Asymmetric contests over resources for survival and migration: a field experiment with bluethroats

Abstract:
The relative importance of prior occupancy and of asymmetries in both value of winning and in resource-holding potential for the outcome of animal contests was tested. Bluethroats, Luscinia s. svecica, were offered food ad libitum in the field. Prior to migration, at a moulting site, their weight increased rapidly and peaked after a few days. Lean birds won significantly more interactions than fat birds, probably owing to higher motivation to fight for the food. Dominance between individuals shifted and was not correlated with size or prior occupancy. During the subsequent migration, at a stopover site, the bluethroats gained weight throughout their stay. Dominance between individuals was constant and positively correlated with size, whereas fat reserves and prior occupancy had no effect on contest resolution. Fat deposition rates were positively correlated with dominance status. The different dominance patterns are explained in terms of different gain curves of fat for birds putting on fat for survival and migration, respectively. Though prior occupancy had no general effect on dominance patterns, such an uncorrelated asymmetry may have been used to settle single contests between fat birds at the moulting site. The fact that dominance (acting through differences in size) influences fat deposition rates may be important for the spatial and temporal pattern of migration in birds that compete for resources at stopover sites.