Abstract:
Regulation of energy and water are by necessity closely linked in
avian nectarivores, because the easily available sugars in nectar are
accompanied by an excess of water but few electrolytes. In general,
there is convergence in morphology and physiology between three
main lineages of avian nectarivores that have evolved on different
continents – the hummingbirds, sunbirds and honeyeaters. These
birds show similar dependence of sugar preferences on nectar
concentration, high intestinal sucrase activity and rapid absorption of
hexoses via mediated and paracellular routes. There are differences,
however, in how these lineages deal with energy challenges, as well
as processing the large volumes of preformed water ingested in
nectar. While hummingbirds rely on varying renal water reabsorption,
the passerine nectarivores modulate intestinal water absorption
during water loading, thus reducing the impact on the kidneys.
Hummingbirds do not generally cope with salt loading, and have
renal morphology consistent with their ability to produce copious
dilute urine; by contrast, as well as being able to deal with dilute diets,
honeyeaters and sunbirds are more than capable of dealing with
moderately high levels of added electrolytes. And finally, in response
to energy challenge, hummingbirds readily resort to torpor, while the
passerines show renal and digestive responses that allow them to
deal with short-term fasts and rapidly restore energy balance without
using torpor. In conclusion, sunbirds and honeyeaters demonstrate a
degree of physiological plasticity in dealing with digestive and renal
challenges of their nectar diet, while hummingbirds appear to be
more constrained by this diet.
