Hummingbirds fascinate with their tiny size and the rapidity of their wing beats making their wings invisible in flight. Their size, which varies from 2.3 inches (6 cm) for 0.07 oz (2 grams) in the hummingbird-bee (Mellisuga helenae) - the smallest of all birds - to 8.2 inches (21 cm) for 0.7 oz (20 grams) in the giant hummingbird (Patagona gigas), is the cause of many physiological problems. The unfavorable ratio between their weight and their body surface leads to a significant loss of heat. Hummingbirds, therefore, need a lot of energy to maintain their internal heat and consume three to eight times more oxygen at rest than a small sparrow. As a result, their heart stands for 2.5 % of their body mass and beats up to 1,200 times per minute. Hummingbirds' small size, extremely swift flight, and long beak, on the other hand, protect them from regular predators, even though the larger species can be preyed upon by hawks or tree snakes and the smaller ones can be caught in large spider webs or captured by dragonflies, praying mantises, fish, and frogs.