The extra “excess” egg is possibly laid either due to exploit the possibility of elevated food abundance (as seen in the Blue-footed Booby, Sula nebouxii) or due to the chance of sterility in one egg.
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The closely related Masked and Nazca Boobies (Masked Booby and Nazca Booby) are both obligately siblicidal species, while the Blue-footed Booby is a facultatively siblicidal species.
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Cattle Egrets, Bubulcus ibis, exhibit asynchronous hatching and androgen loading in the first two eggs of their normal three-egg clutch.