The blue-footed booby (Sula nebouxii) is a marine bird native to subtropical and tropical regions of the eastern Pacific Ocean.

This results in a growth inequality and size disparity between siblings, leading to facultative siblicide in times of food scarcity.

The blue-footed booby was described by the French naturalist Alphonse Milne-Edwards in 1882 under the current binomial name Sula nebouxii.

[2] The blue-footed booby is on average 81 cm (32 in) long and weighs 1.5 kg (3+1⁄4 lb), with the female being slightly larger than the male.

[9] The Peruvian booby is similar in appearance, but has grey feet, whiter head and neck, and white spots on its wing coverts.

[13] Since the blue-footed booby preys on fish by diving headlong into the water, its nostrils are permanently closed, and it has to breathe through the corners of its mouth.

The blue-footed booby is distributed among the continental coasts of the eastern Pacific Ocean from California to the Galápagos Islands south into Peru.

[13] A booby may use and defend two or three nesting sites, which consist of bare black lava in small divots in the ground, until they develop a preference for one a few weeks before the eggs are laid.

Since their parents had successfully raised chicks to reproductive age, their nest site must have been effective, either by providing cover from predation and parasitism, or by its suitability for taking off and landing.

[16] The blue color of the blue-footed booby's webbed feet comes from structures of aligned collagens in the skin modified by carotenoid pigments obtained from its diet of fresh fish.

Boobies that were experimentally food-deprived for 48 hours experienced a decrease in foot brightness due to a reduction in the amount of lipids and lipoproteins that are used to absorb and transport carotenoids.

[17] As blue feet are signals that reliably indicate the immunological and health condition of a booby, coloration is favored through sexual selection.

The smaller second eggs contained less yolk concentration, which could influence embryo development, hatching success, and subsequent chick growth and survival.

[20] As androgen plays an important role in chick survival, the experiment suggested female blue-footed boobies use the attractiveness and perceived genetic quality of their mates to determine how much resources they should allocate to their eggs.

When the lead bird sees a fish shoal in the water, it signals to the rest of the group and they all dive in unison, pointing their bodies down like arrows.

[23] The courtship of the blue-footed booby consists of the male flaunting his blue feet and dancing to impress the female.

[24] The dance also includes "sky-pointing", which involves the male pointing his head and bill up to the sky while keeping the wings and tail raised.

[5] Like other sexually size-dimorphic birds, female blue-footed boobies usually favor the smaller sex during times of food scarcity.

Booby chicks do not show clear differences in size based on sex, but females do grow faster than males, which means they require greater parental investment.

Blue-footed boobies display behavior that is described in the flexible investment hypothesis, which states that a female adjusts the allocation of resources to maximize her lifetime reproductive success.

Subordinate chicks in asynchronous broods die more quickly, thus relieving the parents of the burden of feeding both offspring when resources are insufficient to properly do so.

[30] Blue-footed booby chicks practice facultative siblicide, opting to cause the death of a sibling based on environmental conditions.

Experiments in which the necks of chicks were taped to inhibit food ingestion showed that sibling aggression increased sharply when the weight of the A-chicks dropped below 20-25% of their potential.

A steep increase in pecking occurred below that threshold, indicating that siblicide is, in part, triggered by the dominant chick's weight, and not simply by the size difference between the siblings.

Booby parents even appear to facilitate the demise of the younger sibling by creating and maintaining the inequality between the two chicks.

Blue-footed booby parents make steep-sided nests that serve to deter the early ejection of the younger chick by the older sibling.

This is in direct contrast to the masked booby, a species in which siblicide is obligate due to the ease in which older siblings can eject younger chicks from their flat nests.

[35] Egg-mass analysis shows that in clutches produced at the beginning of the breeding season, the second egg in a nest were, on average, 1.5% heavier than the first.

Although dominant A-chicks grow faster and survive past infancy more often than subordinate B-chicks, no difference in reproductive success is seen between the two types of siblings during adulthood.

The decline is feared to be long-term, but annual data collection is needed for a firm conclusion that this is not a normal fluctuation.