Abstract:
A nectar diet is simple in nutritional composition
and easily digested, but may vary greatly in its proportions of
sugar and water. Here, we apply the geometric framework, a
modelling approach for investigating how animals balance
nutrient needs in multidimensional and dynamic nutritional
environments, to captive whitebellied sunbirds (Cinnyris
talatala). We address the question of how these small birds
(»8 g) prioritise sugar and water intake, and how dietary salt
content interacts with sugar and water intake. Sunbirds kept
at 20°C and provided with moderate to high sucrose concentrations
(¸1 M), together with supplementary water, converge
on an intake target of 2.79 g day¡1 of sucrose and
7.72 g day¡1 of water: equivalent to 0.85 M sucrose. When
the birds are given more dilute sucrose concentrations, they
defend their sugar intake by over-ingesting water, up to a
ceiling of 47 g day¡1. Sugar intake thus gets priority over
water intake, but the birds have a Wnite capacity to overingest
water to gain the target level of sugar. Regulation
appears to be less precise when birds are given a choice
between two sucrose solutions than when they choose
between a sugar solution and supplementary water. Intake
targets vary in response to internal and external factors, and
sunbirds increase their sugar intake in response to increased
activity and cold, irrespective of nectar concentration. They
also compensate for interruptions in foraging activity,
whether overnight or during the day. Interactive eVects
become evident when sodium is included as a third nutrient:
on very dilute nectar (·0.1 M), where sunbirds lose body
mass, the addition of sodium to the diet helps to achieve the
carbohydrate intake target, while raising the ceiling on water
intake. This analysis provides a new perspective on nectarivory,
while adding to the comparative database on nutrient
regulation and emphasising water as a nutrient.
