Bird pollination syndrome is the plant's adaptation to ornithophily, but nectarivorous birds are not so selective
Abstract
Many tropical plants are pollinated by birds and several bird phylogenetical lineages have specialised to a nectar diet. The long-assumed, intimate ecological and evolutionary relationship between ornithophilous plants and phenotypically specialised nectarivorous birds has nevertheless been questioned in recent decades, where such plant-pollinator interactions have been shown to be highly generalised. In our study, we analysed two extensive interaction datasets: bird-flower and insect-flower interactions, both collected on Mt Cameroon, west-central Africa. We tested if:
1) insects and birds interact with distinct groups of plants;
2) plants with a typical set of ornithophilous floral traits (i.e. bird pollination syndrome) interact mainly with birds;
3) birds favour plants with bird pollination syndrome and;
4) if and how the individual floral traits and plant level nectar production predict bird visitation. Bird-visited plants were typically also visited by insects, while approximately half of the plants were visited by insects only. We confirmed the validity of the bird pollination syndrome hypothesis, as plants with bird-pollination syndrome traits were visited by birds at a higher rate and mostly hosted a lower frequency of visiting insects. However, these ornithophilous plants were not more attractive than the other plants for nectar-feeding birds. Nectar production per plant individual was a better predictor of bird visitation than any other floral trait traditionally related to the bird pollination syndrome. Our study thus demonstrated the highly asymmetrical relationship between ornithophilous plants and nectarivorous birds.