Fine-scale life-history variation in sociable weavers in relation to colony size

Summary

• 1
  This study investigates the mechanistic and ecological basis of phenotypic sorting in the sociable weaver Philetairus socius (Latham). In this colonially and communally breeding African bird, morphology, condition, age-specific survival rates and reproductive investment vary with colony size.
• 2
  This variation might arise from non-adaptive constraints imposed by density dependence, or from adaptive life-history responses to colony size-dependent selection pressures. To attempt to distinguish these, the environments in which adults made reproductive decisions and in which offspring developed were manipulated.
• 3
  When food supplementation improved the pre-breeding environment of adults, they bred earlier but did not change investment in eggs or incubation.
• 4
  Nestling and subsequent adult phenotypes were unaffected by supplementary feeding, although post-fledging survival improved. Nestling origin and not rearing environment predicted phenotype in a cross-fostering experiment between colonies.
• 5
  Phenotypic differences among colonies hence seemed not to be plastic responses to resource availability. Life-history differentiation, mediated either genetically or maternally, might have taken place over relatively narrow temporal and spatial scales.
• 6
  Adaptive variation in reproductive investment could thus mitigate costs of large colony size, by reducing costs of reproduction and hence improving adult survival. If so, then fine-scale life-history adjustment could help to maintain stable variation in colony size.