Male Calypte anna - used with permission from


Anna’s hummingbird is unique organism because of its relatively small size.  Only weighing 3-6 grams and measuring about 10 centimeters long, Anna’s hummingbird is much smaller than the typical flying vertebrate (The Cornell Lab of Ornithology 2011).  Their bills are short and straight that connects to a long, flat forehead.  Anna’s hummingbirds have a green iridescent back and a gray colored stomach (BirdWeb).  As observed in many species, the males are brightly colored and easy to identify because their entire heads and throats are colored a flashy iridescent red (BirdWeb).  The females have a small amount of red on their throats but have a mostly green head (BirdWeb). 

Though Calypte anna is a medium-sized hummingbird, it is much smaller than almost every other species of bird creating challenges that the organism must evolutionarily overcome.  One huge challenge faced by Anna’s hummingbird is regulating their metabolism because of their small size and fast wingbeat.  Another challenge that Anna’s hummingbirds face is flying in the rain, which is problematic because of their small size yet again.  

Female Calypte anna in sustained hovering flight - obtained from wikipedia.comHummingbirds’ exhibit extremely high mass-specific metabolic rates partially because of their small size and partially because of their unique attribute of being able to sustain hovering flight (Evangelista et al. 2010).  With being only about 10 cm long and weighing only 3-6 grams, Anna’s hummingbird faces the challenge of sustaining adequate body temperature because their surface area is much greater than their volume, meaning they have more area for heat lost (The Wild Bird Store 2014).  A hummingbird often has to eat its weight in nectar every day in order to keep up with their high metabolic demands (The Wild Bird Store 2014)!  One way hummingbirds can overcome this massive heat loss is by entering nocturnal torpor, a significantly lowered body temperature at night (Evangelista et al. 2010).  Another way that is believed to compensate for this costly mass-specific metabolic rate is that Anna’s hummingbird passively absorbs glucose in their intestines instead of relying solely on cell mediated transport, saving them energy when digesting food (McWhorter et al. 2005).  Sustained hovering is a very demanding and costly form of flying that requires high metabolic input (Evangelista et al. 2010). 

Sustained hovering requires both aerobic and anaerobic metabolism, demonstrating the demanding energetic cost of this action (Altshuler et al. 2010).   Selection has favored hummingbirds that have fast oxidative-glycolytic muscle fibers in their wings which allows for quick response and fatigue resistence (Altshuler et al. 2010).  The hummingbird has also developed (likely through selection) a high degree of synchrony within the pectoral muscles and a specialized motor endplate pattern which includes two curivilinear bands which concentrate the motor endplates to the center of the muscles (Altshuler et al. 2010). This differs from the regular grid-like pattern observed in all other birds and allows for the rapid beating of the hummingbirds wings (Altshuler et al. 2010).

Another structural adaption acquired by Anna’s hummingbird is hydrophobic wings (Ortega-Jimenez, V.M. and R. Dudley 2012).  Due to their high metabolic demands, hummingbirds cannot go more than a day without feeding, requiring adaptions to all weather conditions including the heaviest of rains.  Rain poses a challenge to flight because the water droplets can alter the mass of the wings and body of birds, impacting the wing’s inertia as well as requiring more power to stay airborne against the downward force of the rain drops (Ortega-Jimenez, V.M. and R. Dudley 2012).  Anna’s hummingbird has wings which are both water repellant and wings that deter water penetration which allows them to remain highly active without any loss of control during even the heaviest of rain (Ortega-Jimenez, V.M. and R. Dudley 2012).

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