Orchid–bird interactions: a case study from Angraecum (Vandeae, Angraecinae) and Zosterops (white-eyes, Zosteropidae) on Reunion Island
Although angraecoid orchids are famous for their spectacular flowers, pollinated by hawkmoths in Madagascar, two species of Angraecum endemic to the Mascarenes (sect. Hadrangis ) have atypical unscented and short-spurred flowers that are pollinated by birds. We focused on the pollination and reproductive success of Angraecum bracteosum Balf.f. & S.Moore, a member of sect. Hadrangis. We investigated the breeding system by performing a set of hand-pollination experiments and by observing natural pollinators using video-captures. Pollinator efficiency (pollen removal and deposition) and reproductive success (fruit set) were quantified once a week in two natural island populations during three consecutive flowering seasons (2003–2005). The orchid is self-compatible but requires pollinator services to achieve reproduction. The two white-eye song-bird species (Zosteropidae) that occur on Reunion, Zosterops borbonicus Boddaert and Zosterops olivaceus L., were recorded as pollinators. However, while pollination events were rare and occasional following flower visits by Z. borbonicus, foraging of Z. oliveaceus resulted in a high frequency of successful pollinations. White-eyes typically perched on inflorescences, and probed most of the “fresh-looking” flowers for nectar. This study documents a probable case of bird-pollination specialization within Angraecum section Hadrangis, and sheds light on the role played by pollinators during the radiation of section Hadrangis in the Mascarene Archipelago.