Birds abandoning or rejecting their young, known as chick ejection, is a relatively common phenomenon in the avian world. There are several reasons why parent birds may get rid of or leave behind some of their chicks.
To Increase Survival Chances of Remaining Chicks
One of the main reasons parent birds may reject some chicks is to increase the survival chances of the remaining chicks in the brood. Birds often lay more eggs than they can realistically raise to adulthood. This overproduction of eggs is a hedge against high mortality rates caused by starvation, predators, and accidents. However, when food becomes scarce or the brood gets too large, parent birds may selectively get rid of the smaller, weaker chicks to focus their care on the stronger individuals that are most likely to survive.
Siblicide, where stronger chicks attack and kill smaller siblings, is common in birds of prey like eagles, hawks, and owls. Parent raptors may encourage this sibling rivalry by not bringing enough food to the nest. The aggression helps thin out the brood so the parents can effectively provide for the survivors. Seabirds like boobies and pelicans also regularly abandon the weaker later hatchlings that have less chance of competing with older, larger siblings.
To Reduce Strain of Raising Chicks
Rearing chicks is extremely demanding for parent birds. The continuous hunting for food and feeding of chicks requires a huge expenditure of time and energy. Birds may abandon some chicks to lighten the burden of raising an entire brood. This is more likely to happen when food resources are scarce or if the parents themselves are in poor physical condition after a tough migration or long winter.
In some bird species, chicks beg loudly and incessantly for food. The constant noise and activity in the nest can take a toll on breeding adults. Getting rid of particularly loud and aggressive chicks reduces parental stress levels and allows the parents to focus on raising the best candidates for survival.
To Cut Losses With Weak or Defective Chicks
Parent birds investing time and energy to raise sick, weak, or disabled chicks that have little chance of surviving to adulthood or reproducing may be an exercise in futility. By abandoning low-quality chicks, the parents conserve resources and increase their lifetime reproductive success. Birds may reject chicks that are undersized, injured, have deformities, or show signs of disease. Culling these doomed individuals early maximizes the parents’ reproductive output.
To Avoid Alerting Predators
Abandoned chicks left in the nest cry loudly, which may attract predators to the area. However, parent birds may intentionally leave some chicks behind to distract predators away from the rest of the brood. While tragic for the rejected chicks, sacrificing some offspring may enable the parents to sneak away with the other chicks and raise them safely.
When Adoption Fails
Sometimes parent birds reject foreign chicks that appear in their nest if they do not recognize them as their own. Adoptions can fail because the parents imprinted on their biological chicks early on. Any sudden chick additions later may disrupt the bonding process and lead to rejection.
When Chicks Fall Out of the Nest
Young nestlings may accidentally fall or fledge prematurely before they are able to fly or take care of themselves. If the parent birds do not or cannot retrieve the fallen chicks, they are often left to die of starvation, exposure, or predation. However, chick ejection is not the cause in these situations.
Due to Reproductive Errors
In rare cases, birds may mistakenly lay eggs in the wrong nest or confuse broods when communally nesting. This can lead to rejection if the parents do not recognize the foreign chicks. Confused males may also discard eggs believing they are not the true parents.
When Is Chick Rejection Normal?
While chick rejection may seem harsh, it is a natural behavior evolved to maximize reproductive success. However, there are situations where too much chick ejection may indicate a wider problem:
- High egg or chick mortality across entire clutch suggests environmental issues like pollution, low food supply, high predator population, or rampant disease.
- Abandonment by otherwise good parents could signal the adults are unwell and unable to care for offspring.
- Ejection of almost an entire brood may point to inexperienced first-time parents.
- Egg rejection can result from chemical signals of infertility or embryos that died in the egg.
How Common Is Chick Abandonment?
Chick rejection occurs in over 1,300 species of birds. It is especially prevalent in seabirds, birds of prey, and precocial birds like ducks and chickens. Here are rejection rates for some example species:
Species | Chick Rejection Rate |
---|---|
Great Blue Herons | 38% |
Boobies | 65% |
Pelicans | 48% |
Storks | 12% |
Eagles | 75% |
Owls | 43% |
Ospreys | 33% |
Cuckoos | 100% (always lay eggs in other birds’ nests) |
Rejection in Different Bird Groups
Seabirds
Seabirds like gulls, terns, and tube-nosed birds have the highest rates of chick rejection. Having to nest in crowded colonies and compete for limited food sources puts pressure on parents to selectively raise the strongest chicks. Mass rejections of up to 95% of chicks can occur during times of low fish abundance.
Birds of Prey
Raptors like eagles, hawks, and owls commonly practice siblicide where older siblings kill younger ones. The parents facilitate this by not bringing enough food, leading to intense rivalry. Weaker chicks often starve or are forcibly ejected from the nest by stronger ones.
Ratites
Large flightless birds like ostriches, emus, and rheas are known to lay more eggs than they can rear. They may abandon part of the clutch, especially if eggs are lost or damaged. Survival rates for ostrich chicks can be as low as 25% in the wild.
Waterfowl
Ducks, geese, and other waterfowl are precocial birds, meaning chicks can feed themselves soon after hatching. This enables parents to abandon stray ducklings. But the chicks may survive by joining other broods or living independently.
Passerines
Songbirds and perching birds have lower rates of rejection than seabirds and raptors. However, food shortages may cause passerine parents to trim brood size by ejecting less competitive nestlings.
Cuckoos
Cuckoos are obligate brood parasites, meaning they always lay eggs in the nests of other species. The young cuckoos often evict or outcompete the host’s chicks. Some species like honeyguides even destroy the host’s eggs when parasitizing a nest.
Does the Father or Mother Reject Chicks?
Both parents may reject offspring, but the bulk of chick provisioning and feeding usually falls on the mother in most species. Mothers spend more time at the nest and have higher energy demands from nurturing young. This could make them more likely to abandon chicks to maintain their own body condition.
Males are more likely to reject in polygamous species where they provide little or no parental care. Some father birds may kill chicks sired by other males. Infanticide by males occasionally happens in albatrosses, eagles, and other raptors.
Do Birds Mourn or Care for Rejected Chicks?
Parent birds do not feel sentimental attachment to chicks and will not mourn abandoned offspring. Their focus is on investing resources in the best candidates with highest reproductive potential. However, bird parents may continue minimal care like sheltering, warming, and defending rejected chicks. The parents only fully cut off care when chicks are terminally weak or dead.
What Behaviors Lead to Chick Rejection?
Parent birds use various cues to select which chicks to reject:
- Size – Smaller and slower growing chicks are often abandoned first.
- Age – Younger later hatchlings lose out to older siblings.
- Begging – Loud or aggressive begging is favored over weaker begging signals.
- Bright gapes – Chicks with vibrant mouth coloring get preferentially fed.
- Plumage – Feather quality indicates health and growth rates.
- Injuries – Chicks with disabilities or deformities tend to get rejected.
What Are the Outcomes for Rejected Chicks?
Abandoned chicks rarely survive long without parental care. Common fatal outcomes include:
- Starvation – Chicks are unable to self-feed and die without provisioning.
- Exposure – Chicks get chilled and succumb to cold weather.
- Predation – Discarded chicks are taken by predators.
- Elements – Chicks may fall from the nest or get forcefully thrown out.
- Siblicide – Older siblings directly attack and kill younger ones.
- Trampling – Chicks get trampled and crushed in the nest by adults or siblings.
In rare cases, rejected chicks may get adopted by other parents or survive independently. But the vast majority perish unless taken into human care.
Can Rejected Chicks Be Saved?
Chicks abandoned at a very young age have poor survival prospects on their own. However, human intervention can rescue rejected chicks in some situations:
- Place chick back in nest if accidentally knocked out.
- Provide supplemental heating and feeding.
- Foster chick with surrogate wild parents.
- Take chick to wildlife rehabilitator.
- Hand-rear the chick if re-nesting fails.
But caution should be used before intervening too early. Parent birds may be deliberately rejecting chicks based on health and chances of survival. Removing chicks that would naturally die may disrupt the reproductive strategy of the species.
Preventing Rejection
While chick rejection is a natural behavior in birds, the following steps may help reduce unnecessary abandonment under human care:
- Remove excess eggs from the nest.
- Provide adequate nesting space.
- Supplement feed parents if needed.
- Isolate aggressive chicks temporarily.
- Minimize disturbances and nest visits.
- Incubate eggs well to prevent weakened chicks.
- Let parents raise chicks without interference.
Key Takeaways
- Chick rejection allows parent birds to increase survival of remaining chicks.
- Rearing offspring imposes high energy demands, so abandoning some chicks reduces parental effort.
- Parents preferentially provide for chicks most likely to thrive and reproduce.
- Ejection of weak, small, or defective chicks cuts losses on futile parenting.
- Chick abandonment is common in seabirds, raptors, ratites, and waterfowl.
- Rejected chicks usually die quickly without sustained parental care.
Conclusion
While chick rejection may seem cruel, it is simply an evolutionary adaptation to maximize birds’ reproductive success. Culling less fit chicks allows parent birds to focus effort on the best offspring. This natural ability to trim brood size is important for species facing environmental fluctuations, food shortages, high chick mortality, and other challenges. Understanding the reasons behind chick abandonment provides insights into the breeding strategies and parental decision-making of birds.