A nectar diet is simple in nutritional compositionand easily digested, but may vary greatly in its proportions ofsugar and water. Here, we apply the geometric framework, amodelling approach for investigating how animals balancenutrient needs in multidimensional and dynamic nutritionalenvironments, to captive whitebellied sunbirds (Cinnyristalatala). We address the question of how these small birds(»8 g) prioritise sugar and water intake, and how dietary saltcontent interacts with sugar and water intake. Sunbirds keptat 20°C and provided with moderate to high sucrose concentrations(¸1 M), together with supplementary water, convergeon an intake target of 2.79 g day¡1 of sucrose and7.72 g day¡1 of water: equivalent to 0.85 M sucrose. Whenthe birds are given more dilute sucrose concentrations, theydefend their sugar intake by over-ingesting water, up to aceiling of 47 g day¡1. Sugar intake thus gets priority overwater intake, but the birds have a Wnite capacity to overingestwater to gain the target level of sugar. Regulationappears to be less precise when birds are given a choicebetween two sucrose solutions than when they choosebetween a sugar solution and supplementary water. Intaketargets vary in response to internal and external factors, andsunbirds increase their sugar intake in response to increasedactivity and cold, irrespective of nectar concentration. Theyalso compensate for interruptions in foraging activity,whether overnight or during the day. Interactive eVectsbecome evident when sodium is included as a third nutrient:on very dilute nectar (·0.1 M), where sunbirds lose bodymass, the addition of sodium to the diet helps to achieve thecarbohydrate intake target, while raising the ceiling on waterintake. This analysis provides a new perspective on nectarivory,while adding to the comparative database on nutrientregulation and emphasising water as a nutrient.
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