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

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.
Methods
All the methods are described in detail in the scientific paper associated to this dataset: González-Varo et al. 2021 Oikos 10.1111/oik.08106

种子传播互利共生（seed-dispersal mutualisms）领域的研究长期以来高度偏向植物类群，很大程度上忽视了果实资源对食果动物（frugivorous animals）适合度（fitness）的影响。此外，尽管诸如张口限制（gape limitation）这类形态不匹配现象可能会减少某一食果动物物种实际可获取的果实丰度，但从食果动物视角出发探讨性状匹配（trait-matching）意义的相关证据仍极为匮乏。本研究对近年提出的资源供给模型（resource-provisioning models）进行优化，以全面检验果实丰度与性状匹配对食果鸟类种群的食性类型和身体状况（适合度的替代指标）的联合效应。研究对象为栖息于10个肉质果实丰度与组成存在差异的地中海森林中的撒丁岛林莺（Sardinian warblers，Curruca melanocephala）。我们提出如下假说：果实资源丰度对林莺的食果比例与身体状况具有正向效应，且当同时考虑性状匹配（即可获取果实）与资源供给（即可获取果实中的能量）时，该效应会更为显著。研究发现存在一个明确的阈值：随着当地可获取果实丰度的提升，林莺的食性会从几乎不含果实的饮食模式转向以食果为主的饮食模式。此外，我们还发现可获取果实丰度与林莺的身体状况（体质量（body mass）与残差体质量（residual body mass））之间存在显著的正向关联，且该效应在雌性个体中比雄性更为显著。尽管纳入性状匹配因素能更好地预测食性类型与身体状况，但加入资源供给因素并未提升果实资源的解释能力。我们在繁殖季前数周就检测到果实资源对身体状况存在显著且依赖于性别的效应，这一结果表明果实资源可能会影响繁殖的时机与成功率，该问题值得进一步开展研究。本研究结果为理解种子传播互利共生对食果动物的适合度影响提供了新的视角。
研究方法
本数据集关联的学术论文已详细阐述所有研究方法：González-Varo 等人，2021年，《Oikos》，DOI:10.1111/oik.08106