This paper shows how our knowledge of the evolution, ecology and conservation of birds can be improved through the analysis of external morphological traits. After giving a short history of morphological studies of birds, we discuss the pros and cons of such data in exploring within-species variation and describe the main patterns and hypotheses related to the factors affecting bird size and shape. We describe the usefulness of external measurements (including body mass and feather morphology) of live birds for inferring population differentiation or intraspecific variation in body condition. Bird morphology monitoring is conceptually similar to other programs aimed at recording the distribution of species and their habitats. However, it has one additional advantage: the same data used to describe variation can be used to infer the processes underlying observed changes by testing geographical or ecomorphological predictions. Morphological approaches may be implemented in the context of national ringing schemes, in which thousands of birds are measured each year. They may be particularly illustrative in bird species with populations distributed between regions of contrasting ecology, or wherever man-made environmental stressors affect bird populations.