Variable foraging and flower probing behaviour of sunbird pollinators of the South African <i>Pelargonium fulgidum</i>

Whether foraging sunbird pollinators hover or perch, the type of perch used, and the direction of flower probing are generally thought to be determined by matching shapes of decurved bird bills and curved flower tubes, as well as inflorescence architecture. However, few studies have objectively quantified aspects of sunbird foraging behaviour to test predictions based on adaptive trait matching. I investigated sunbird foraging in a population of <i>Pelargonium fulgidum</i>, a species with slightly upcurved flowers borne on upward-oriented peduncles, which grows along the west coast of South Africa. Long-range photographs revealed that <i>Cinyris chalybeus</i>, a sunbird species with a slightly decurved bill, hovers or perches during foraging, and uses both the inflorescence and alternative vegetation structures as perch. Contrary to previous suggestions, both male and female sunbirds probe flowers from either of two alternative directions (sternotribic versus nototribic) in more or less equal proportions, and frequently display flexibility in both foraging behaviour and flower probing direction. Predominant or complete sternotribic and nototribic flower probing were explained by hovering versus use of an inflorescence perch respectively, whereas use of an alternative perch resulted in similar proportions of each mode of flower probing. The number of flowers per visited inflorescence and number and proportion of probed flowers did not vary with foraging behaviour or flower probing direction. Although certain foraging behaviours resulted in predictable flower probing directions, variation in foraging behaviour and the presence of many alternative perches provided by vegetation surrounding <i>P. fulgidum</i> plants resulted in variability in flower probing direction that appears not to affect foraging efficiency. Relatively weak reciprocal curvature of flower tube and sunbird bill, resulting in easy access to nectar from multiple feeding directions, likely facilitates flexible foraging. This study shows that environmental context may facilitate more complex foraging behaviour than that predicted by adaptive trait matching.人们普遍认为，觅食的传粉太阳鸟（sunbird）是悬停还是停栖、所使用的栖木类型以及探花方向，均由鸟类下弯喙（decurved bill）与弯曲花管的形状匹配度，以及花序结构（inflorescence architecture）共同决定。然而，鲜有研究客观量化太阳鸟觅食行为的相关特征，以验证基于适应性性状匹配的相关预测。 本研究针对南非西海岸分布的亮叶天竺葵（<i>Pelargonium fulgidum</i>）种群开展太阳鸟觅食行为调查，该物种的花朵略微上弯，着生于向上延伸的花序梗（peduncle）上。 长焦摄影结果显示，钢蓝太阳鸟（<i>Cinyris chalybeus</i>，一种喙部略微下弯的太阳鸟物种）在觅食时会悬停或停栖，同时将花序与其他植被结构作为栖木。 与此前的研究结论相悖，雌雄太阳鸟均以大致相等的比例从两种可选方向（腹侧传粉式（sternotribic）与背侧传粉式（nototribic））探花，且在觅食行为与探花方向上均常表现出灵活性。 以悬停方式觅食时，太阳鸟多采用完全或主导性的腹侧传粉式探花；而借助花序栖木时，则多采用背侧传粉式探花；但若使用其他植被作为栖木，则两种探花模式的占比相近。 每到访花序的花朵数量、已探花朵的数量与占比，均未随觅食行为或探花方向发生变化。 尽管部分觅食行为可对应可预测的探花方向，但太阳鸟觅食行为的变异，以及亮叶天竺葵周边植被提供的大量可选栖木，使得探花方向存在差异，而这种差异似乎并未影响觅食效率。 花管与太阳鸟喙部的相互弯曲度相对较弱，使得鸟类可从多个取食方向轻松获取花蜜，这可能促进了灵活觅食行为的产生。 本研究表明，环境背景可能催生比适应性性状匹配预测更为复杂的觅食行为。