J. Avian Biol. 40: 461465, 2009 doi: 10.1111/j.1600-048X.2008.04689.x # 2009 The Authors. J. Compilation # 2009 J. Avian Biol. Received 19 August 2008, accepted 8 December 2008 A comparative study of migratory behaviour and body mass as determinants of moult duration in passerines Iva ´n de la Hera, Jose ´ A. Dı ´az, Javier Pe ´rez-Tris and Jose ´ Luis Tellerı ´a I. de la Hera (correspondence), J. A. Dı ´az, J. Pe´rez-Tris and J. L. Tellerı ´a, Dept. de Zool. y Antropol. Fı ´sica, Facultad de Biol., Univ. Complutense de Madrid, E-28040 Madrid, Spain. E-mail: idelahera@bio.ucm.es Birds moult to maintain plumage function through life, but the factors that determine moult duration are poorly understood. In temperate areas, variation in moult duration could be largely associated with between-species differences in migratory behaviour (migrants have less time for moulting after breeding), and body mass (because the aerodynamic cost of rapid moult increases allometrically with body size). Moreover, if the energetic cost of transport favours a smaller body size in migratory species, then the effects of migratory behaviour and body mass on moult duration could be confounded. We conducted a comparative study of the duration of adult complete moult in 48 European passerine species, in relation to body mass and migratory behaviour (sedentary, short-distance migrants and long-distance migrants). Lighter and more migratory species moulted faster than heavier and more sedentary species, but migration was not associated with body mass. If accelerated moult compromises the success of migration, changes in the physiology or phenology of moult in migratory birds are better interpreted as adaptive responses to compensate for such costs. Moult is essential for birds to maintain plumage function through life (Jenni and Winkler 1994). In the temperate zone, most passerines undergo a complete post-breeding moult each summer since their second year of life, which lasts from 28 to 182 days depending on the species (Ginn and Melville 1983, Jenni and Winkler 1994). However, the factors that may explain such a wide variation in moult duration among species are far from being identified (Kjelle ´n 1994), a gap in our knowledge which is especially relevant because moult duration can greatly affect the structure and function of feathers (Dawson et al. 2000, Hall and Fransson 2000). It is common knowledge that migratory behaviour and body mass of birds are two major determinants of moult duration. Migration is costly in terms of both energy and time (Alerstam 1990, Berthold et al. 2003), and therefore birds tend to separate migration from other costly activities, such as moult (Jenni and Winkler 1994). As a consequence, migratory birds have less time available for post-breeding summer moult before migration, which might select for rapid moult in migratory but not in sedentary species (Kjelle ´n 1994, Piersma et al. 2005). Besides, larger bird species may take longer to complete their moult than smaller ones, because their larger feathers may take longer to reach full size, and the aerodynamic costs of moult increase allometrically with increasing body mass (Jenni and Winkler 1994). The latter is particularly important because wing gaps formed during moult impair flight performance (Williams and Swaddle 2003), and fast-moulting birds have larger wing gaps than slow-moulting birds (Bensch and Grahn 1993). Interestingly, heavy species will face larger flight costs than lighter species with equivalent wing gaps given their size (Jenni and Winkler 1994, Carrascal and Polo 2006), which might select for slow moult in heavy species. Although the existence of relationships among moult duration, migratory behaviour and body mass of birds seems intuitive, the shape of such relationships is far from being explicitly evaluated. In particular, it is important to determine how migratory behaviour and body mass interact as possible determinants of moult duration. For example, heavy birds might face an increased cost of transport during migratory flights (Hedenstro ¨m and Alerstam 1998, Bowlin and Wikelski 2008), which might have favoured the evolution of reduced body mass in migratory species (Calmaestra 2000). In turn, a detailed knowledge of the relationships among body size, migratory behaviour, and moult duration is required for understanding the evolu- tion of different moult durations observed among bird lineages. We conducted a comparative analysis of 48 species of European passerines for which data of moult duration, migratory behaviour and body mass were available from the literature. We tested for differences in moult duration and body mass among three groups of species with different migratory behaviours (sedentary, short-distance migration, and long-distance migration), with specific interest in the effect of migration on moult duration controlling for variation in body mass. 461