As a bird owner, you may have noticed the odd behavior of your feathered friends pushing an egg out of the nest. This unusual act certainly warrants an explanation.
If you’re short on time, here’s the quick answer: Birds sometimes eject eggs from their nests if the eggs are infertile or contain dead embryos. It’s an instinctive behavior designed to keep the nest sanitary and prevent diseases.
In this article, we’ll explore this topic in depth, covering reasons why different bird species dispose of eggs, how they identify eggs to remove, the evolutionary basis of this behavior, and what it means for pet birds.
Common Reasons Why Birds Discard Eggs
Birds have valid biological reasons for removing eggs from their nests. While it may seem cruel, they actually do this to preserve resources for the healthiest possible offspring. Understanding why birds toss out eggs can provide insight into avian behavior and reproduction.
Infertile Eggs
Infertile eggs contain no embryo, as they were never fertilized by male sperm or failed to develop properly. Since these eggs will never hatch, many bird species remove them from nests. This conserves the parents’ energy and attention for fertile eggs and hatched chicks instead of wasted on duds.
Studies show some birds can detect infertility early. For example, research on Zebra Finches revealed females laid replacement eggs within days of discarding infertiles. Removing them promptly optimizes the chance of fertile clutches. Their sharp instincts guide this energy-saving behavior.
Eggs with Dead Embryos
Even fertilized eggs may not develop normally. Estimates indicate 10-15% of songbird embryos die within 4-5 days. If parents sense no movement inside, they assume chick mortality and eject the egg. This also applies if embryos expired later on.
Avian parents cannot afford squandering precious feeding efforts on non-viable eggs. Their swift decision-making abilities help target care toward eggs which still might produce offspring. By continually evaluating development, adult birds demonstrate an astute capacity to recognize troubles early.
Eggs That Don’t Hatch in Time
Hatching success depends partly on synchronization between egg-laying and incubation start dates. Many species only commence warming eggs after the clutch reaches completion. If initial eggs cool too long before brooding, their embryos may sustain damage.
Likewise, late additions to a clutch can create issues. Parent birds often reject these outliers if already nurturing older eggs. Their body heat gets focused on eggs closest to hatch-date instead of stragglers. These patterns illustrate the intricate timing constraints around avian breeding.
In all cases, removing laggard eggs enables concentrating effort on stronger individuals. As creatures with finite energy, breeding birds must make pragmatic choices to rear as many viable offspring as environmental conditions allow.
How Birds Identify Eggs to Remove
Visual cues signaling infertility
Birds have excellent vision and can pick up on subtle visual differences between eggs. If an egg has an unusual shape, size, or coloration compared to the others, the parents may remove it. Cracked or dented eggs are often rejected.
Clear or translucent eggs typically signal infertility, so birds instinctively remove them from the nest. Amazingly, some species can even detect very slight variations in egg pigmentation unseen by human eyes that indicate lack of viability.
Vocalization differences
Communication between parent birds and their chicks begins even before hatching. Unhatched chicks will “chirp” from inside fertilized eggs, whereas unfertilized eggs remain silent. Parent birds seem to be able to hear these pre-hatching vocalizations and preferentially incubate the “talking” eggs.
If an egg fails to chirp after a few days, the parents may abandon it or push it out of the nest.
Eggs that feel different
Birds use their highly sensitive sense of touch to identify eggs that don’t seem right. When turning and repositioning the eggs in the nest, parents can detect subtle differences in weight, shape, surface texture, and shell thickness.
Lighter or oddly smooth eggs often signal something is amiss developmentally, prompting the birds to reject them. Some birds even “shake” suspicious eggs next to their ear and listen for sloshing liquid inside, an indicator the egg has gone bad.
The Evolutionary Basis of Egg Rejection
The evolutionary theory provides critical insights into why some bird species have developed the behavior of removing eggs from their nests. This seemingly puzzling practice likely emerged as an adaptive response to brood parasitism – when other birds sneak their own eggs into another bird’s nest to be raised by foster parents.
Species such as cuckoos and cowbirds are notorious brood parasites, imposing substantial reproductive costs on host birds by reducing their own offspring’s survival. Over evolutionary time, this selective pressure led certain hosts, like American robins, to evolve egg discrimination abilities.
Robins can recognize differences between their pale blue eggs mottled with reddish-brown spots and the darker eggs of parasitic cowbirds. When they discover the foreign eggs, robins swiftly remove them by grasping them in the beak and tossing them out of the nest.
Egg rejection behaviors confer a major fitness advantage in the evolutionary arms race between hosts and parasites. Rather than waste precious resources raising unrelated chicks, robins can invest more in their own genetic progeny. So natural selection strongly favors keen egg recognition skills.
Thus, the propensity to actively eject foreign eggs from nests seems perplexing from an ethical human standpoint but makes perfect evolutionary sense. It is a powerful adaptation that has evolved across over 200 host species – enabling birds to police the paternity of the eggs in their nest and winnow out cheating brood parasites.
What It Means for Pet Birds
Birds pushing eggs out of their nests can seem confusing or even cruel to us as pet owners. However, this behavior is completely natural for wild birds and can happen occasionally with pet birds as well. Understanding why birds do this can help us provide better care for our feathered friends.
Natural Instincts
In the wild, birds have an innate drive to protect their offspring and ensure the survival of their species. Sometimes this means recognizing when an egg is inviable and removing it from the nest. If a bird senses something is wrong with an egg, they may push it out to conserve their energy and focus on viable eggs and chicks.
Some common reasons a wild bird may reject an egg include:
- Infertility
- Developmental issues
- Damage or cracks
- Parasites or bacteria
- Incorrect incubation temperature
While this behavior may seem harsh, it’s simply nature’s way of ensuring the strongest chicks survive. For wild birds, the health of the brood outweighs the loss of a single egg.
What Pet Birds Do
Pet birds may occasionally exhibit this same nesting behavior. However, there are usually different factors at play than in the wild. Since pet birds receive ample food, veterinary care, and a controlled nesting environment, the reasons for rejecting eggs aren’t always the same.
Some reasons pet birds may push out eggs include:
- Stress or change in environment
- Lack of bonding between mates
- Inexperience with breeding
- Disturbances to the nest
- Illness, malnutrition, or vitamin deficiencies
With pet birds, rejection is more likely tied to behavioral or environmental factors than the egg itself. So it’s important not to immediately assume something is wrong with the egg.
What You Can Do
If your pet bird is pushing eggs out of the nest, here are some tips:
- Leave rejected eggs in the cage for a few days in case she changes her mind
- Minimize stress by limiting noise, visitors, and changes to her environment
- Ensure she has a calcium-rich diet for healthy egg development
- Provide a private, secure nesting area away from busy household activity
- Consult an avian vet to rule out any medical issues
With some adjustments to care and environment, you can encourage natural nesting behaviors and bonding between bird parents. However, repeatedly rejected or broken eggs may indicate a need for veterinary guidance to ensure the health and wellbeing of your birds.
While we may not like to see our feathered friends rejecting their own eggs, it’s important to remember this is simply instinctual behavior. With proper care and attention, we can work to create the best possible nesting conditions for our pet birds.
Conclusion
While the behavior of casting eggs out of nests may seem peculiar, it serves an important purpose for bird health and breeding success. Hopefully this breakdown has shed some light on why birds do this instinctively.
Paying attention to egg activity can also help pet bird owners identify potential fertility issues.
