African sunbirds hover to pollinate an invasive hummingbird-pollinated plant

Abstract

Why do hummingbirds hover while Old World nectar-feeding birds perch? A unique opportunity to explore this question is presented by the invasion into Africa of a plant adapted for pollination by hovering hummingbirds. Like other hover-pollinated plants of the New World, the flowers of the tree tobacco Nicotiana glauca lack perches and are oriented towards open space. We find that Old World nectarivores, especially the malachite sunbird, Nectarinia famosa, hover 80% of the time when taking nectar from these flowers. They hover for up to 30 s, and are able to sustain this hovering lifestyle in an area where native nectar plants are absent. Nicotiana glauca greatly increases the local abundance of sunbirds compared with uninvaded areas. In turn, flowers visited by sunbirds formed significantly more capsules and set significantly more seed than sunbird-excluded flowers, possibly facilitating the invasion. The results suggest a prominent role for plant, rather than bird traits in determining the occurrence of hover-pollination, begging the question of why plants adapted for hover pollination do not occur outside the New World.