NECTAR CONSUMERS - Nectaries and Nectar - page 316

Figure 5. Scale-throated hermit Phaetornis eurynome , Brazil. (Photo: C. Purchase.)
more accurate than saying that bird flowers are red to exclude bees (Chittka
& Waser, 1997). As already mentioned, bees often visit bird flowers, in spite
of dilute nectar. Birds will also take concentrated nectar, such as the sunbirds
that forage extensively on the viscous nectar (60% w/w) of Lobelia telekii in
Kenya (Evans, 1996). Interestingly, hummingbirds are commonly provided
with artificial nectar in both North and South America (Fig. 5), a practice
that is influencing bird densities, with associated effects on the plants they
pollinate (Streisfeld & Kohn, 2006).
A diet of dilute nectar, varying in concentration in time and space, has a
major impact on the energy and water balance of birds, requiring close integra-
tion of the intestinal and renal systems (Beuchat et al., 1990). Nectarivorous
birds exhibit compensatory feeding, increasing the volume of food consumed
in response to diet dilution or when energy demands increase due to low
temperatures (reviewed by Martínez del Rio et al., 2001). This is illustrated
in Fig. 6 for white-bellied sunbirds, Nectarinia talatala , which maintain con-
stant energy intake on sucrose solutions varying tenfold in concentration
from 0.25 to 2.5 M (Nicolson & Fleming, 2003a). The ingestion of large vol-
umes of dilute nectar results in extreme water fluxes in birds, up to five
times body mass per day, and chronic diuresis. Alhough it was predicted by
Beuchat et al. (1990) that hummingbirds would be able to modulate the rate
of intestinal water absorption according to diet concentration, and thus