Biology Reference
In-Depth Information
and corolla length usually determines whether nectar-feeding birds are legiti-
mate pollinators or nectar robbers, although the two categories are not distinct
(Maloof & Inouye, 2000). When nectar is scarce, honeydew and sugary exu-
dates from plants may be alternative carbohydrate sources for birds (Paton,
1980; Gaze & Clout, 1983). Stiles (1981) contributed an excellent and wide-
ranging review of geographical differences in bird-flower associations. The
small size of nectar-feeding birds seems to be determined by co-evolution
with flowering plants and competition with the largest insect pollinators,
bees and hawkmoths, which are also endotherms; and it may be the use of
torpor that has enabled hummingbirds to be significantly smaller than other
nectar-feeding birds (Brown et al., 1978; Cotton, 1996).
Bird flowers are usually large and robust, although sometimes massed
small flowers can provide an equivalent reward (Castro & Robertson, 1997).
While tubular shapes are very common, generalist brush blossoms are also
important nectar sources (Stiles, 1981), especially for passerine nectarivores
visiting flowers of Myrtaceae and Proteaceae in Australia and southern Africa.
The copious nectar of bird flowers is characteristically dilute compared to that
of insect-pollinated flowers. For example, Nicolson and Fleming (2003b) sum-
marized data from the literature and obtained means of 25% (w/w) for the
nectars of 255 hummingbird species and 21% for 158 sunbird species. These
can be compared with 36% for the nectars of 156 species of bee flowers
(Pyke & Waser, 1981). Various hypotheses have been proposed to account for
the relatively low sugar concentration of bird nectars:
•
The viscosity of nectar increases exponentially with concentration, and low
viscosities enable more efficient extraction of nectar by bird tongues, espe-
cially during hovering (Baker, 1975).
•
Low concentrations serve to discourage bees (Bolten & Feinsinger, 1978).
•
Dilute nectars are necessary to meet the water requirements of birds, es-
pecially at high ambient temperatures (Baker, 1975; Calder, 1979).
•
Nectars of “bird” flowers remain dilute because they are protected from
evaporation by tubular corollas (Plowright, 1987).
•
Dilute nectars may encourage birds to visit more flowers by not satiating
their energy requirements immediately (Martínez del Rio et al., 2001).
•
Dilute nectars are a consequence of hydrolysis of sucrose to glucose and
fructose: this maintains the gradient for sucrose transport and the in-
creased osmolality draws additional water from the nectary (Nicolson,
1998, 2002).
The question is still unresolved, and a combination of these factors is
probably involved. Saying that nectar is dilute to discourage bees may be no
