Interactions among environmental stress, body condition, nutritional status, and dominance in Great Tits

LUIS M. CARRASCAL,1,3 JUAN CARLOS SENAR,2 INGRID MOZETICH,1 FRANSEC URIBE,2 AND JORDI DOMENECH

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 the study period. The percentage of time tits spent at feeders was higher at El 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 El Ventorrillo and were positively and consistently related to dominance at both localities. Growth rate of induced feathers was slower at El Ventorrillo but was not directly related to dominance in either locality. Only dominant adult males at El 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 to play an important role as energy stores to overcome periods of food scarcity or increased energetic demands (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 only after both glycogen and lipid reserves are nearly or completely exhausted (see Blem 1990). Therefore, they are thought to measure long-term body condition (Gosler 1991, Newton 1993). Feather growth is linked with protein reserves that are used to obtain amino acids that can be assembled into feather proteins (Murphy 1996a). The width of feather growth bars is increasingly being used as an indicator of long-term nutritional status in winter populations (e.g. Grubb and Cimprich 1990, White et al. 1991, Hogstad 1992, Nilsson et al. 1993, Grubb 1995, Ekman et al. 1996). Moreover, fluctuating asymmetry on the growth of right and left side feathers has been suggested to be a good measure of long-term body condition due to its sensitivity to environmental stress and response to differences in phenotypic quality (Clarke 1992, 1995, Polak and Trivers 1994). Feathers are regenerated at a faster rate and to a greater total length and mass at higher ambient temperatures (Grubb et al. 1991; but see Grubb 1995), and growth is more rapid in summer than in winter (Grubb et al. 1991). Dominance also seems to have a strong effect on daily growth rates, with dominant individuals growing wider bars than subordinates (Grubb 1989, Grubb and Cimprich 1990, Hogstad 1992, Witter and Swaddle 1995). Habitat differences, probably related to habitat-specific nutritional condition, have also been described as a relevant factor affecting feather growth (Grubb and Yosef 1994).