Correlates and Consequences of Body Size in Nectar-Feeding Birds

Nectar-feeding birds are among the smallest birds and the largestpollinators. Energetic costs of maintenance, temperature regulation,foraging and reproduction increase in direct proportion to bodymass raised to fractional exponents, which may vary from 0.5to 1.0; overall costs probably vary with an exponent of 0.75.Avian nectarivores acquire most of their energy from flowernectar; in so doing they compete with other nectar feeders andpollinate plants. Larger pollinators are more reliable and movepollen greater distances, but to attract them plants must secretemore nectar and protect it from utilization by smaller animals.Minimum body size of avian nectarivores (2g) appears to reflectboth competition with insects and the limited capacity of thesmallest birds to acquire and store energy relative to the demandsof fasting, temperature regulation, and reproduction. Hummingbirdshave attained significantly smaller size than other nectar feedingbirds because lower metabolic rates and use of hypothermic torporreduce their energy expenditure relative to income. Maximumbody size of avian nectarivores (approximately 80g) apparentlyreflects the upper limit of plant energy expenditure for reliable,long distance pollination. Between these limits, size variationreflects divergence to reduce interspecific competition andcoevolution with plants to promote specificity