Correlates of variation in flight feather quality in the Great Tit Parus major

The most important function of moult in birds is the renewal of deteriorated feathers. Worn and holey remiges may incur fitness costs, yet little is known about the factors responsible for the degree of feather wear and sources of variation in feather hole incidence. Here, we report results on variation in feather quality based on a study of three consecutive annual cycles of a Great Tit Paws major population. We found that month, age, sex, weight and the presence of feather holes are the main sources of variation in degree of wear. Juveniles and individuals with lower feather quality (measured through rachis diameter) and with fault bars present had higher feather hole loads. Feather abrasion peaked in the breeding season and was higher in females probably due to higher workload. The lower feather quality of juveniles compared to adults probably arises because of fast ontogeny in contrast to adults' prolonged moult. Our results indicate that feather deformities are positively interrelated and can be used as proxies of feather quality. Our findings on the correlates of feather holes seem partially contradictory with the widespread view that holes are feeding marks of chewing lice. We propose that feather holes are instead minor feather handicaps, at least in Great Tits.