Delayed post-juvenile moult in malaria-infected European blackcaps

Parasites have their strongest impact on fitness when host defences limit the quantity of resources available for other critical life-history stages, such as development, breeding or migration. One greatly neglected stage that could be altered by parasites is the post-juvenile moult (PJM) of birds, through which inexperienced yearlings replace their weak and fast-generated juvenile feathers by adult-like feathers. The earlier juvenile birds complete PJM, the earlier they will be prepared to withstand forthcoming adverse conditions, especially if they migrate short after moulting. We used data from 435 juvenile European blackcaps (Sylvia atricapilla) sampled during three years in 26 localities spanning the wide range of environmental conditions across Iberian Spain to test whether haemosporidian infections (presence and abundance in blood of parasites of the genera Haemoproteus, Plasmodium and Leucocytozoon) were related to delayed PJM. Controlling for body condition, sex, year and date of capture, infected blackcaps (single-infected or coinfected) had lower moult scores indicative of delayed moult, especially when birds had Plasmodium infections or higher intensity of Haemoproteus parasites. Our results broaden the range of fitness costs that haemosporidian parasites may have on birds, as delayed plumage maturation prolongs the period during which juvenile birds lack a fully functional plumage.