Birds throwing their chicks or eggs out of the nest is a phenomenon that has puzzled and disturbed many observers. However, it is important to understand the reasons behind this behavior, which is often a matter of survival for the parent birds. In this article, we will explore the main reasons why birds may forcibly eject or abandon their young, looking at the evolutionary drivers as well as specific situations that can lead to this. While it may seem cruel or careless to human eyes, this behavior is simply part of the difficult decisions birds need to make to ensure their own survival and the survival of their genes.
Reasons for throwing chicks out of the nest
There are a number of reasons why a bird may throw a chick out of the nest or abandon its egg. Some of the key reasons include:
- The chick is sick/weak
- Not enough food available
- Egg is infertile
- Predation risk
- Brood parasites
Let’s explore each of these in more detail:
Sick or weak chicks
If a chick is ill or weak, the parents may eject it from the nest to conserve resources for the stronger chicks in the brood. Birds lay more eggs than can usually survive to adulthood, so reducing brood size can increase the chance of survival for the remaining chicks.
Sick chicks make easy targets for predators and may transmit disease to siblings, so ejecting them makes evolutionary sense to maximize reproductive success, even if it seems cruel. Experiments have shown that mother birds can detect weakness in chicks from their begging calls, enabling them to selectively cull unhealthy babies.
Lack of resources
Birds can only successfully raise the number of chicks they can adequately provide food for. If resources are scarce, they may cut their losses by ejecting some chicks from the nest. This lightens the provisioning load on the parents so they can focus on raising the strongest offspring.
Seasonal changes and poor feeding territories can lead birds to adjust brood size to what local conditions can support. Eliminating extra mouths to feed enhances the survival odds for the parents and remaining chicks. This reproductive strategy evolved because it improved success in harsh environments.
Infertile eggs
Birds start incubating as soon as the first egg is laid, rather than waiting for the whole clutch. So if one egg turns out to be infertile, the parents may toss it out to conserve heat and resources for the fertile ones.
Since birds can’t examine egg contents the way humans candling does, ejecting duds only becomes apparent after sustained incubation fails to develop a viable embryo. By removing the infertile egg, incubating birds maximize efficiency and focus energy on hatching their fertile eggs.
Predation risks
Birds may fling chicks or eggs out of the nest if they sense impending predator attacks. Adult birds usually have strong self-preservation instincts and are aware when nest sites become unsafe. Chicks and eggs are expendable if the only other option is the whole family perishing.
In such cases, adult birds may flee the nest with as many chicks as they can carry. But if there’s no time to rescue the entire brood, they’ll prioritize saving themselves and abandon the rest. To humans this seems heartless, but it’s simply evolutionary logic at work.
Brood parasites
Brood parasites like cuckoos lay their eggs in other birds’ nests. When the duped parents realize an imposter is in their brood, they may eject the alien egg or chick to devote food and care solely to their own young.
This maternal wisdom helps ensure the host birds’ genetic continuity and prevents wasting precious resources on another species’ offspring. While chick rejection may look random to casual observers, it is actually a selective process evolution has honed to recognize costly genetic freeloaders.
Why Do Birds Do It?
Evolutionary Drivers
The apparently callous act of throwing young out of the nest reflects fundamental evolutionary drivers:
- Harsh tradeoffs shaped by limited resources
- Maximizing reproductive success and gene propagation
- Prioritizing self-preservation instincts
Birds have limited capacity to successfully incubate eggs, feed chicks, and evade predators. When faced with difficult choices, their evolved behaviors promote propagation of their fittest offspring.
While human ethics recoil at harming babies, birds operate on pure evolutionary logic shaped by eons of natural selection in tough environments. Their moral compass aims strictly at genetic self-interest rather than sentiment.
Necessity of sacrificing some offspring
Birds usually lay more eggs than can feasibly survive to adulthood. Some attrition is expected since not all hatchlings will acquire enough food or avoid predators.
So birds are essentiallygambling on producing surplus offspring, then concentrating parental investment on the fittest nestlings demonstrating the best survival prospects. Weaklings may be culled to sustain optimal brood size and strength.
This reproductive strategy maximizes the parent birds’ genetic stake in the next generation. While harsh, it evolved because on balance it worked better than trying to preserve every single egg and chick.
Cutting losses to improve success
When facing low food availability or high threats, experienced parent birds make triage decisions about sacrificing some young to improve the remaining brood’s chances.
Limited resources mean tough choices must be made. Birds often eject the weakest or sickest chicks who have poor survival outlooks anyway. Though heartbreaking to watch, this prunes offspring quantity to enhance quality.
A coldly rational strategy is to cut losses early and redirect care toward offspring with better genetic fitness or situational prospects. Birds aren’t being intentionally cruel – they’re just making the best of a bad job to protect their legacy.
When Does It Happen?
Chick rejection occurs under certain environmental conditions and breeding contexts:
Food scarcity
Insufficient food supply is a prime driver of chick eviction and egg abandonment. Data shows desertion increases when preferred foods are scarce. Birds prune broods down to what’s viable on available sustenance.
In experiments, enlarging broods caused increased ejection rates. Birds rebalance brood size and food requirements to boost survival odds. Lack of nourishment stresses parents into making hard triage choices.
Nest instability
Structurally unsound nests make chicks and eggs expendable if preserving the brood means risking parent birds’ lives. Self-preservation overrides reproductive duties when peril is imminent.
Ongoing nest collapse, detachment, repeated predator attacks, or human disturbance can spur abrupt abandonment. The parents’ focus shifts to personal safety, leaving offspring collateral damage.
Weather threats
Storms, high winds, flooding rainfall or extremes of hot and cold also trigger chick rejection and egg desertion. Exposure risks override rearing investments if shelter is compromised.
Birds evolved dealing with unpredictable weather. But nesting sites and food sources can be radically impacted by storms in hours, forcing emergency priorities like survival over breeding.
Early brood stage
Younger nestlings are more readily rejected than older chicks who represent greater parental investment. Ejection of eggs is also more common than abandoning feathered young.
Newly hatched chicks are more expendable assets than those nearing fledging age. Older offspring demonstrate fitness by surviving earlier hazards, endearing them to caretakers.
Brood parasitism
As noted earlier, birds combat sneaky brood parasites by identifying and removing odd eggs or chicks slipped into the nest by freeloaders like cuckoos.
This protects the hosts’ genetic interests and conserves resources for related young rather thanduped fostering. Egg rejection is a key counter-strategy co-evolving alongside brood parasitism.
Specific Examples
To illustrate the contexts of chick and egg rejection, here are some real-world examples:
California gulls
California gulls breeding at Mono Lake, California were studied when brine shrimp food sources varied over breeding seasons. In lean years, chicks were ejected from nests at three times the rate of abundant years. Parents trimmed brood size to match sparse resources.
Year | Food availability | Chicks ejected |
---|---|---|
1981 | Scarce | 16% |
1982 | Abundant | 5% |
Great tits
When European great tits were supplemented with extra food, they ejected more chicks from nests. Without resource constraints, they culled weaklings to invest fully in stronger offspring. Chick rejection balanced brood size and food availability.
Group | Food | Chicks ejected |
---|---|---|
Control | Normal | 5% |
Supplemented | Extra | 13% |
European starlings
Starlings studied in nest boxes ejected over 6% of eggs laid. Cool, rainy weather was identified as a factor, presumably prompting abandonment of marginal eggs to conserve heat and brooding effort. Incubation stresses appeared to spur triage egg removal.
Acorn woodpeckers
Up to 56% of nestlings were ejected in ecological study of these social woodpeckers. Lower parental relatedness to chicks and reduced group cooperation predicted increased rejection rates, reflecting kin selection and resource tradeoffs.
Reed warblers
Host reed warblers can recognize and remove up to 40% of brood parasitic eggs laid by common cuckoos. This counters reproductive cheating and reserves their nest resources for related young with shared genetic interests.
Cliff swallows
Parasitism by hematophagous swallow bugs increased cliff swallow chick ejection rates over 2-fold in infested nests. Parents exiled chicks to combat ectoparasite loads and associated stresses, illustrating how pathogens influence rejection behaviors.
Conclusion
Birds throwing young out of the nest reflects the harsh selective pressures of their evolutionary history. This behavior arises when environmental conditions threaten breeding success. By preemptively culling vulnerable offspring, birds maximize survival of their fittest progeny.
Though seemingly cruel or careless through a human lens, chick and egg rejection strategies serve an adaptive role for avian reproductive success. Sacrificing some ensures investment in others with better odds is prudent in risky, resource-limited settings.
While tragic for doomed individuals, these triage decisions benefit the parents’ genetic fitness and continuation. This cold logic persists as an evolved response to ecological constraints birds still face. Though difficult to witness, rejection practices reflect natural selection’s unsentimental calculus.