Sula sula is the smallest booby, and is well suited for its long flights out to sea in search of prey. It has strong neck muscles, and a wedge-shaped tail. It has a long, tapering bill with serrated cutting edges to help catch and eat its prey. Its external nostrils are closed to allow for plunge-diving, but it has developed secondary nostrils beside the mouth which are automatically covered by flaps when it plunges. The skin around its face is generally bare. The eyes are beside the bill, face forward, and have very pale irises. The wings are long, pointed, and situated fairly far back on the body. They help the bird to fly in high winds (which it does by alternating powerful flaps with gliding) and also to dive. When diving, the wings close around the body of the bird, making it more aerodynamic. The legs are short and strong, and the feet are large and totipalmate (webbed between all four toes). The feet are highly vascular because it is through the feet that heat is transmitted from the parent to the offspring. The legs are set far back on the body, helping it to swim. The feet and legs are also red, thus giving the bird its name. Sula sula is very buoyant, having developed a series of air-sacs which are extensions of the bronchi. They help in its plunge-diving.
The plumage of Sula sula must serve two purposes, camouflage and body temperature maintenance. The underpart of the bird is generally white, so as to provide countershading to help prevent its prey from seeing it. Also, "in an environment where both salt and the sun intensify the effects of feather abrasion, dark, melanin-rich feathers on the upper parts help to provide stronger resistance" (del Hoyo et al., 1992). The plumage is molted continuously so as not to hinder the bird's flight capability at any one time, however, molt is suspended during breeding. Feathers are renewed 1 to 2 times yearly. Sula sula lacks a brood patch because it would interfere with streamlining and insulation. There are several morphs of this bird, often making it difficult to identify without noting the color of its feet. The morphs are: the white-tailed brown morph, the white headed and white tailed brown morph, the white morph (which has black on its wings), the black-tailed white morph (found on the Galapagos Islands), and the golden white morph (found on the Christmas Islands).
Red-footed boobies are 70 to 71 cm long, on average and have a wingspan of 91 to 101 cm. They weigh from 900 to 1003 g. Females tend to be slightly larger than males.
Range mass: 900 to 1003 g.
Range length: 64 to 76 cm.
Average length: 70-71 cm.
Range wingspan: 91 to 101 cm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry ; polymorphic
Sexual Dimorphism: female larger