A Pollinator's Perch

Credit: Image courtesy of Oxford University Press (OUP)

Small South African irises in the genus Babiana have evolved an upright perch to attract sunbirds to pollinate them.

The research consisted of fieldwork across the western Cape, examining Babiana and making observations of the sunbirds that pollinate them. Barrett said: "What we found is when sunbirds visit Babiana ringens, they always land on this perch and then turn upside down to feed on nectar from the the ground-level flowers. In contrast Babiana carminea doesn't have a perch, and here we found that sunbirds land on the ground and probe flowers to feed on nectar. Ground level flowers are highly unusual in bird-pollinated plants because of the likely predation risk and to find two contrasting feeding strategies among related species is remarkable."

"Babiana avicularis has a perch, but these plants are only visited by the southern double collared sunbird. This is a smaller sunbird, and it looks like Babiana avicularis may have evolved a specialised relationship with it. Larger malachite sunbirds don't visit this plant but are the exclusive pollinators of Babiana ringens which has larger flowers. No one had seen birds pollinate this plant before and it's now one of the smallest bird-pollinated plants known."

The image above shows a sunbird feeding on a Babiana flower from one of those evolved perches. There is also some variation in the size of the perch, with the perch length being longer wherever sunbirds visited Babiana more frequently.

"What we also found with Babiana ringens is that the perch length was smaller in the east of our survey area. This is the region where we saw fewest sunbird visits to plants. So with fewer pollinators around there's less natural selection to maintain large perches, and it also makes sense to have self-pollination as a back-up when the alternative may be no pollination at all."

Barrett is pleased that the work provides insight into the most specialized bird perch in the plant kingdom, but warns that there is still much that we do not know about how the perch evolved in the first place: "Our research shows a clear functional basis between perches and bird pollination, but that's not all it shows. At the same time we're also seeing self-pollination and it's clear Babiana can adopt different mating strategies in different locations depending on the availability of sunbirds."

The study was published in Annals of Botany.