Why do masked boobies and brown boobies usually raise only one chick after laying two eggs?

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Three eastern white pelicans with black-tipped wings take off from a muddy shoreline, wings outstretched, with blurred green vegetation in the background. — Pelecaniform European white pelicans (*Pelecanus onocrotalus*) in flight.

Masked and brown boobies typically raise only one chick because the older nestling usually forces the younger one out of the nest shortly after hatching.1

While these species lay two eggs, the second egg serves primarily as an insurance policy against the infertility, breakage, or loss of the first egg. This behavior is common among these ground-nesting boobies, whereas other species, such as the tree-nesting red-footed booby, face lower risks and typically lay only a single egg.1