The timing of primary‐feather development in relation to growth in body‐mass varies considerably between altricial species. This variation relates, at least in part, to differences in nesting habits: species nesting in relatively well‐protected nest sites, such as holes, start primary‐feather development later than altricial species using open‐cup nests. For the latter ‘open‐nesting’ species, the timing of primary‐feather development also tends to show an allometric correlation with adult body‐size. Since primary‐feather development takes longer, rerative to growth in body‐mass, in small species than in large ones, these data suggest that, for small species, there will be an important selective advantage in using relat