* 1Measures of the quality of an individual are important in the study of proximate and ultimate factors in biology. Records of developmental history are particularly desirable, as many phenotypical traits are influenced by conditions during growth. Conspicuous irregularities in feathers, known as fault bars, result from a variety of stresses that occur during feather growth. The frequency of fault bars was evaluated on primary and tail feathers (grown 1 year previously) of 1919 adult American kestrels from a breeding population in Canada (1990–97). * 2Most (91·5%) birds exhibited some fault-barring, although females had significantly more feathers with fault bars than males (17% vs. 14%, respectively). Body size, intensity of haematozoan infections and leucocyte differentials were all unrelated to fault bars; however, birds with many fault bars were in poor body condition during prelaying (males) and incubation (males and females). * 3Individual kestrels tended to be consistent in the number of feathers with fault bars from year to year. * 4The percentage of feathers with fault bars was not associated with the timing of arrival in spring or prey abundance per territory; however, birds of both sexes with many bars were less likely to breed. Birds paired non-randomly, as mates tended to have a similar frequency of fault bars. Males and females with many bars had significantly later clutch initiation dates, but there were no negative consequences regarding clutch size or egg size. * 5Female kestrels with many fault bars had lower survival probabilities. Both sexes were also less likely to be recaptured in years following initial banding if they had many bars, suggesting that they were more likely to emigrate from the study area temporarily. * 6Fault bars on feathers appear to be indicative of an individual’s susceptibility to stress, and are useful in predicting components of fitness. The use of fault bars is a promising tool as they are easy to evaluate and can be assessed on live or dead birds, on moulted feathers and on individuals repeatedly over time.
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