Nectar intake and foraging efficiency : the responses of sunbirds to flower morphology

Abstract

Floral traits vary greatly between plant species, and determine which pollinators are physically capable of accessing floral rewards and carrying out effective pollination. Research on the responses of nectarivorous birds to different flower morphologies has been largely restricted to hummingbirds, while other flower specialists, the sunbirds and honeyeaters, remain relatively unstudied. We investigated how flower morphology influences the foraging behaviour of White-bellied Sunbirds, Cinnyris talatala. Using artificial flowers made of clear polyvinyl chloride tubing and filled with 20% w/w sucrose solution, we measured maximum extraction depths of sunbirds foraging at 3- and 5-mm-wide flowers at both upward and downward orientations. Flower width, but not orientation, strongly influenced maximum extraction depth, with the birds probing deeper at wider flowers. Foraging bout dynamics were tested at these two diameters and at two corolla lengths, 14 and 27 mm; birds fed faster at wider and at shorter flowers, when length and diameter were tested separately. In combination treatments, sunbirds fed fastest at the short and narrow flowers where the fixed volume of nectar was easier to reach. When given a choice between two floral lengths or two floral diameters, birds preferred short flowers, with diameter having no effect. Floral length appears to be more important than width in determining sunbird foraging behaviour, and White-bellied Sunbirds should be effective pollinators of short tubular flowers. The lack of effect of orientation is perhaps surprising when many sunbird-pollinated plants have downward-facing flowers.