Negative experiences during feather growth can result in fault bar formation. Fault bars are malformations perpendicular to the rachis of the feather caused by stressful experiences during feather growth. However, there are little data on the causal effect of psychological stress on the formation of fault bars in chickens. The objective of this study was to examine the effect of acute stress in domestic chickens (Gallus gallus domesticus) on fault bar formation to validate this measure as a welfare indicator. Thirty broiler breeder pullets were housed in six cages at 21 days of age. Three cages were exposed to an acute stress protocol while the other three were the unstressed control. Feathers were marked as close as possible to the growing follicle at 21 (wing feathers) and 60 (all feathers) days of age. Acute stress came in the form of three procedures (unpredictable feed delivery, induction of tonic immobility, and crowding) repeated twice, 3-8 days apart and randomly, from 28 to 60 days of age. Wing, tail, and cover feathers were removed and measured at 60 days of age for weight, length, and number of fault bars. Exposure to acute, unpredictable stress increased the number of fault bars in wing feathers of chicks with a high number of initial fault bars. Feather growth decreased for the stressed group compared to the control. These results suggest that feather traits, including fault bars and feather growth, can be used as indicators of negative welfare in chickens.

Infection by parasites or pathogens can have marked physiological impacts on individuals. In birds, infection may affect moult and feather growth, which is an energetically demanding time in the annual cycle. Previous work has suggested a potential link between clinically visible Trichomonas gallinae infection and wing length in turtle doves Streptopelia turtur arriving on breeding grounds. First, T. gallinae infection was characterized in 149 columbids from 5 species, sampled on turtle dove wintering grounds in Senegal during the moulting period, testing whether infection by T. gallinae is linked to moult. Trichomonas gallinae prevalence was 100%, so rather than testing for differences between infected and uninfected birds, we tested for differences in moult progression between birds infected by different T. gallinae strains. Twelve strains of T. gallinae were characterized at the internal transcribed spacer 1 (ITS1)/5.8S/ITS2 region, of which 6 were newly identified within this study. In turtle doves only, evidence for differences in wing length by strain was found, with birds infected by strain Tcl-1 having wings nearly 6 mm longer than those infected with strain GEO. No evidence was found for an effect of strain identity within species on moult progression, but comparisons between infected and uninfected birds should be further investigated in species where prevalence is lower.

Prenatal feather growth development in the chicken was studied in 7 body regions in HH stages 27–45, using direct measurements, specific histological and immunohistochemical methods, and scanning electron microscopy. The results from measurements of absolute length values, and, particularly, growth rate development in each HH stage revealed a distinct phase of most intensive growth in HH stage 40–41, which was preceded by feather follicle insertion and accompanied by the occurrence of α-keratins in barbule cells. Specific regional evaluation demonstrated that growth in the feather follicles of abdominal skin generally showed the slowest progression from absolute values and that in the feather filaments of the developing wings the most rapid progression occurred during HH stage 40–41 from growth rate values.