GEOGRAPHIC VARIATION AND REASSESSMENT OF SPECIES LIMITS IN THE "MASKED" BOOBIES OF THE EASTERN PACIFIC OCEAN

Abstract

Two distinct forms of Masked Booby (S&a dactyl&-a) occur in the eastern Pacific: (1) a yellow-billed form that includes a population on Clipper-ton Island and islands off western Mexico (S. d. cal- ifornica), and another, unnamed, population on Las Islas Desventuradas, Chile, and (2) an orange-billed form (S. (d.) grunti) that nests almost exclusively on the islands of the Galapagos and on Malpelo Island, Colombia. Quantitative comparisons, including discriminant function analysis (DFA) of standard morphological characters indicated that yellow-billed populations are only marginally different from one another, and neither is consistently separable from 5. d. personata, a yellow-billed form that ranges over most of the tropical Pacific. Further, we found no consistent differences in bare-part coloration or plumage among yellow-billed populations. In contrast, DFA indicated morphological differences between orange- and yellow-billed populations. The orange-billed bird is smaller with a significantly shorter, shallower bill, shorter tarsus, and longer wings and tail. It is also more sexually dimorphic and has distinct plumage characters. Biological observations also support the distinctness of orange-billed birds. They typically nest on cliffs and steep slopes, whereas yellow-billed forms nest mainly on low, flat areas. A difference in habitat preference at sea resulted in a parapatric distribution: orange-billed birds away from colonies concentrated in nearshore waters off the coast of the Americas, whereas the yellow-billed forms foraged much farther offshore. Most importantly, orange- and yellow-billed birds paired assortatively where they nested sympatrically. Thus, based on morphological and biological differences, including positive assortative mating, we recommend that SuZa granti be recognized as a separate species, the Nazca Booby. Received 24 May 1997, accepted 30 March 1998. Geographic variation in the Masked (or Blue-faced) Booby (Sulu ductylutru) has been over-described but under-studied. Most of the seven proposed races of this common pan- tropical seabird date from an era of excessive splitting and were based almost entirely on foot and bill coloration. Plumage and size (with one exception) were ignored, perhaps because few collections contained enough ma- terial to allow study of variation in those char-