The closely related masked and Nazca boobies are both "obligately siblicidal" species, while the blue-footed booby is a "facultatively siblicidal" species. In a facultatively siblicidal species, aggression occurs between siblings but is not always lethal, whereas in an obligately siblicidal species, aggression between siblings always leads to the death of one of the offspring. All three species have an average brood size of two eggs, which are laid within approximately four days of each other. In the few days before the second egg hatches, the first-born chick, known as the senior chick or A-chick, enjoys a period of growth and development during which it has full access to resources provided by the parent bird. Therefore, when the junior chick (B-chick) hatches, there is a significant disparity in size and strength between it and its older sibling.