Quick Answer
Infanticide, where baby birds kill each other in the nest, does sometimes occur in certain bird species but is not common overall. The level of siblicide, as it is known scientifically, varies greatly between different species based on factors like food availability, birth order, and number of offspring in the nest. In most songbirds, siblicide is rare, though it can be seen more often in birds of prey like eagles or owls. Generally, siblicide only occurs when resources are scarce and gives a competitive advantage to the surviving nestling.
Siblicide in Birds of Prey
Birds of prey like eagles, hawks, and owls are among the most likely to commit siblicide. This is due to their hatching pattern where the first chick hatches days before the second, leading to a large size disparity between siblings. The older nestling has a major advantage in competing for food brought by the parents and will often kill the younger chick if food is scarce.
For example, in bald eagles, the first eaglet typically hatches 3-5 days before the second. During times of famine, the older and larger firstborn may kill its younger sibling, eliminating a competitor for the limited food supply brought by the parents. This increases the chance of survival for the remaining nestling. Similar patterns are seen in other raptor species like great horned owls. Siblicide allows the strongest nestling to survive periods of low food availability.
Facultative vs. Obligate Siblicide
There are two main types of siblicide seen in birds of prey:
Facultative Siblicide: The dominant nestling kills the subordinate one only when resources like food are scarce. This optional siblicide maximizes survival in tough conditions.
Obligate Siblicide: The killing of nestmates happens invariably, even when food is plentiful. The slightly older firstborn is hardwired to kill the second hatchling. This obligate siblicide is seen in some eagle and owl species.
So in most cases, siblicide is a facultative response to starvation rather than obligatory behavior. The parents may try to prevent it by separating chicks or ensuring adequate food.
Siblicide in Other Bird Species
While less common, siblicide can occur in some other bird families besides raptors:
– Seabirds: In times of food shortage, Nazca booby chicks and blue-footed booby chicks may kill their siblings in the nest. Similar behavior is seen in pelicans, skuas, and gulls.
– Songbirds: Scarce resources can sometimes lead to fatal sibling aggression in certain songbirds too. Examples include the house wren and great reed warbler.
– Black egrets: Older chicks will peck and kill younger siblings, often when the brood size is too large for the parents to adequately feed.
– White storks: When population densities get too high, white stork nestlings will kill their siblings to reduce competition for limited food from the parents.
So while not as common as in raptors, siblicide can arise in a range of bird taxa when environmental conditions like food availability deteriorate. It allows the strongest in the brood to survive.
What Triggers Siblicide in Birds?
Several key factors can lead to increased rates of siblicide and nestling aggression:
– Hatching asynchrony – When chicks hatch days apart, size disparities enable older siblings to easily kill younger ones. This is a major trigger in most raptors.
– Food scarcity – When the parents cannot provide enough food, aggressive siblings will act to reduce competition.
– Brood size – Larger broods make adequate provisioning harder, increasing siblicide risk. Parents may lay fewer eggs to compensate.
– Nest space – Cramped nests can raise stress and trigger fatal fights between chicks.
– Adverse weather – Cold, wet, or otherwise extreme weather can make food scarce for the parents, leading to siblicide.
– Population density – High density and intraspecific competition for resources can increase siblicide rates.
– Birth order – Being younger and smaller in species with hatching asynchrony raises vulnerability.
By adjusting these factors, parents can reduce the chance of deadly sibling rivalry emerging.
Do Both Parents and Offspring Benefit from Siblicide?
Siblicide presents an evolutionary puzzle – how can a behavior that reduces reproductive success be adaptive for the parents? While nestlings clearly benefit by reducing competition, why would parents favor offspring that limit their reproductive output by killing siblings?
Several evolutionary hypotheses have been proposed:
– Brood reduction hypothesis – Siblicide represents a form of adaptive brood reduction that maximizes parental fitness in tough times by sacrificing some offspring to ensure adequate resources for the remainder.
– Parent-offspring conflict – Nestlings are acting solely in their own genetic interests, against those of parents trying to raise as many chicks as possible.
– Insurance egg hypothesis – Parents may benefit from extra ‘insurance’ eggs and chicks, even if they expect some losses to siblicide.
– Signal of need hypothesis – Siblicide may signal hunger levels to parents, causing them to increase feeding rates.
The benefit to parents likely depends on the ecological context and is still debated. But for nestlings, siblicide improves survival when resources are scarce.
How Common is Siblicide Overall in Birds?
While vividly exemplified in certain bird families, siblicide is relatively uncommon across most bird species:
– It is rare in songbirds, which make up over half of all bird species. Parents often separate chicks if aggression arises.
– In many colonial seabirds, food is abundantly available, so there is little benefit to siblicide.
– In many precocial species like ducks, chicks feed themselves and are less in direct competition.
– Most passerines fledge nestlings at a similar size, reducing disadvantage for later chicks.
– Many tropical species can raise multiple broods in a season, reducing pressure.
So while spectacular examples like eagles exist, parental strategies in most birds seem to mitigate or prevent serious siblicide. It is an infrequent reproductive strategy overall, occurring only when specific ecological and life history factors align to favor it. But where it does occur, it can help optimize parental fitness in challenging conditions.
Does Siblicide Happen in Other Animal Species?
Fatal sibling aggression is seen in a range of other animal taxa as well:
– Hyena cubs will attack and kill weaker siblings shortly after birth. This is related to their unusual reproductive biology.
– Shark pups will cannibalize each other in utero, a practice known as intrauterine oophagy.
– Predaceous mite hatchlings will readily eat neighboring eggs and siblings when they hatch.
– Several groups of fish like salmonids exhibit extreme siblicide and cannibalism of juveniles by larger individuals.
– Many spiders, including black widows, will cannibalize siblings before dispersing from the egg sac.
– Tiger salamander larvae have been observed killing and eating nearby siblings when resources are scarce.
So siblicide exists across diverse animal taxa, but is especially prominent among certain birds of prey like eagles and owls where specific ecological factors favor it. While shocking, it can serve to optimize survival in challenging conditions.
Conclusion
In summary, siblicide or fatal sibling rivalry does occur in some bird species, especially raptors like eagles and owls. It is triggered primarily by hatching asynchrony and food scarcity which gives older nestlings a major advantage over younger siblings. By eliminating rivals, siblicide maximizes resource access for the remaining chick and optimizes the reproductive success of the parents under ecological constraints. However, such behavior is not common or inevitable across most bird families. Through strategies like brood reduction, synchronized hatching, supplementary feeding, and hatching only compatible brood sizes, parents in most species are able to avoid serious siblicide and raise offspring successfully. So while an important reproductive strategy in certain taxa, cannibalistic nestling behavior is relatively rare in birds overall and seen only under specific challenging conditions.
