Biology Reference
In-Depth Information
when these are offered at realistically low volumes, resembling the volumes
found in hummingbird flowers (Roberts, 1996). This is far greater than the
average concentration of bird nectars (Pyke & Waser, 1981; Nicolson &
Fleming, 2003b). Sunbirds fed high sucrose concentrations will drink sup-
plementary water if it is available, diluting their food to around 30%
(Nicolson & Fleming, 2003a). There is still no clear explanation for the dif-
ferences between the birds' preferences in laboratory tests and the nectar
properties of their food-flowers. Biophysical models of optimal nectar con-
centration for hummingbird feeding are briefly discussed by Nicolson and
Thornburg (2007, Chapter 5 in this volume), and Table 3 shows that sucrose
intake is fastest at concentrations of 30-45% for representatives of all three
main nectarivore families.
So far I have considered only the water and sugar content of bird nectars.
Amino acid levels in some passerine bird nectars (
Erythrina
and
Aloe
) are
surprisingly high, sometimes exceeding 100 mM in total concentration (S.W.
Nicolson in preparation), but white-bellied sunbirds (
Nectarinia talatala
) are
generally indifferent to the inclusion of amino acids in their diets (C.D.C.
Leseigneur and S.W. Nicolson, in preparation), confirming the findings of a
much older study on hummingbirds (Hainsworth & Wolf, 1976). Despite
low daily maintenance nitrogen requirements, nectarivorous birds are appar-
ently unable to meet their nitrogen needs from the amino acids in nectar, and
require insects or pollen as additional sources (Roxburgh & Pinshow, 2000;
Van Tets & Nicolson, 2000). Pollen ingestion by birds is usually uninten-
tional, except in the case of lorikeets, which harvest large quantities of
pollen (Churchill & Christensen, 1970). Arthropod foraging by humming-
birds in Costa Rica was examined in detail by Stiles (1995). Although
sunbirds feed on both nectar and arthropods, their young receive only ar-
thropod prey. However, the rate at which female Palestine sunbirds
(
Nectarinia osea
) provision their chicks increases in proportion to the sugar
concentration available to the adults (Markman et al., 2002).
Another component of nectar chemistry has been shown to affect the
feeding choices of
N. osea
: this is the alkaloid nicotine. It has a deterrent
effect at the average levels measured in the nectar of
Nicotiana glauca
(So-
lanaceae) (Tadmor-Melamed et al., 2004); this is, however, an invasive plant
in Israel and the responses of the plant's native hummingbird pollinators to
nicotine are not known. Little is known about the effect of secondary com-
pounds in nectar on foraging by avian nectarivores. Nectar of the South
African
Aloe vryheidensis
is dark, with a bitter taste due to phenolic com-
pounds, and attracts larger birds such as bulbuls and white-eyes, which are
more effective pollinators than sunbirds (Johnson et al., 2006).
