Predation is a major factor in the survival of young birds, causing the failure of a majority of nesting attempts in many species. Baby birds are uniquely vulnerable because they are confined to a stationary location for days or weeks, lacking the mobility to escape danger. Their dependence on parents makes the nest a concentrated, predictable target for a wide array of predators, from large mammals to tiny insects.
Mammalian Hunters of Nests
Mammals represent a significant threat to bird nests, using climbing skill, opportunistic feeding habits, and a keen sense of smell to locate prey. Raccoons are highly effective nest raiders, often operating at night when parent birds are less alert. These nocturnal hunters can climb trees or use their nimble paws to reach into nest boxes, sometimes called “dipping,” to pull out eggs or nestlings.
Domestic and feral cats are major predators, especially of inexperienced fledglings. The hunting drive in cats is instinctual, and they are responsible for the mortality of millions of wild birds annually. Gray squirrels are opportunistic omnivores that consume eggs and young from cavity nests, using agility to access high spots. Foxes and coyotes, primarily ground hunters, target nests built low to the ground, relying on their sense of smell to locate the brood.
Avian and Aerial Predators
Birds of prey and opportunistic omnivores use their aerial advantage and superior eyesight to survey for nests. Corvids, including crows, ravens, and jays, are intelligent and successful nest predators. They rely on keen observation to track parent birds back to their nests before consuming the young or eggs.
Raptors, such as hawks and owls, readily take vulnerable nestlings and fledglings. Hawks employ daylight assaults on nests spotted from above. Nocturnal owls hunt silently at night, using exceptional hearing to snatch young birds directly from the nest cup. Opportunistic gulls also raid the ground nests of other seabirds in coastal areas, consuming unattended eggs and chicks.
Reptiles and Amphibians
Reptiles, particularly snakes, are exceptionally successful and stealthy nest predators, often surpassing mammals in predation rates in certain habitats. Rat snakes are highly documented predators, known for their remarkable climbing ability that allows them to ascend rough bark and fence posts to reach high nests. These slender reptiles move silently, often leaving the nest structure intact after consuming the eggs or nestlings whole.
Many snakes, including rat snakes, corn snakes, and king snakes, are active both day and night, exploiting nests when parent birds are foraging or sleeping. Their elongated bodies allow them to access narrow cavities, posing a significant threat to birds utilizing nest boxes or tree hollows. The success of a snake’s raid is often measured by the absence of the brood without any sign of struggle or nest destruction, differentiating them from many mammalian predators.
Invertebrate and Insect Threats
Smaller, often overlooked invertebrates pose a significant threat to helpless nestlings, particularly those in ground nests. Certain species of fire ants are notorious for swarming a nest, overwhelming newly hatched chicks unable to move or defend themselves. These ants inflict fatal bites and stings, consuming the nestlings in place.
Parasitic insects, while not direct predators, cause significant harm and mortality. Bird mites and parasitic flies feed on the blood of nestlings, leading to anemia, weakness, and a compromised immune system. These tiny threats can weaken the young until they succumb to disease or are too weak to survive until fledging. Large predatory spiders and certain wasps may also opportunistically attack and kill small hatchlings when the parents are away.