Interactions among Environmental Stress, Body Condition, Nutritional Status, and Dominance in Great Tits

Abstract

--We studied body condition and feather growth rate in Great Tits (Parus major) in relation to dominance in two localities during late autumn and early winter. The two localities differed in elevation, ambient temperature, and arthropod availability. We supplemented the two study areas with food (husked peanuts) throughout he study period. The percentage of time tits spent at feeders was higher at E1 Ventorrillo (the locality that was colder and had lower natural food availability) and was associated with dominance only at this locality. The number of aggressive displacements per hour experienced by each individual was 150 times higher in the area with lower arthropod availability and lower temperatures. Protein reserves (measured as pectoralis muscle thickness) were higher at E1Ventorrillo and were positively and consistently related to dominance at both localities. Growth rate of induced feathers was slower at E1 Ventorrillo but was not directly related to dominance in either locality. Only dominant adult males at E1 Ventorrillo compensated for the environmental harshness at this locality by attaining a higher feather growth rate than the other sex/age classes. Feather-mass asymmetry of induced feathers during autumn was not associated with body condition, did not change between localities, and was inversely and consistently related to dominance at both localities. The covariation among variables describing bird size, access to supplemental food, body condition, feather growth rate, and asymmetry was different at the two localities. Larger, dominant Great Tits spent more time foraging at feeders, had a thicker pectoralis muscle (i.e. body condition), and grew induced feathers at a higher rate only at the locality with colder temperatures and lower food availability. Received 9 September 1997, accepted 2 February 1998. SOCIAL DOMINANCE, food availability, and environmental stress (e.g. declining temperatures and/or high snow cover) are thought to influence winter survival of small passerines through their effects on body condition and access to food (Gauthreaux 1978, Lundberg 1985, Piper and Wiley 1990). The role of these variables in food access has been studied extensively (Ekman and Askenmo 1984, Millikan et al. 1985, Theimer 1987, Enoksson 1988, Hogstad 1988, Caraco et al. 1989, Desrochers 1989). Body condition generally has been defined in a very broad sense to indicate the ability of an individual to cope with present and future physiological stress, and therefore, the ability to enhance fitness. Fat reserves are thought o play an important role as energy stores to overcome periods of food scarcity or increased energetic demands 3 E-mail: mcnc152@fresno.csic.es (Blem 1990, Bednekoff et al. 1994, Bednekoff and Houston 1994, Rogers 1995, Gosler 1996, Lilliendahl et al. 1996). However, fat reserves may have costs in terms of winter survival diminished maneuverability, and increased predation risk (e.g. Lima 1986, Witter and Cuthill 1993, Ekman and Lilliendahl 1993, Witter et al. 1994, Clark and Ekman 1995, Gosler et al. 1995, Metcalfe and Ure 1995). Therefore, fat storing may be subjected to selection pressures not directly related to body condition. The complex interaction between fat reserves and environmental conditions also is complicated by the possible inverse relationship between fat stores and dominance in winter (Ekman and Lilliendahl 1993, Witter and Swaddle 1995, Gosler 1996). Protein reserves are not viewed as short-term energy stores because they are not as easily mobilized as fat and are not maintained as special depots. Muscle proteins are used for energy