Plausible causes of seed preferences and diet composition in seed-eating passerines

We evaluated whether seed mass, handling time, handling efficiency and profitability account for (a) preferences in controlled experiments, and (b) field-diet composition of four bird species of the Monte desert, Argentina. The question of whether birds maximise their energy intake rates while feeding on seeds is assessed. We used feeding experiments with six native seed species of 0.07 – 0.75 mg (i.e. the seed-size range consumed in nature), which account for 0.59 – 0.84 of the field diet of the four birds. We measured seed-handling times, and used published information on bird preferences and diets, and on seed chemistry, for further calculations. Bird preferences were always positively related to seed mass, and also to seed profitability in the two intermediate-sized birds. Diet composition correlated positively with seed mass and negatively with seed profitability in three species, but some birds also showed a flexible behaviour eating the most attractive seeds according to their availability. This behaviour is not genuinely opportunistic because it only focuses on a restricted fraction of the total seed species present in the field. Contrary to expectations of species coexistence due to resource partitioning, small and large birds showed similar feeding efficiencies when eating the smaller and the larger seeds. The positive association between seed mass and profitability in several studies suggests that most birds can maximise their energy reward, on average and in the long term, by preferring the larger seeds. A combination of potential feeding optimisation with certain flexibility in the field may characterise the feeding ecology of desert seed-eating birds.

本研究旨在评估种子质量、取食处理时长、取食处理效率与取食收益，能否解释（a）受控实验中的鸟类取食偏好，以及（b）阿根廷蒙特沙漠四种鸟类的野外饮食组成。同时，本研究也探讨了鸟类在取食种子时是否能够最大化其能量摄入速率这一科学问题。
本研究选取0.07~0.75mg（即野外自然取食的种子尺寸范围）的6种本土种子开展取食实验，此类种子占四种鸟类野外饮食组成的59%~84%。我们测定了种子处理时长，并结合已发表的鸟类取食偏好、饮食组成及种子化学组成相关数据开展后续计算分析。
研究结果显示，鸟类的取食偏好始终与种子质量呈正相关；在两种体型中等的鸟类中，取食偏好还与种子取食收益呈正相关。三种鸟类的野外饮食组成与种子质量呈正相关，与取食收益呈负相关；但部分鸟类也会根据种子可获得性，灵活取食吸引力最强的种子。该行为并非真正的机会主义取食，因为其仅局限于野外存在的全部种子物种中的一小部分。
与基于资源分化的物种共存理论预期相悖的是，无论取食小型还是大型种子，体型大小迥异的鸟类均表现出相近的取食处理效率。多项研究证实种子质量与取食收益呈正相关，这表明平均而言且从长期来看，多数鸟类可通过偏好取食大型种子，最大化其能量收益。兼具潜在取食优化策略与野外取食灵活性的特征，或许正是荒漠食籽鸟类取食生态的典型特点。