Ecological fitting is a sufficient driver of tight interactions between sunbirds and ornithophilous plants

A generalized bird pollination system in Schotia brachypetala (Fabaceae)

Bird pollination systems are diverse, ranging from narrow-tubed flowers pollinated by specialist nectarivores such as hummingbirds and sunbirds, to relatively open flowers pollinated by opportunistic (i.e. generalist) nectarivores. The role of opportunistic avian nectarivores as pollinators has historically been under-appreciated. A key aspect to understanding the importance of opportunistic birds as pollinators is to investigate how efficiently they transfer pollen among flowers. Here, we document the pollination and breeding systems of Schotia brachypetala, a southern African tree known as the 'weeping boer-bean' on account of its prolific production of dilute hexose-dominated nectar. The cup-shaped flowers of this tree attract a large number of bird species, including both opportunistic and specialist nectarivores. We identified floral visitors using observations and camera traps and quantified the floral traits responsible for animal attraction. We documented the breeding system, used selective pollinator exclusion to test the contribution of birds to fecundity, and performed supplemental pollination to test for pollen limitation. Single-visit pollen deposition trials were undertaken to determine the efficacy of bird pollinators. Controlled hand-pollination experiments showed that S. brachypetala is genetically self-incompatible and therefore dependent on pollinators for seed production. Supplemental hand-pollination experiments showed that natural fecundity is limited by either the amount and/or the quality of pollen on stigmas. Flowers from which birds but not insects were experimentally excluded set fewer seeds than open control flowers. Opportunistic birds deposited more pollen per visit than did specialist sunbirds. We conclude that S. brachypetala has a generalized bird pollination system that mainly involves opportunistic nectarivores.

The ornithophily of Impatiens sakeriana does not guarantee a preference by sunbirds

In recent decades, the tight mutual specialization between nectarivorous birds and ornithophilous plants has been questioned, and instead, high degrees of generalization and interaction asymmetry have been highlighted. Here, we studied interactions among two sunbirds and four plant species in two Mount Cameroon forests, with two plant species from each forest. First, we investigated whether sunbirds differ in frequencies of visitation to target plant species in natural conditions. Second, using a cage experiment, we investigated whether sunbirds prefer various plant species, plants with which they are more familiar and that occur in the habitat where they were caught and/or the only studied ornithophilous plant, Impatiens sakeriana. In natural conditions, the short-billed sunbird, Cinnyris reichenowi, fed more on flowers with shorter tubes than the long-billed sunbird, Cyanomitra oritis. Likewise, sunbirds differed in their experimental preferences. Local plants were generally preferred. This was most obvious in the case of I. sakeriana, which was often visited by both sunbirds, but only in the habitat where it grows naturally. This study supports the importance of associative learning. Together with other studies, we suggest that the signalling traits of flowers with bird pollination syndromes evolved to filter out other visitors rather than to attract bird pollinators.

A geographical perspective on the relationship between Impatiens spur lengths and bill lengths of sunbirds in Afrotropical mountains

Trait matching-a correlation between the morphology of plants and their pollinators-has been frequently observed in pollination interactions. Different intensities of natural selection in individual regions should cause such correlations to be observable across different local assemblages. In this study, we focused on matching between spur lengths of the genus Impatiens and bill lengths of sunbirds in tropical Africa. For 25 mountain and island locations, we compiled information about the composition and traits of local Impatiens and sunbird assemblages. We found that assemblage mean and maximum values of bill lengths were positively correlated with mean and maximum spur lengths across locations. Moreover, our results suggest that the positive correlations hold only for forest sunbird assemblages sharing the same habitat with Impatiens species. We further show that long-billed sunbirds seem to locally match the morphology of multiple Impatiens plant species, not vice versa. Our observation implies that trait matching significantly contributes to structuring of Impatiens-sunbird pollination systems. We suggest that special habitat preferences together with spatial isolation of mountain environment might play a role in this case.