Adults are quite distinctive with their bold dark head and breast cleanly separated from the white belly. Immature boobies of other species are very similar to Brown Boobies. Young Brown Boobies have darker bellies than Masked and Blue-footed Boobies. As Brown Boobies age, the distinctive breast band becomes more apparent. Young Red-footed Boobies are also quite dark but are smaller, and have even darker bellies and underwings. First-year gannets are larger and lack the breast demarcation line.