Our understanding of bird pollination systems has changed dramatically in the last few
years. A long-standing paradigm was that hummingbirds and passerine birds select for
different nectar properties in flowers (phylogenetic hypothesis). However, specialist
passerines, such as sunbirds, have similar nectar preferences to hummingbirds and nectar
in plants pollinated by these two bird groups is strongly convergent. Thus, as an alternative
to the existing paradigm, it has been argued that the most useful distinction that can be
drawn is that between specialist and generalist avian nectarivores (feeding niche
hypothesis). This was supported by phylogenetically-controlled analyses that show that
nectar in plants pollinated by specialist avian nectarivores (whether hummingbird or
passerine) tends to have a lower volume, higher concentration and higher sucrose content
than that in plants pollinated by generalist avian nectarivores. The aim of this thesis was to
determine if these trends can be explained by the nectar preferences of avian nectarivores,
and to determine whether the generalist-specialist dichotomy can be applied to Kniphofia,
a largely bird-pollinated African plant genus.
This thesis consists of two sections. In the first, I present data from equicaloric
choice experiments to determine sugar preferences of both specialist and occasional nectar
feeding birds across a range of concentrations. In addition, I determine apparent sugar
assimilation efficiencies and concentration preferences for a range of occasional nectar
feeding birds. Using Malachite Sunbirds (Nectarinia famosa), Dark-capped bulbuls
(Pycnonotus tricolor), Speckled Mousebirds (Colius striatus) and Red-winged Starlings
(Onychognathus morio) (plus Village Weavers (Ploceus cucullatus) in an appendix) as
representatives of the major groups of nectar feeding birds in South Africa, I show that
some differences occur between occasional nectar feeding birds and all specialist nectar
feeding birds studied so far. Specialists show a switch from hexose preference at low (5%)
concentrations, exhibit no preference at medium concentrations (10-20%) and prefer
sucrose or show no preference at high concentrations (25%). However, specialists are
unable to maintain energy balance at low concentrations, and always select the higher
concentration when given choices within this range. Occasional nectar feeding birds prefer
hexose solutions at low and medium concentrations (5 and 10%), but show varied
preferences at higher concentrations. Occasional nectar feeding birds are able to maintain
energy balance at low concentrations, and either show no concentration preference, or a preference for lower concentrations when given a choice. Occasional nectar feeding birds
show a range in ability to digest sucrose, although some species, like the Dark-capped
Bulbul, are quite proficient at it. Surprisingly, no significant difference is found between
specialist and occasional nectarivores sugar preferences when analysed globally, even
when phyllogeny is accounted for. Instead, I found a significant relationship between body
size and bird sugar preference at different concentrations, from which I build the body size
hypothesis, which I suggest is a better predictor to use than bird diet type (specialist or
occasional nectarivores – feeding niche hypothesis).
In the second section of the thesis, I examine the associations between plant traits
and nectarivore nectar preferences. This section focuses on flower morphology and nectar
characteristics in Kniphofia species, and ecotypes within species, pollinated by specialist
versus occasional avian nectarivores. I show that apart from sugar type, which appears to
be phylogenetically constrained, flower morphology and nectar characteristics appear to
diverge according to whether plants are pollinated by generalist or specialist nectar feeding
birds. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10779 |
| Date | 27 May 2014 |
| Creators | Brown, Mark. |
| Contributors | Johnson, Steven D., Downs, Colleen Thelma. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0022 seconds