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.

中文翻译：

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水果丰度和性状匹配决定了食果鸟类种群的饮食类型和身体状况

种子传播共生关系的研究对植物高度不平衡，很大程度上忽略了水果资源对食果动物的适应性影响。此外，尽管像裂口限制这样的形态不匹配可能会减少食果动物物种实际上可以获得的水果的丰度，但从食果动物的角度来看，关于性状匹配影响的证据很少。在这里，我们改进了最近的资源供应模型，以全面测试水果丰度和性状匹配对食果鸟类种群的饮食类型和身体状况（健康的替代指标）的联合影响：撒丁岛莺Curruca melanocephala栖息在十个地中海森林中，肉质水果的丰度和成分各不相同。我们假设丰富的水果资源对莺的食果程度和身体状况产生积极影响，并且在考虑性状匹配（可获得的水果）和资源供应（可获得的水果中的能量）时，这种影响更为明显。我们发现了一个明显的阈值，在这个阈值上，莺从很少或什至没有水果的饮食转变为以水果为主的饮食，增加了当地可获得的水果的丰度。我们还发现，可获取水果的丰度与莺的身体状况（体重和剩余体重）之间存在很强的正相关关系，这种影响在雌性中比在雄性中更为明显。尽管在考虑性状匹配时可以更好地预测饮食类型和身体状况，但考虑资源供应并没有提高水果资源的解释力。我们在繁殖季节前几周检测到水果资源对身体状况的强烈和性别依赖性影响这一事实表明，水果资源可能会影响繁殖的时间和成功，这是一个值得进一步研究的问题。我们的研究结果为种子传播共生对食果动物的适应性后果提供了新的见解。我们在繁殖季节前几周检测到水果资源对身体状况的强烈和性别依赖性影响这一事实表明，水果资源可能会影响繁殖的时间和成功，这是一个值得进一步研究的问题。我们的研究结果为种子传播共生对食果动物的适应性后果提供了新的见解。我们在繁殖季节前几周检测到水果资源对身体状况的强烈和性别依赖性影响这一事实表明，水果资源可能会影响繁殖的时间和成功，这是一个值得进一步研究的问题。我们的研究结果为种子传播共生对食果动物的适应性后果提供了新的见解。