The act of birds pushing young from the nest is an umbrella term for several extreme, adaptive survival strategies. This aggressive elimination of offspring is not random but a calculated consequence of intense competition or a mechanism to maximize reproductive success. The behavior is driven by distinct biological phenomena, including exploitation by another species (brood parasitism), lethal rivalry among siblings (siblicide), and deliberate culling by the parents themselves (parental removal). These actions highlight the harsh realities of resource limitation and natural selection.
Brood Parasitism: Outsiders Forcing Removal
The most direct form of removal is avian brood parasitism. Parasites lay their eggs in the nests of other birds (hosts), delegating all parental duties. Obligate brood parasites, such as the Common Cuckoo, have lost the ability to build their own nests or raise their young. The female parasite deposits an egg, often removing a host egg to avoid detection.
The Common Cuckoo chick has a specialized adaptation: a sensitive depression on its back. Within hours of hatching, the parasite is driven by instinct to systematically hoist the host’s eggs or newly hatched chicks onto its back and heave them over the rim of the nest. This physical ejection eliminates all competition for the host parents’ resources and attention.
Other parasites rely on different methods. The Brown-headed Cowbird in North America out-competes the host’s young. Cowbird eggs hatch earlier, and the nestling grows faster and larger, monopolizing the food and starving the host’s smaller chicks. Honeyguides, another obligate parasite, hatch with sharp, temporary hooks on their beaks, which they use to fatally attack the host’s young.
A less extreme form is facultative brood parasitism, seen in species like the Bank Swallow. These birds can raise their own young but occasionally lay eggs in a neighbor’s nest of the same species. This reproductive gamble increases the total number of offspring without increasing parental investment.
Siblicide: Lethal Competition Among Nestmates
Siblicide is an internal form of elimination where a nestling actively kills its own sibling due to intense competition for parental resources. This behavior is common in species like raptors and boobies, where parents practice asynchronous hatching. This means incubation begins before all eggs are laid, resulting in a size and age difference that gives the first-hatched chick a significant advantage.
Siblicide is categorized as either obligate or facultative. Obligate siblicide, seen in species such as the Black Eagle and the Nazca Booby, means the death of the younger sibling is almost guaranteed. Black Eagle chicks often peck their younger sibling to death shortly after hatching.
Facultative siblicide, observed in the Blue-footed Booby and the Great Egret, is contingent on environmental conditions, typically resulting in death only when food is scarce. The older chick’s aggression ensures that available food is channeled to the strongest survivor. Parents rarely intervene, tolerating the brood reduction to ensure at least one healthy chick survives.
Parental Removal: Culling for Nest Survival
Parent birds sometimes directly or indirectly cause the removal of their own young, a behavior known as filial infanticide or selective abandonment. This resource optimization strategy is usually triggered by environmental stress, such as famine or drought, when parents recognize they cannot adequately feed the entire brood. Parents may cease feeding the smallest or weakest chick, or actively remove it from the nest.
Removal can also be a preventative measure to maintain the health of the remaining brood. Parents may remove a sick or deceased nestling to prevent the spread of disease or parasites, a form of nest hygiene. Pigeons, for instance, sometimes neglect or abandon a squab that appears sickly or has low viability.
In species like the White Stork, parents have been observed performing infanticide directly by killing the smallest offspring. Sacrificing the chick with the lowest chance of survival conserves the parents’ limited energy and resources for the healthier chicks. This action increases the probability of successfully fledging the remaining young.
The Evolutionary Logic Behind Infanticide
The existence of brood parasitism, siblicide, and parental removal is a direct result of evolutionary pressure to maximize reproductive success. All three mechanisms serve as a form of brood reduction, ensuring limited resources are allocated effectively to offspring with the highest chance of survival.
The underlying evolutionary logic, described by the brood reduction hypothesis, is that it is better to raise a small number of high-quality, well-fed young than a larger brood of weak chicks unlikely to survive. By eliminating the weakest or most resource-intensive member, the remaining young receive sufficient food and parental care. This selective culling enhances the fitness of the surviving offspring and increases the parents’ probability of surviving to breed again.