Split moult: time constraints or endogenous strategy?

Plumage renewal is an important process in the avian yearly cycle, yet knowledge about the factors that shape moult strategies is far from complete. A varying proportion of individuals of several Palaearctic passerine species divide their flight-feather moult between breeding and wintering areas. Here we focus on split moult, which includes retaining some or all the secondary feathers during the main moult period (interrupted moult) and later renewal of these feathers (complementary moult), after migration or breeding. We built a large moult database (213 species, 64 076 individuals) to describe split moult pattern in Palaearctic passerines, identify its frequency and test whether it is more common in long-distance migrants than in short-distance migrants and sedentary species. We found that first-year individuals, tested shortly after the main moult period, exhibited interrupted moult, yet individuals tested later in the annual cycle also retained non-moulted secondaries. In contrast, in most species, adults tested later in the annual cycle had fully renewed flight-feathers. This would suggest that age may be a factor associated with the probability of exhibiting split moult, as this strategy was more common in adults than in first-year birds. Furthermore, we found that interrupted and split moults occur more often among long-distance migrants than among sedentary or short-distance migrants. Our results probably imply that the split flight-feather moult is not initiated in the first winter of the bird's life, as has been suggested in previous studies. In addition, the occurrence of split moult among long-distance migrants in which the main moult takes place during the winter and among sedentary or short-distance migratory species, suggests that split moult probably does not represent an evolutionary transition between summer and winter moult; in both species groups, an evolutionary transition towards winter moult is probably impossible. Based on our findings, we suggest that split moult may be a moult strategy that allows birds to cope with time and energy constraints during the main moult period by retaining part of their flight-feathers and resuming their moult later in the yearly cycle.

Automated summary

Our study shows that split moult is more common in long-distance migrants than in short-distance migrants and sedentary species. Age may be a factor associated with the probability of exhibiting split moult, as this strategy was more common in adults than in first-year birds. Split moult may be a strategy that allows birds to cope with time and energy constraints during the main moult period by retaining part of their flight-feathers and resuming their moult later in the yearly cycle.