Effects of fruit novelty on feeding preference in four globally invasive frugivorous birds

Invasive fruit-eating animals (hereafter frugivores) can form novel
mutualisms with fruiting plants and alter seed dispersal. Plant-frugivore
interactions are often linked to frugivore preference for certain fruit
traits, but for invasive frugivores, it is uncertain if novelty—whether or
not a fruit is familiar—impacts foraging decisions. We experimentally
tested fruit preferences of the four most abundant and frugivorous bird
species on O‘ahu, a Hawaiian Island, all of which are globally invasive
songbirds. With captive wild birds, we tested for preference in relation
to fruit size, color, nutrients, and bird sex. We also tested how novelty
of fruits affected trait-based preferences, with fruits from plant species
either established in the wild or novel. Using arrays that offered
multiple fruit species, we conducted 252 trials with 111 plant species,
59.5% of which were considered novel. From all fruits tested, three bird
species preferred smaller fruits, and color preferences varied among bird
species. Novelty of fruits influenced preference in three ways. First,
birds preferred fruits from plants that were established in the wild over
novel fruits. Second, repeated exposure to novel fruits increased the
probability of interactions. Third, preferred novel fruits were smaller
and redder than preferred fruits from established plants. Finally, bird
sex and fruit nutrient content did not affect preference. Together, these
results suggest that preference for fruit traits varies among invasive
bird species and can be flexible when fruits are novel to birds,
increasing the likelihood of novel mutualisms and making it increasingly
difficult to predict invasion impacts.

入侵性食果动物（fruit-eating animals，下文简称食果动物，对应英文frugivores）可与结果植物形成全新的互利共生关系，并改变种子传播过程。食果动物与结果植物的互作关系通常与其对特定果实性状的偏好相关，但对于入侵性食果动物而言，果实的新颖性——即该果实是否为动物所熟悉——是否会影响其觅食决策，目前尚无定论。本研究针对夏威夷瓦胡岛上数量最多的4种食果性鸟类开展了果实偏好实验，这4种鸟类均为全球入侵性鸣禽。研究采用圈养野生鸟类，针对果实大小、颜色、营养成分以及鸟类性别等因素开展偏好性测试；同时，我们还以野外定植植物的果实和新颖植物的果实为材料，探究果实新颖性如何影响基于性状的偏好选择。通过设置包含多种果实的实验阵列，我们共开展了252组实验试次，涉及111种植物，其中59.5%的植物被归类为新颖物种。在所有受试果实中，有3种鸟类偏好体型更小的果实，且不同鸟类的颜色偏好存在差异。果实新颖性通过三种途径影响鸟类的偏好选择：其一，相较于新颖果实，鸟类更偏好野外定植植物的果实；其二，反复接触新颖果实会提升鸟类与该果实发生取食互动的概率；其三，相较于野外定植植物的偏好果实，鸟类偏好的新颖果实体型更小、颜色更偏红色。最后，鸟类性别与果实营养成分并未对偏好选择产生显著影响。综合来看，本研究结果表明，入侵性鸟类对果实性状的偏好存在物种间差异，且当果实对鸟类而言属于新颖物种时，其偏好性会发生改变，这提升了形成新型互利共生关系的可能性，同时也使得预测该类入侵物种的生态影响变得愈发困难。