Balancing moult, migration, and breeding in a long-lived partially migrant raptor

Moult, breeding, and migration are the three major life-history events in the annual cycle of birds. All are energetically demanding processes that rarely overlap. In large raptors such as the Egyptian vulture Neophron percnopterus, completing a full moult may take more than one year, requiring birds to balance this process with other life-cycle events. We analysed 740 moult cards from across the species’ distribution, including migratory, sedentary, and mixed subpopulations, to test predictions about trade-offs between the physiological demands of migration and moult. Juveniles began moulting in January-February of their second calendar year and continued "year-round" until their fifth calendar year. Adults, however, suspended moult from the months preceding spring migration until the late breeding phase. Subadults from the Canary Islands started moulting earlier and completed it faster than those from Western Europe, while adults from Western Europe initiated moult later than adults from the Canary Islands, India, and Oman. Sedentary subpopulations, particularly those from the Canary Islands, showed a greater moult extent and replaced more primaries annually than migratory and mixed subpopulations, whereas the wintering subpopulation of southwestern Spain (Extremadura) exhibited the smallest moult extent. Moult symmetry decreased from the second plumage onwards and differed between the Canary Islands and Oman subpopulations. Geographic differences in moulting patterns likely arise from population-specific life-history trade-offs, reflecting local adaptations or responses to environmental constraints. These patterns highlight the influence of migratory strategy on moult progression and suggest that sedentary lifestyles may allow more extensive feather renewal in long-lived raptors.

Methods
We created moult cards from high-definition and high-quality pictures (sufficient to compare feather wear and changes in tone and colour) of wild birds taken by research teams during trapping events (n = 265), wildlife photographers (n = 251), and two online citizen science repositories, the Macaulay Library (www.macaulaylibrary.org/) and iNaturalist (www. inaturalist.org/) (n = 81 and 143, respectively) (moult_EV_Raw_Database.csv). To avoid pseudoreplication, data on single individuals from different sources were not used, although we used different pictures of the same individual to cross-check the moult cards. In the few cases where individuals from the same area were selected, individual marks and moult state were checked to avoid resampling of the same bird. Scoring and recording moult on cards for all photographs was conducted by a single, trained researcher (IZ, see Zuberogoitia et al. 2013). Birds in hand were photographed with extended wings (normally taking two photos per wing, one per side). For 452 birds, we had photographs of both wings, and for 288, only one wing was photographed. A moult card was filled out for each wing with a photo, and feather generation was identified based on wear, shape, colour, age pattern, and growth of the remiges (i.e. help_outlineprimaries and secondaries; Zuberogoitia et al. 2018). This provided data for 10 primaries and 20 secondaries for each wing. Primaries were numbered in descending order (from inside to outside), and secondaries were numbered in ascending order (from outside to inside).To calculate moult scores, we followed Ginn and Melville (2000) and Newton (2009), using a standard recording system, based on the growth stage of primaries and secondaries: old feathers were scored as 0, fully grown new feathers as 5, and growing feathers as 1–4 for intermediate stages of development. These individual moult scores were then summed to give an overall moult score between 0 and 50 (for 10 primaries), and 0 and 100 (for 20 secondaries). However, moult in large birds, whichrequirese more than one year to replace all their remiges, occurs in ‘waves’ as a process termed ‘serial moult’ (Edelstam 1984) or ‘Staffelmauser’ (Zuberogoitia et al. 2018). Following Zuberogoitia et al. (2013), feathers scored as 0 were also recorded as ‘A’ (juvenile), ‘K’ (moulted in the previous moult year), ‘M’ (moulted two years earlier), and ‘O’ (moulted three years earlier) (Supporting information)