Correlates and Consequences of Body Size in Nectar-Feeding Birds

synopsis. Nectar-feeding birds are among the smallest birds and the largest pollinators. Energetic costs of maintenance, temperature regulation, foraging and reproduction increase in direct proportion to body mass raised to fractional exponents, which may vary from 0.5 to 1.0; overall costs probably vary with an exponent of 0.75. Avian nectarivores acquire most of their energy from flower nectar; in so doing they compete with other nectar feeders and pollinate plants. Larger pollinators are more reliable and move pollen greater distances, but to attract them plants must secrete more nectar and protect it from utilization by smaller animals. Minimum body size of avian nectarivores (2g) appears to reflect both competition with insects and the limited capacity of the smallest birds to acquire and store energy relative to the demands of fasting, temperature regulation, and reproduction. Hummingbirds have attained significantly smaller size than other nectar feeding birds because lower metabolic rates and use of hypothermic torpor reduce their energy expenditure relative to income. Maximum body size of avian nectarivores (approximately 80g) apparently reflects the upper limit of plant energy expenditure for reliable, long distance pollination. Between these limits, size variation reflects divergence to reduce interspecific competition and coevolution with plants to promote specificity.