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Nectar preferences of specialist and occasional avian nectarivores, and their role in the evolution of floral traits.

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.

Identiferoai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10779
Date27 May 2014
CreatorsBrown, Mark.
ContributorsJohnson, Steven D., Downs, Colleen Thelma.
Source SetsSouth African National ETD Portal
Languageen_ZA
Detected LanguageEnglish
TypeThesis

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