Hummingbird

Hummingbirds are New World birds that constitute the family Trochilidae. They are among the smallest of birds, most species measuring 7.5–13 cm (3–5 in). Indeed, the smallest extant bird species is a hummingbird,
the 5-cm bee hummingbird weighing less than a U.S. penny (2.5 g).
They are known as hummingbirds because of the humming sound created by their beating wings which flap at high frequencies audible to humans. They hover in mid-air at rapid wing-flapping rates, typically around 50 times per second, allowing them also to fly at speeds exceeding 15 m/s (54 km/h; 34 mph).
Hummingbirds have the highest metabolism of any homeothermic animal. To conserve energy when food is scarce, and nightly when not foraging, they go into torpor, a state similar to hibernation, slowing metabolic rate to 1/15th of its normal rate.


Sexual dimorphisms
Hummingbirds exhibit sexual size dimorphism according to Rensch's rule, in which males are smaller than females in small species, and males are larger than females in large-bodied species.The extent of this sexual size difference varies among clades of hummingbirds. For example, the Mellisugini clade exhibits a large size dimorphism, with females being larger than males. Conversely, the Lophomithini clade displays very little size dimorphism; males and females are similar in size.Sexual dimorphisms in beak size and shape are also present between male and female hummingbirds, where in many clades, females have longer, more curved beaks favored for accessing nectar from tall flowers. For males and females of the same size, females will tend to have larger beaks.
Sexual size and beak differences likely evolved due to constraints imposed by courtship because mating displays of male hummingbirds require complex aerial maneuvers and are costly in terms of energy. Males tend to be smaller than females, allowing conservation of energy to forage competitively and participate more frequently in courtship. Thus, sexual selection will favor smaller male hummingbirds.
Female hummingbirds tend to be larger, requiring more energy, and their beaks longer to access preferred flowers. Female hummingbirds tend to have longer beaks that allow for more effective reach into crevices of tall flowers for nectar.Thus females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size. Directional selection will thus favor the larger hummingbirds in terms of acquiring food.
Another evolutionary cause of this sexual bill dimorphism is that the selective forces from competition for nectar between the sexes of each species are what drive the sexual dimorphism. Depending on which sex holds territory in the species, it is advantageous for the other sex to have a longer bill and be able to feed on a wide variety of flowers, decreasing intraspecific competition. For example, in species of hummingbirds where males have longer bills, males do not hold a specific territory and have a lek mating system. In species where males have shorter bills than females, males defend their resources and therefore females must have a longer bill in order to feed from a broader range of flower.