A structural comparison of fault bars with feather defects known to be nutritionally induced

Abstract

Fault bars are narrow, translucent bands in feathers where barbs lack barbules. It has been widely assumed that fault bars result from malnutrition and are therefore a record of the bird's nutritional status while growing or molting. We compared the microstructure of fault bars with that of defects correlated with experimental malnutrition (sulfur amino acid deficiency or deficiency of a balanced diet) in White-crowned Sparrows (Zonotrichia leucophrys gambelii). The structural defects in fault bars (mainly missing barbules) are unlike defects that accompany frank malnutrition (all structures present, but barbules too short or weak to interlock). We also subjected White-crowned Sparrows to 36 h of total fasting (12 to 19% loss of body mass). Fault bars or spots occurred in the rectrices of 19 of 44 starved birds. Some bars coincided with days on which birds were handled (19 of a total of 41 bars), others (22) occurred erratically in relation to handling or starvation days, but none coincided with starvation days. From these data and a review of previous investigations we conclude that there is little support for the notion that malnutrition always causes fault bars. Many kinds of stimuli are associated with fault bars. We suggest that the peripheral barbule cells are damaged by contractions of feather muscles that squeeze or crimp the soft feather sheath. Fault bars are not a reliable index of a bird's nutritional status while it is growing or molting.