Variable foraging and flower probing behaviour of sunbird pollinators of the South African Pelargonium fulgidum.

Whether foraging sunbird pollinators hover or perch, the type of perch used, and the direction of flower probing are generally thought to be determined by matching shapes of decurved bird bills and curved flower tubes, as well as inflorescence architecture. However, few studies have objectively quantified aspects of sunbird foraging behaviour to test predictions based on adaptive trait matching. I investigated sunbird foraging in a population of Pelargonium fulgidum, a species with slightly upcurved flowers borne on upward-oriented peduncles, which grows along the west coast of South Africa. Long-range photographs revealed that Cinyris chalybeus, a sunbird species with a slightly decurved bill, hovers or perches during foraging, and uses both the inflorescence and alternative vegetation structures as perch. Contrary to previous suggestions, both male and female sunbirds probe flowers from either of two alternative directions (sternotribic versus nototribic) in more or less equal proportions, and frequently display flexibility in both foraging behaviour and flower probing direction. Predominant or complete sternotribic and nototribic flower probing were explained by hovering versus use of an inflorescence perch respectively, whereas use of an alternative perch resulted in similar proportions of each mode of flower probing. The number of flowers per visited inflorescence and number and proportion of probed flowers did not vary with foraging behaviour or flower probing direction. Although certain foraging behaviours resulted in predictable flower probing directions, variation in foraging behaviour and the presence of many alternative perches provided by vegetation surrounding P. fulgidum plants resulted in variability in flower probing direction that appears not to affect foraging efficiency. Relatively weak reciprocal curvature of flower tube and sunbird bill, resulting in easy access to nectar from multiple feeding directions, likely facilitates flexible foraging. This study shows that environmental context may facilitate more complex foraging behaviour than that predicted by adaptive trait matching. [ABSTRACT FROM AUTHOR]