Developmental stability is most accurately assessed by measuring the small asymmetries between left and right elements of bilateral traits, i.e. fluctuating asymmetry. Although there has been much recent interest in fluctuating asymmetry, as low asymmetry may reflect high fitness, relatively little is known concerning the developmental origins of these minor discrepancies. Understanding the mechanisms that determine fluctuating asymmetry is crucial to interpreting much of the recent literature, for example, it has often been claimed that asymmetry reflects properties of an individual's genome. Therefore, in this study, we have examined the ontogeny of fluctuating asymmetry in the primary feathers of European starlings (Sturnus vulgaris) and compared our data with six published hypotheses of the mechanisms of asymmetry development. We found that signed asymmetries were not consistently biased toward either the left or the right side among feathers on the same individual; growth increments (measured every two days) were also not sided; and both absolute and relative asymmetry decrease as the feathers develop. These data are most consistent with a developmental regulatory system that involves some elements of feedback between left and right sides and episodes of compensational growth to correct large asymmetries. We discuss the possible differences in developmental mechanisms that determine asymmetry in traits of varying functional importance.