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Sociobiol. 20: 153 – 160. JOURNAL OF AVIAN BIOLOGY 33: 446–450, 2002 Two blue tit Parus caeruleus populations from Corsica differ in social dominance Ce ´line Braillet, Anne Charmantier, Fre ´de ´ric Archaux, Anabelle Dos Santos, Philippe Perret and Marcel M. Lam- brechts (correspondence), Centre d’Ecologie Fonctionnelle et Eolutie, UPR 9056 du CNRS, 1919 route de Mende, F -34293 Montpellier Cedex 5, France. E -mail: marcel.lambrechts@cefe.cnrs -mop.fr Although the causes and consequences of social dominance have been examined extensively, avian studies have rarely focused on between-population differences in social dominance. On the is- land of Corsica, two resident blue tit Parus caeruleus popula- tions 25 km apart differ significantly in body size measures, timing and effort of reproduction, and song structure, and some of these population differences have a genetic basis. Because earlier avian studies have shown that social dominance is influ- enced by body size or mass, we predicted that individuals from these two blue tit populations would also differ in their ability to dominate other individuals. Consistent with this prediction, we found that male blue tits of these two populations differ in social dominance, and that heavier or larger individuals dominate lighter or smaller ones in aviary experiments. We propose that social dominance may serve to maintain phenotypic population differentiation at a micro-geographic scale by acting as a barrier to dispersal. Social dominance describes the capacity of one individ- ual to cause another individual to retreat in agonistic interactions. Dominant individuals have better success in acquiring resources in intraspecific competition, and therefore may have better survival and reproduction than subordinates (e.g. Kikkawa 1980, Arcese and Smith 1985, Piper 1997, Stahl et al. 2001). For instance, dominant individuals can force subordinates to settle in less suitable habitat causing non-random distributions of individuals within or across populations (e.g. Ketter- son and Nolan 1976, Ulfstrand et al. 1981, Ekman and Askenmo 1984, Hogstad 1989, Marra 2000, Burton and Evans 2001, but see Lemel 1989, Rogers et al. 1989). Social dominance is not a fixed trait because dominance status often changes with age, site-familiarity, experi- ence, body condition and mating status (e.g. Krebs 1982, Arcese and Smith 1985, Lundberg 1985, Piper and Wiley 1989, Cristol et al. 1990, Dearborn and Wiley 1993, Martin et al. 1997, Piper 1997, Hogstad 1999, Stahl et al. 2001). In many cases, older individu- als dominate less experienced juveniles. Prior occu- pancy may also often confer a dominance advantage (e.g. Krebs 1982, Sandell and Smith 1991, Piper 1997), whatever the quality of the individuals involved (e.g. Lambrechts and Dhondt 1988). However, because of consistent individual variation in intrinsic traits, some individuals have a higher capacity to dominate others (Piper 1997). For instance, social dominance ability may be influenced by aggression that has a genetic basis (Verbeek et al. 1996), males most often dominate fe- males (e.g. Lundberg 1985, Wiedenmann and Rabenold 1987, Hogstad 1999), or larger individuals dominate smaller ones (e.g. Dearborn and Wiley 1993, Burton and Evans 2001, Stahl et al. 2001, but see Lambrechts and Dhondt 1986). If local populations differ in pheno- typic traits, such as body size, the ability to dominate may differ between populations, a supposition that has rarely been tested experimentally (e.g. Foster 1999). Two resident blue tit Parus caeruleus ogliastrae pop- ulations on the island of Corsica, only 25 km apart, differ phenotypically, and these phenotypic differences may have a genetic basis (e.g. Blondel et al. 1999, Lambrechts et al. 1999). Blue tits in a valley with patches of rich broad-leaved deciduous oak (valley Mu) breed one month earlier, lay more eggs, and are larger and heavier than blue tits from a valley dominated by poor evergreen habitat (valley Pi) (Lambrechts et al. 1997, Blondel et al. 1999). The two populations also differ in the song types they sing (Doutrelant et al. JOURNAL OF AVIAN BIOLOGY 33:4 (2002) 446