# Avian Siblicide (DD-587)

> 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.

- Department: Nature
- First published: 2025-06-02
- Last updated: 2025-06-02
- Permalink: https://detailenjoyer.com/case/avian-siblicide/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-587-F01**: In obligate siblicide, one chick almost always kills the other. _(Write-up · Exhibit A (Obligate or facultative))_
- **DD-587-F02**: The insurance egg hypothesis says a second egg is a backup in case the first fails. _(Write-up · Exhibit D (The insurance egg))_

## Write-up

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

1. **Obligate or facultative**: In obligate siblicide a chick almost always dies. In facultative siblicide it depends on conditions.
2. **Boobies**: Nazca and masked booby A-chicks usually kill or eject their sibling within two days.
3. **A head start**: Eggs hatch days apart, so the first chick is bigger and stronger.
4. **The insurance egg**: The second egg may be a backup in case the first fails.
5. **Parents matter**: Blue-footed booby parents suppress siblicide in the chicks they raise.

## Related case files

- [Albatross Chicks (DD-599)](https://detailenjoyer.com/case/albatross-chicks/): I paired it with another seabird chick story
- [Murmuration (DD-653)](https://detailenjoyer.com/case/murmuration/): I linked another bird behavior that looks strange until it's explained
- [Naturally Selected (DD-432)](https://detailenjoyer.com/case/naturally-selected/): I linked more sibling rivalry in birds

## Reel vs. record

- **Note**: 'Commonly seen in species like eagles, herons, and some seabirds': correct. Boobies are the best-studied seabirds, and egrets and herons are classic facultative cases.
- **Note**: 'Ensures that limited parental resources are concentrated on the chick most likely to survive': I'd add that in obligate species like the Nazca booby, it happens even when food is plentiful. In facultative species, it mostly happens when food is short.
- **Missing**: why lay a second egg at all? The leading explanation is the insurance egg hypothesis: the extra egg is a backup in case the first fails.
- **Note**: My Reel vs. Record note: the footage shows a hatchling heaving a nestmate out of a small cup nest. That's an exemplar of a chick ejecting a nestmate, and I didn't try to identify the species.

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

- Duration: 23.0 s · Published: 2025-06-02
- File: https://detailenjoyer.com/assets/videos/DKaXhCuSP3s.mp4

### Transcript

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!

## Questions & answers

**Q: What is obligate siblicide?**  
A: Whatever the food supply. _(Source: Write-up · Exhibit A (Obligate or facultative) · Wikipedia: Siblicide)_

**Q: Which seabird is a classic obligate siblicide species?**  
A: 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))_

**Q: Why is the first chick usually stronger?**  
A: That's called hatching asynchrony. _(Source: Write-up · Exhibit C (A head start) · Wikipedia: Siblicide)_

**Q: What does the insurance egg hypothesis say?**  
A: 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))_

**Q: What happened when masked booby chicks were raised by blue-footed booby parents?**  
A: Parents' behavior affects siblicide. _(Source: Write-up · Exhibit E (Parents matter) · Lougheed & Anderson, Parent blue-footed boobies suppress siblicidal behavior of offspring (1999))_

## Sources

- [Wikipedia: Siblicide](https://en.wikipedia.org/wiki/Siblicide)
- [Anderson, Evolution of obligate siblicide in boobies, The American Naturalist (1990)](https://doi.org/10.1086/285049)
- [Lougheed & Anderson, Parent blue-footed boobies suppress siblicidal behavior of offspring (1999)](https://doi.org/10.1007/s002650050535)
- [Clifford & Anderson, Experimental demonstration of the insurance value of extra eggs (2001)](https://doi.org/10.1093/beheco/12.3.340)

## 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/