Fault bars timing and duration: the power of studying feather fault bars and growth bands together

Growth bands and fault bars, widespread features of feathers that form during regeneration, have largely been studied independently. Growth bands result from normal regeneration: each pair of dark/light bands forms every 24 h. Fault bars are a response to stress during regeneration, creating a translucent line that can break the feather. We studied the relative position and width of these two structures in feathers of nestling and adult white storks Ciconia ciconia . We first confirmed that one growth band represents 24 h of feather regeneration. Fault bars did not occur at random within growth bands: 65.7% (in nestlings) and 45.6% (in adults) of them occurred in one out of six defined segments within a growth band, namely that segment corresponding to the first one‐third of night time hours. The width of fault bars relative to growth bands suggested that fault bars were produced during a median (range) of 7.0 h (2.7–27.0) in nestlings and 3.7 h (1.8–7.9) in adults. Fault bars were concentrated at feather tips in nestlings, but at central locations in adult feathers. Our results suggest that, in general, fault bars are a discrete event of a finite duration occurring mainly during the night (particularly in nestlings). This, along with current knowledge, suggests that acute stressors, rather than chronic ones, are responsible for fault bar formation. Thus, such acute punctual stressors (a matter of minutes) can have long‐lasting (months−years) physiological effects due to the wing load increase from feather breakage caused by fault bars.