Effects of wing area reduction on winter body mass and foraging behaviour in coal tits: field and aviary experiments

Theoretical and experimental evidence suggests that an increase in flight costs will decrease flight performance, and that birds should trade-off the benefits of body reserves to minimize these costs. Alternatively, birds could avoid starvation by increasing food intake, thereby maintaining body reserves, and/or decreasing flight activity to compensate for the greater per unit flight costs. To test the effect of increased flight costs on body mass regulation and on flying and feeding activity, we experimentally manipulated wing area in a free-ranging wintering population of coal tits, Periparus ater, and in captive birds living in a less restrictive environment (large outdoor aviaries). In the field, body mass decreased when wing area was reduced, but heavier birds lost more weight than lighter birds as a consequence of an allometric increase in flying costs. However, wing area reduction had no effect on body mass in the aviaries. Birds also flew less when wing area was reduced and those with higher wing loadings decreased flying frequency more markedly. We suggest that the goal of small resident birds living in a Mediterranean montane climate would be to maintain daily fat reserves within narrow limits during autumn and winter, even under contrasting ecological conditions.