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Thank you Lil, kmunroe, Jude, Ashley, Eric, Don, Socrate, Conrad and David.
All the boobies are closely related to the gannets. And they are similarly sized.
A native species of hawk is about the only predator on the Galapagos, and they can take anything small- including fairly large iguanas. But the greatest danger to booby eggs and the very young is the heat from the sun- so they are always protected at the nest by one or the other of the parents.
Siblicide isn't uncommon. i.e. bald eagle newborns sometimes practice the procedure- depending on whether there's enough food. I've seen pics where the biggest baby is swallowing its sibling. Again- often the second egg is just an insurance - in case the first-born isn't healthy. When birds live long lives, over-production of babies is not critical to the survival of the species. Theoretically, each mating pair needs to raise two babies to reproduction age for the species to survive.