Nazca boobies are common residents of Galapagos, with an estimated population between 15,000 – 20,000 pairs. They were formerly treated as a subspecies of the Masked Booby (sula dactylatra), which is why they are often still referred to as Masked boobies. However, they now hold full species status.
Nazca Boobies breed throughout the year, with colonies on different islands nesting at different times. They lay eggs on Genovesa Island between August and November and on Española Island between November and February. Like their blue-footed relatives, they nest on the ground. Males and females are alike, although females tend to be slightly larger with a duller bill. This is the only black and white booby with an orange-yellow bill and has distinctive bright yellow eyes.
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Nazca boobies are known for practicing siblicide. This means that they lay two eggs, several days apart, and if both eggs hatch, the elder chick will kill its sibling by pushing it out of the nest and leaving it to die of hunger or cold. Parent boobies will not intervene in this natural process of selection, and the younger chick will unavoidably die. Scientists believe that this species lays two eggs as an assurance that at least one survives, in case the other dies soon after hatching or is eaten by predators.