In migratory bird species, juveniles normally have shorter and more rounded wings than adults. The causes of this age-specific difference in wing morphology, however, are largely unknown. Here, we used longitudinal data collected over 3 years from a Pied Flycatcher Ficedula hypoleuca population to assess whether age-related differences in wing morphology are a consequence of ontogenetic changes or of selection favouring birds with longer and more pointed wings. Our study provides evidence of ontogenetic changes in wing length and shape, whereby birds grow longer and more pointed wings as they grow older. Age-dependent variation is likely to be adaptive and may partly explain age differences in spring migration phenology and breeding success.