Why Do Birds Throw Eggs Out Of The Nest?

It can be alarming to peer into a bird’s nest and find broken eggs and shells scattered on the ground below. If you’ve witnessed this and wondered why birds sometimes eject their own eggs from the nest, you’re not alone.

If you’re short on time, here’s a quick answer: Birds mainly throw out eggs that are infertile or contain dead embryos to conserve resources for raising healthy chicks.

In this comprehensive article, we’ll explore the reasons behind this unusual bird behavior in depth. We’ll look at how birds assess egg viability, why removing eggs may benefit the rest of the clutch, how some species use egg ejection to choose sex ratios, and what you should do if you find rejected eggs.

Assessing Egg Viability

When birds throw eggs out of the nest, it is often a result of instinctual behavior aimed at ensuring the overall health and survival of the nest. One of the key factors that birds consider when deciding to remove an egg is its viability.

Birds have evolved various mechanisms to assess the viability of their eggs, including detecting infertile eggs and identifying embryo death.

Detecting Infertile Eggs

One way birds assess the viability of their eggs is by detecting whether they are infertile. This is particularly important for species that lay a large number of eggs, as it allows them to invest their energy and resources in eggs that have the potential to develop into healthy offspring.

Birds have a remarkable ability to sense fertility in their eggs, often relying on their sense of smell or the appearance of the eggshell. For example, some bird species can detect the absence of a developing embryo by smelling the egg, while others can identify infertile eggs based on their color or texture.

According to a study published in the Journal of Avian Biology, researchers found that certain bird species are able to discriminate between fertile and infertile eggs based on differences in volatile compounds emitted by the eggs.

This fascinating ability allows birds to selectively remove infertile eggs from the nest, ensuring that only viable eggs receive the necessary attention and care.

Identifying Embryo Death

Birds also possess the ability to identify embryo death in their eggs. This is crucial to prevent the spread of diseases or genetic abnormalities within the nest. When an embryo dies, it can release harmful bacteria or fungi that could pose a threat to the remaining eggs or the adult birds themselves.

By detecting the death of an embryo, birds can quickly remove the egg from the nest, reducing the risk of contamination and increasing the chances of successful hatching for the remaining eggs.

Researchers at the Royal Society for the Protection of Birds have conducted studies on various bird species, such as the common tern, to understand how they detect embryo death. They found that birds can sense changes in temperature and chemical composition within the egg, which can indicate embryo death.

This remarkable ability allows birds to remove non-viable eggs promptly and focus their energy on nurturing the healthy ones.

Conserving Resources

When it comes to the behavior of birds throwing eggs out of the nest, one of the main reasons behind this seemingly strange action is the need to conserve resources. Birds are intelligent creatures that have evolved to make efficient use of their limited resources in order to ensure the survival of their offspring.

By throwing eggs out of the nest, they are able to conserve resources such as food, energy, and space, which are crucial for the successful rearing of their young.

Brood Reduction

One reason birds throw eggs out of the nest is to practice a phenomenon known as brood reduction. Brood reduction is a strategy employed by many bird species to ensure the survival of the strongest and healthiest offspring.

By removing some eggs from the nest, parents are able to allocate more resources to the remaining eggs or chicks, increasing their chances of survival. This ensures that the limited resources available are used most effectively, resulting in a higher overall reproductive success.

Did you know? The phenomenon of brood reduction has been observed in various bird species, including raptors such as eagles and falcons, as well as passerines like sparrows and finches.

Freeing Up Space

Another reason birds may throw eggs out of the nest is to free up space. Nests are typically small and confined spaces, especially for cavity-nesting species. By removing excess eggs, birds can create more room for the remaining eggs or chicks to grow and develop properly.

This allows for better ventilation and reduces the risk of overcrowding, which can lead to increased competition for resources and higher mortality rates.

Fun fact: Some bird species, such as the Common Murre, are known to lay their eggs on narrow cliff ledges. These ledges can become crowded, so the parents may push eggs that are not viable or healthy off the ledge to create more space for the remaining eggs.

It is important to note that not all bird species throw eggs out of the nest. The behavior of throwing eggs is more commonly observed in species that lay multiple eggs in a single clutch. Each species has its own unique strategies for resource allocation and ensuring the survival of their young, and throwing eggs out of the nest is just one of the fascinating ways in which birds have adapted to their environment.

For more information on bird behavior and conservation, visit: https://www.audubon.org/

Influencing Sex Ratios

One of the reasons why birds throw eggs out of the nest is to influence the sex ratios of their offspring. This behavior is more common in species that have temperature-dependent sex determination (TSD).

TSD means that the sex of the bird is determined by the temperature at which the eggs are incubated.

Temperature-Dependent Sex Determination

In species with TSD, the temperature at which the eggs are incubated during a critical period of development determines whether the offspring will be male or female. For example, in some turtle species, lower temperatures result in the development of males, while higher temperatures result in females.

Birds that practice egg throwing may do so to regulate the temperature of the eggs and manipulate the sex ratios of their offspring.

By throwing out certain eggs, birds can control the overall temperature inside the nest. They may remove eggs that are in areas of the nest that are too warm or too cold, depending on the desired sex ratio.

This behavior allows birds to increase or decrease the number of males or females in their brood, depending on various ecological or social factors.

Facultative Manipulation

In addition to temperature-dependent sex determination, some bird species also have the ability to facultatively manipulate the sex ratios of their offspring. This means that they can adjust the sex ratio based on environmental conditions or other factors.

By throwing out eggs, birds can fine-tune the sex ratio to maximize their reproductive success in a particular environment.

For example, if resources are scarce, a bird may choose to throw out eggs of one sex to ensure that the remaining offspring have a better chance of survival. This strategic decision allows the parent birds to optimize their investment in reproduction and increase the overall fitness of their offspring.

It is important to note that not all bird species engage in egg throwing behavior. This behavior is more commonly observed in species with TSD or facultative sex ratio manipulation capabilities. The specific reasons for egg throwing may vary among different bird species, and further research is needed to fully understand the underlying mechanisms and evolutionary significance of this behavior.

What To Do If You Find Ejected Eggs

Understanding the Behavior of Birds

It can be quite alarming to stumble upon a nest with ejected eggs. However, it’s important to understand that this behavior is not uncommon in the avian world. Birds may throw eggs out of their nests for various reasons, including:

  • Protecting the nest from predators
  • Removing infertile or damaged eggs
  • Creating more space for healthier eggs
  • Reducing competition among siblings

By ejecting eggs, birds are making calculated decisions to increase the chances of a successful breeding season.

Leave the Eggs Alone

If you come across ejected eggs, it’s best to resist the urge to intervene. It’s important to remember that wild birds are protected by laws in many countries, and it is illegal to disturb their nests or eggs without proper permits.

Additionally, interfering with the natural processes of wildlife can have unintended consequences and disrupt the delicate balance of ecosystems.

Observe from a Distance

While it may be tempting to investigate further, it is advisable to observe the nest from a safe distance. This will allow you to learn more about the birds’ behavior and potentially witness their return to the nest.

By keeping your distance, you can ensure that you do not inadvertently cause stress or harm to the birds.

Report Your Observation

If you come across ejected eggs, you can contribute to scientific knowledge by reporting your observation to local birdwatching or wildlife organizations. They may be interested in gathering data on bird behavior and population trends.

Your report may help researchers gain a better understanding of bird species and their nesting habits.

Encourage Bird-friendly Environments

To support bird populations, create bird-friendly environments in your own backyard. Plant native trees and shrubs that provide food and shelter for birds. Install bird feeders and birdbaths to attract a variety of species.

By creating a welcoming habitat, you can help birds thrive and potentially provide alternative nesting sites for them.

Remember, birds have evolved over millions of years to successfully raise their young. Although it may be disconcerting to find ejected eggs, it is best to respect their natural behavior and allow them to continue their reproductive process without interference.


Finding cracked and rejected eggs underneath a bird’s nest can certainly be puzzling. However, this unusual behavior is often the bird’s strategy for optimizing its reproductive success. By expelling certain eggs, birds conserve resources and invest more in healthy offspring.

While ejecting eggs may seem harsh, it is an adaptation that ensures the survival of the fittest chicks.

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