Black noddies are medium-sized terns with uniformly dark, sooty plumage and a white cap, the reverse of the common tern color pattern. The white cap blends gradually into the grey of the body. They measure 35 to 40 cm in length, 65 to 72 cm in wingspan, and weigh from 85 to 140 grams. They have small, white markings on the lower and upper rims of their eyelids. The bill is black and the legs and feet are reddish-brown to orange. The mouth lining and tongue is orange-yellow in the northwestern Hawaiian Islands and Atlantic populations, yellow in Caribbean Islands, and the mouth lining is pink and the tongue yellow in Australia. The tail is 105 to 130 mm and is wedge-shaped with a central notch. Males and females are similar, juveniles are also similar, but are slightly more pale in color and the white cap is sharply differentiated from the gray body plumage.
There is some slight geographic variation in size, plumage color, and foot color of black noddies. Seven subspecies are recognized: A. m. americanus (Caribbean populations), A. m. atlanticus (tropical Atlantic), A. m. diamesus (Clipperton and Cocos Islands), A. m. melanogenys (main island of Hawaii, also called the "light phase" form), A. m. marcusi (northwest Pacific, including northwestern Hawaiian islands, also called the "dark phase" form), A. m. worcesteri (Sulu and Java seas), and A. m. minutus (Flores Sea and southwestern Pacific).
Black noddies are considered conspecific with their close relatives, lesser noddies (Anous tenuirostris), by some authors. Under this classification scheme the two species are known as white-capped noddies, A. tenuirostris. However, black noddies are distinguishable from lesser noddies by size, plumage color, and wing quill formula, as well as their separate nesting habits.
Black noddy basal metabolic rate has been estimated at 54.8 kj/d. Mean oxygen consumption is 1.28 cubic centimeters/g/hr. Mean daytime body temperature of adults while incubating is 39.5 to 41.9 degrees Celsius (in shade and sun, respectively).
Range mass: 85 to 140 g.
Range length: 35 to 40 cm.
Range wingspan: 65 to 72 cm.
Average basal metabolic rate: 1.28 cm^3 oxygen/hour.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike