The implications of partial moult
Partial replacement of juvenile, nest-grown wing feathers during the first year of life is a common strategy among passerines due to constraints involving available time for moulting or food resources that must be acquired for this process (Jenni & Winkler 1994; Kiat & Izhaki 2016). These constraints can occur in extremely northern moult areas (Kiat & Sapir 2017) or as a result of long-distance migration (Kiatet al. 2019a), higher body mass (de la Hera et al. 2009; Kiat & Izhaki 2016) or later hatching dates (Bojarinova et al.1999). These effect are more severe for juvenile than for adult birds due to the juveniles’ lower foraging proficiencies (Marchetti & Price 1989) and anti-predator capabilities (Hanson & Coss 1997; Rajalaet al. 2003). Partial moult affects bird appearance and thereby may affects sexual and social interactions. As a result, juveniles can be easily identified by their partial moult and nest-grown feathers, which signal their low competitive abilities (Grant 1990). An extensive partial moult may increase the attractiveness of males during the first breeding season and may positively affect bird fitness through sexual selection. However, individuals that moult extensively and consequently have a more adult-like appearance may also be subjected to more aggressive behavior from competing adults (Senar et al. 1998).