanting. None the less, changing patterns of land use, associated with the appropriation of increasing proportions of net primary productivity by the human population, seem likely not simply to have reduced the diversity of life, but also to have reduced the carrying capacity of the environment in terms of the numbers of other organisms that it can sustain. Here, we estimate the size of the existing global breeding bird population, and then make a first approximation as to how much this has been modified as a consequence of land–use changes wrought by human activities. Summing numbers across different land–use classes gives a best current estimate of a global population of less than 100 billion breeding bird individuals. Applying the same methodology to estimates of original land–use distributions suggests that conservatively this may represent a loss of between a fifth and a quarter of pre–agricultural bird numbers. This loss is shared across a range of temperate and tropical land–use types.