Bird pollination syndrome is the plant’s adaptation to ornithophily, but nectarivorous birds are not so selective

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.