Pitman and Jehl have shown that yellow-billed forms of the Masked Booby (Sula dactylatra) are only marginally different from one another. The orange-billed form S. (d.) granti is smaller with a proportionately shorter and shallower bill, shorter tarsus, and longer wings and tail.

It, moreover, shows a stronger sexual dimorphism, and typically nests on cliffs and steep slopes, rather than on low, flat areas. Most importantly, orange- and yellow-billed birds paired assortatively where they nested sympatrically.

Nazca Booby     Sula granti
Rothschild 1902
Ecuador: Galapagos Islands; Colombia: Malpelo Island
split from: Sula dactylatra
insert after: Sula dactylatra
R.L. Pitman & J.R. Jehl,
Geographic variation and reassessment of species limits in the "Masked" Boobies of the eastern Pacific Ocean
Wilson Bulletin 110, 2 (1998): 155-170