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Avian Siblicide

Short answer: Siblicide is the killing of a young animal by its siblings, and it is best known in birds. Biologists split it into two kinds.

In obligate siblicide, one chick almost always kills the other, whatever the food supply. Masked boobies (Sula dactylatra) and Nazca boobies (Sula granti) are classic examples: the first-hatched A-chick starts pecking its sibling as soon as it hatches and usually kills it or pushes it out of the nest scrape within two days. In facultative siblicide, siblings fight but killing depends on conditions. Blue-footed booby (Sula nebouxii) chicks, for instance, usually kill a sibling only when food is short. More bird species are facultatively siblicidal than obligately so. Hatching asynchrony sets up the contest: the eggs are laid days apart, so the older chick gets a head start in size and strength. Cattle egrets (Bubulcus ibis) add higher hormone levels in the first eggs. A leading explanation for why parents lay a second egg at all is the insurance egg hypothesis, proposed by Dorward in 1962: the extra egg is a backup in case the first fails. Parents' behavior matters too: in a cross-fostering study, masked booby chicks raised by blue-footed booby parents committed siblicide less often. The footage shows a naked hatchling heaving a nestmate up and over the rim of a small nest.

VideoWrite-up4 sources
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What should you know about Avian Siblicide?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. In obligate siblicide, one chick almost always kills the other.

    DD-587-F01 · Write-up · Exhibit A (Obligate or facultative)

  2. The insurance egg hypothesis says a second egg is a backup in case the first fails.

    DD-587-F02 · Write-up · Exhibit D (The insurance egg)

Video transcript: Avian Siblicide: Some Chicks Kill Their Own Siblings

DID YOU KNOW? Baby birds pushing weaker siblings out of nests is a behavior known as siblicide, commonly seen in species like eagles, herons, and some seabirds. In these cases, the first hatched chick, often stronger and more developed, will aggressively peck at or physically eject younger, weaker nestmates. This brutal strategy ensures that limited parental resources like food, are concentrated on the chick most likely to survive. NOW YOU KNOW!

Burned-in captions, read frame by frame (see transcript.md) · 23.0s Reel ·

Video coming soonAvian Siblicide · Instagram Reel
Avian Siblicide: Some Chicks Kill Their Own Siblings Instagram Reel · · 23.0s

The write-up

Some chicks kill their own siblings. Nazca booby chicks almost always kill their younger sibling.

  1. Exhibit A

    Obligate or facultative

    In obligate siblicide a chick almost always dies. In facultative siblicide it depends on conditions.

  2. Exhibit B

    Boobies

    Nazca and masked booby A-chicks usually kill or eject their sibling within two days.

  3. Exhibit C

    A head start

    Eggs hatch days apart, so the first chick is bigger and stronger.

  4. Exhibit D

    The insurance egg

    The second egg may be a backup in case the first fails.

  5. Exhibit E

    Parents matter

    Blue-footed booby parents suppress siblicide in the chicks they raise.

Questions people ask about Avian Siblicide

What is obligate siblicide?

Whatever the food supply.

Source: Write-up · Exhibit A (Obligate or facultative) · Wikipedia: Siblicide

Which seabird is a classic obligate siblicide species?

Its A-chick usually kills its sibling within two days.

Source: Write-up · Exhibit B (Boobies) · Wikipedia: Siblicide; Anderson, Evolution of obligate siblicide in boobies, The American Naturalist (1990)

Why is the first chick usually stronger?

That's called hatching asynchrony.

Source: Write-up · Exhibit C (A head start) · Wikipedia: Siblicide

What does the insurance egg hypothesis say?

It was proposed by Dorward in 1962.

Source: Write-up · Exhibit D (The insurance egg) · Anderson, Evolution of obligate siblicide in boobies, The American Naturalist (1990); Clifford & Anderson, Experimental demonstration of the insurance value of extra eggs (2001)

What happened when masked booby chicks were raised by blue-footed booby parents?

Parents' behavior affects siblicide.

Source: Write-up · Exhibit E (Parents matter) · Lougheed & Anderson, Parent blue-footed boobies suppress siblicidal behavior of offspring (1999)

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. What is obligate siblicide?

    1. One chick almost always kills the other
    2. Siblings never fight
    3. Parents remove a chick
    4. Chicks share food equally
    Show the answer

    Answer: A. One chick almost always kills the other Whatever the food supply. Source: Write-up · Exhibit A (Obligate or facultative) · Wikipedia: Siblicide

  2. Which seabird is a classic obligate siblicide species?

    1. Nazca booby
    2. Blue-footed booby
    3. Mallard
    4. Puffin
    Show the answer

    Answer: A. Nazca booby Its A-chick usually kills its sibling within two days. Source: Write-up · Exhibit B (Boobies) · Wikipedia: Siblicide; Anderson, Evolution of obligate siblicide in boobies, The American Naturalist (1990)

  3. Why is the first chick usually stronger?

    1. It hatches days earlier
    2. It's always male
    3. It eats the eggshell
    4. It's fed by a different parent
    Show the answer

    Answer: A. It hatches days earlier That's called hatching asynchrony. Source: Write-up · Exhibit C (A head start) · Wikipedia: Siblicide

  4. What does the insurance egg hypothesis say?

    1. The second egg is a backup
    2. Eggs are laid to feed predators
    3. Two eggs keep each other warm
    4. Eggs insure the nest against storms
    Show the answer

    Answer: A. The second egg is a backup It was proposed by Dorward in 1962. Source: Write-up · Exhibit D (The insurance egg) · Anderson, Evolution of obligate siblicide in boobies, The American Naturalist (1990); Clifford & Anderson, Experimental demonstration of the insurance value of extra eggs (2001)

  5. What happened when masked booby chicks were raised by blue-footed booby parents?

    1. Siblicide was less frequent
    2. Siblicide stopped growing
    3. The chicks turned blue
    4. Nothing changed
    Show the answer

    Answer: A. Siblicide was less frequent Parents' behavior affects siblicide. Source: Write-up · Exhibit E (Parents matter) · Lougheed & Anderson, Parent blue-footed boobies suppress siblicidal behavior of offspring (1999)

Cite this case file

Easterling, Marcus. "Avian Siblicide." The Detail Department (DetailEnjoyer), case file DD-587, last updated Jun 2, 2025. https://detailenjoyer.com/case/avian-siblicide/

Permalink: https://detailenjoyer.com/case/avian-siblicide/ · Last updated

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