Confronto delle strategie di muta tra uccelli marini immaturi e adulti in una specie migratrice e longeva

Life-history theory predicts the existence of trade-offs between the simultaneous expression of costly activities under limited energies condition. The replacement of old feathers is considered one of the major events in the avian life history together with reproduction and migration. Because these are energy-demanding activities, they are predicted to be temporally segregated during the annual cycle of migratory birds. Among seabirds, migratory species present some exceptions to this general pattern where the overlap between moult and reproduction can occur. However, few studies have addressed the relationship between moult strategies and individual intrinsic factors such as breeding status, sex, physiological condition and age at the same time, and even less have made comparisons between individuals from different life-history stages: breeders, immatures and sabbatical birds within the same species. To investigate the aforementioned relationships, we studied the moult strategy of a long-lived migratory species, the Cory's Shearwater Calonectris borealis breeding in Selvagem Grande Island. Every year, adult shearwaters come back to their nesting colony that is attended also by non-breeding individuals: sabbatical (adults that skip the current reproductive season) and immature birds (individuals that have never bred before). Because the moult strategies of breeding shearwaters as well as their breeding and migratory phenology are well known, the species is an excellent model for intra-specific comparisons. In fact, while breeding shearwaters show a partial breeding-moult overlap with fail breeders starting moult earlier than successful breeders, virtually nothing is known about how birds without breeding duties that is sabbaticals and immature schedule their moult. On the base of what is known from the literature we predicted that i) sabbatical and immature birds, free from breeding costs, may invest more in moult compared to breeders; ii) due to a remarkable dimorphism in body size and sex-related differential investment in reproduction female birds may renew more feathers than males; iii) a smaller proportion of birds in poor physiological condition may engage in moult; iv) immature birds may start moult progressively later than older conspecifics. To test our predictions, during 2013 and 2014 we scored the moult stage of body, tail and wing feathers of 400 Cory's Shearwaters of known breeding status, sex and age. Moreover, we evaluated the body condition of a subsample of birds and collected blood samples to determine the prevalence of haemospordian parasites Leukocytozoon, Plasmodium and Haemoproteus. The results showed that 50% of sampled birds were engaged in active moult of body feathers during the breeding season regardless of breeding status. Our first prediction was met since a larger proportion of sabbaticals and immatures were moulting tail and wing feathers compared to active breeders. Our second prediction was partially supported with sabbatical-females showing higher total moult scores compared to males of any other breeding status. The lack of a relationship between body condition and moulting bird/moult score as well as of parasite-infected birds did not support our third prediction suggesting no immune-moult trade-offs. Finally, immatures birds which started moult with a delay compared to the other groups of birds also showed a negative and age-related pattern with total moult score thus support our forth prediction.