Raising a brood represents a significant energy investment for parent birds. When eggs fail to hatch, parents must manage these unviable eggs efficiently, as resources are finite and must be allocated to viable offspring. An unhatched egg can become a liability to the remaining clutch. Managing these failures is a necessary part of nesting behavior, and the bird’s response depends on the species, the nest environment, and the stage of reproduction.
How Birds Determine an Egg is Unviable
The decision to remove or abandon an egg is preceded by a diagnostic process that utilizes a bird’s sensory capabilities. One primary cue is visual inspection, particularly among open-cup nesting species. Birds compare the color, size, and pattern of an egg against an internal template established by their own laid eggs. Any significant deviation, such as unusual coloration, a cracked shell, or the presence of fungus, can suggest the egg is foreign or damaged. This visual assessment is highly developed in species that have evolved defenses against parasitic birds, like cowbirds or cuckoos.
A less obvious cue is the egg’s temperature and weight. An egg containing a developing embryo is warmer than a dead or infertile one, and the cessation of embryonic heat can be detected by the brooding parent. Furthermore, an infertile egg will not lose moisture and weight at the same rate as a viable egg, leading to a noticeable difference in mass. Birds may physically manipulate eggs with their beaks to assess this weight difference. The lack of internal activity or sound is another factor, as viable embryos often produce clicks or vibrations before hatching. Eggs that remain beyond the typical incubation period are usually recognized as unviable, prompting the parents to lose interest in providing further care.
Active Ejection and Disposal
Active ejection is a common and observable strategy, especially among passerines, where the unhatched egg is physically removed from the nest area. This action is driven primarily by the need for nest sanitation and hygiene. Removing a dead or rotten egg prevents the spread of bacteria, mold, or attracting disease-carrying insects, which could otherwise harm the healthy nestlings.
The physical act of disposal often involves the parent bird grasping the egg in its beak and flying a short distance away before dropping it. In some cases, particularly with smaller eggs, the bird may simply push the egg over the edge of the nest cup. This behavior is also a defense mechanism against brood parasites, where the host bird removes the foreign egg to protect its own genetic investment. The active removal of unhatched eggs also serves to optimize the clutch size, ensuring that parental resources, such as food and warmth, are focused solely on the viable chicks. If the unhatched egg is large or the parent bird is small, the bird may puncture the eggshell and carry the pieces away one by one. This ensures the nest is clean and that remaining space is available for the growing young.
Passive Retention in the Nest
Retention is the non-action alternative, where the unhatched egg is left in the nest alongside the hatched young. This strategy is common when the cost of removing the egg is higher than the benefit, or when the egg offers utility to the remaining brood.
In some species, the unhatched egg can provide structural support for the nest or act as additional insulation for the newly hatched chicks. This is particularly true for birds with large, deep nests where a small, inert object does not significantly impede the movement of the nestlings. Birds may also leave unhatched eggs to avoid disturbing the rest of the active clutch. If the nest contains living eggs or young, physically manipulating or removing a dead egg could risk damaging the viable offspring or causing unnecessary stress to the nest.
In some instances, the parents may fail to recognize the egg as unviable, or the strong instinct to brood outweighs the need for removal. The unhatched egg may eventually be buried beneath new nesting material as the parents continue to build up the nest structure while caring for the growing young. Cliff-nesting seabirds, such as guillemots, often leave unhatched eggs where they are, as the risk of predators is low in their remote nesting sites. These eggs simply decompose naturally, becoming an inert part of the nest environment until the young fledge and the nest is abandoned.