Nectar preferences of specialist and occasional avian nectarivores, and their role in the evolution of floral traits.
Year: 2009
Type: dissertation
Abstract: Our understanding of bird pollination systems has changed dramatically in the last few \n \nyears. A long-standing paradigm was that hummingbirds and passerine birds select for \n \ndifferent nectar properties in flowers (phylogenetic hypothesis). However, specialist \n \npasserines, such as sunbirds, have similar nectar preferences to hummingbirds and nectar \n \nin plants pollinated by these two bird groups is strongly convergent. Thus, as an alternative \n \nto the existing paradigm, it has been argued that the most useful distinction that can be \n \ndrawn is that between specialist and generalist avian nectarivores (feeding niche \n \nhypothesis). This was supported by phylogenetically-controlled analyses that show that \n \nnectar in plants pollinated by specialist avian nectarivores (whether hummingbird or \n \npasserine) tends to have a lower volume, higher concentration and higher sucrose content \n \nthan that in plants pollinated by generalist avian nectarivores. The aim of this thesis was to \n \ndetermine if these trends can be explained by the nectar preferences of avian nectarivores, \n \nand to determine whether the generalist-specialist dichotomy can be applied to Kniphofia, \n \na largely bird-pollinated African plant genus. \n \nThis thesis consists of two sections. In the first, I present data from equicaloric \n \nchoice experiments to determine sugar preferences of both specialist and occasional nectar \n \nfeeding birds across a range of concentrations. In addition, I determine apparent sugar \n \nassimilation efficiencies and concentration preferences for a range of occasional nectar \n \nfeeding birds. Using Malachite Sunbirds (Nectarinia famosa), Dark-capped bulbuls \n \n(Pycnonotus tricolor), Speckled Mousebirds (Colius striatus) and Red-winged Starlings \n \n(Onychognathus morio) (plus Village Weavers (Ploceus cucullatus) in an appendix) as \n \nrepresentatives of the major groups of nectar feeding birds in South Africa, I show that \n \nsome differences occur between occasional nectar feeding birds and all specialist nectar \n \nfeeding birds studied so far. Specialists show a switch from hexose preference at low (5%) \n \nconcentrations, exhibit no preference at medium concentrations (10-20%) and prefer \n \nsucrose or show no preference at high concentrations (25%). However, specialists are \n \nunable to maintain energy balance at low concentrations, and always select the higher \n \nconcentration when given choices within this range. Occasional nectar feeding birds prefer \n \nhexose solutions at low and medium concentrations (5 and 10%), but show varied \n \npreferences at higher concentrations. Occasional nectar feeding birds are able to maintain \n \nenergy balance at low concentrations, and either show no concentration preference, or a preference for lower concentrations when given a choice. Occasional nectar feeding birds \n \nshow a range in ability to digest sucrose, although some species, like the Dark-capped \n \nBulbul, are quite proficient at it. Surprisingly, no significant difference is found between \n \nspecialist and occasional nectarivores sugar preferences when analysed globally, even \n \nwhen phyllogeny is accounted for. Instead, I found a significant relationship between body \n \nsize and bird sugar preference at different concentrations, from which I build the body size \n \nhypothesis, which I suggest is a better predictor to use than bird diet type (specialist or \n \noccasional nectarivores – feeding niche hypothesis). \n \nIn the second section of the thesis, I examine the associations between plant traits \n \nand nectarivore nectar preferences. This section focuses on flower morphology and nectar \n \ncharacteristics in Kniphofia species, and ecotypes within species, pollinated by specialist \n \nversus occasional avian nectarivores. I show that apart from sugar type, which appears to \n \nbe phylogenetically constrained, flower morphology and nectar characteristics appear to \n \ndiverge according to whether plants are pollinated by generalist or specialist nectar feeding \n \nbirds.
Author: Mark Brown
Language: English
Topic: Plant and animal studies
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1608678960