Fruit abundance and trait matching determine diet type and body condition across frugivorous bird populations

Abstract

Research on seed-dispersal mutualisms has been highly unbalanced towards the plants, largely overlooking the fitness effects of fruit resources on frugivorous animals. Moreover, despite morphological mismatches like gape limitation may reduce the abundance of fruits that are actually accessible to a frugivore species, there is very little evidence on the trait-matching implications from a frugivore's perspective. Here, we refine recent resource-provisioning models to comprehensively test the joint effects of fruit abundance and trait matching on diet type and body condition (a surrogate of fitness) across frugivorous bird populations: Sardinian warblers Curruca melanocephala inhabiting ten Mediterranean forests differing in the abundance and composition of fleshy fruits. We hypothesised the abundance of fruit resources to have positive effects on the degree of frugivory and body condition of warblers, and such effects to be more pronounced when accounting for both trait matching (accessible fruits) and resource provisioning (energy in accessible fruits). We found a sharp threshold over which warblers shifted from a diet with very little or even no fruits to a predominantly frugivorous diet with increasing the local abundance of accessible fruits. We also found a strong positive relationship between the abundance of accessible fruits and the body condition of warblers (body mass and residual body mass), an effect that was more pronounced in females than in males. Although diet type and body condition were much better predicted when accounting for trait matching, accounting for resource provisioning did not improve the explanatory power of fruit resources. The fact that we detected strong and sex-dependent effects of fruit resources on body condition just a few weeks before the breeding season suggests that fruit resources likely affect the timing and success of reproduction, a question that deserves further research. Our findings provide new insight into the fitness consequences of seed-dispersal mutualisms for frugivorous animals.