Discovering a deceased baby bird in its nest is a disheartening experience, yet a natural part of the lifecycle for many bird species. The high mortality rate among nestlings stems from environmental pressures, biological challenges, and external threats.
Harsh Environmental Conditions
Prolonged cold snaps can lead to hypothermia, as young nestlings cannot regulate their body temperature. During cold periods, adult birds may struggle to find sufficient food, like scarce insects, compromising the nestlings’ ability to stay warm. Excessive heat waves can cause hyperthermia and dehydration. Nestlings in open nests are susceptible to direct sunlight; if parents cannot shade them, they may overheat. Extreme heat can compel baby birds to prematurely fledge, leading to injuries or death.
Beyond temperature, severe storms, strong winds, or heavy rain can destroy nests, resulting in nestling fatalities. Nests may be dislodged or collapse, exposing young to elements or falls. Even if intact, prolonged wet conditions can lead to dampness, creating an unhealthy environment. Poor nest construction also contributes to vulnerability, making it susceptible to weather damage.
Predation and Other External Dangers
Predators represent a major cause of nestling mortality. Common nest predators include raccoons, snakes, domestic cats, and larger birds such as jays and crows. Raccoons are adept at raiding nests, using their dexterous paws to access eggs and young birds, often consuming entire clutches. Nocturnal, they may be attracted by the smell of birds. Snakes, including rat snakes, are skilled climbers locating nests through visual cues and scent, often striking at night.
Domestic and feral cats are significant predators, with studies suggesting they kill billions of wild birds annually. Cats are agile hunters, able to stealthily approach nests and capture young birds. Human disturbance can inadvertently lead to nest abandonment. Repeated human presence near a nest can stress adult birds, causing them to leave and exposing eggs or chicks to elements or predators. Intentional nest vandalism or accidental destruction from human activities, such as tree felling, contribute to nestling deaths.
Scarcity of Resources and Sibling Competition
Adequate food and water are crucial for nestling development and survival. When food resources are scarce, due to adverse weather impacting insect populations or reduced parental foraging, nestlings can suffer starvation. Parents may be unable to find enough nourishment for the entire brood, leading to the demise of weaker or smaller nestlings. This is a form of natural selection where the most robust offspring receive priority for limited resources.
Sibling competition, often called brood reduction, is common when resources are limited. In many species, eggs hatch asynchronously, meaning chicks emerge at different times, creating a size hierarchy. Larger or earlier-hatched nestlings often outcompete smaller siblings for food and parental attention. This competition can escalate to aggression, where stronger chicks injure or even kill weaker nestmates, a behavior known as siblicide. Parents may facilitate this process by prioritizing feeding the most vigorous chicks, ensuring the survival of some offspring rather than losing the entire brood.
Disease, Parasites, and Parental Care Challenges
Diseases and parasites can impact nestling health and survival. Bacterial, viral, or fungal infections can weaken young birds, making them susceptible to other threats. External parasites, such as blowfly larvae, are common in bird nests. These larvae attach to nestlings, drawing blood and body fluids, which can lead to anemia and decreased growth. While blowfly infestations do not always cause direct mortality, they can weaken nestlings, particularly when combined with stressors like cold weather or food scarcity.
Challenges related to parental care contribute to nestling mortality. Inexperienced parents may struggle to adequately provide for their brood, leading to insufficient feeding or inadequate brooding, especially during cold periods. The death or disappearance of one or both parents, often due to predation or accidents, leaves nestlings vulnerable and unable to survive. A parent might abandon a nest if they perceive a high risk to their own survival, such as persistent disturbance. If a brood is too large for parents to sustain, they may intentionally neglect or even remove the weakest nestlings to conserve resources for stronger ones.