The breeding season is a period of high vulnerability for many bird species, where mortality rates for young birds are substantial. Predation is the primary cause of failure in a vast number of cases. A nestling is a hatchling dependent on the nest, while a fledgling is a young bird that has left the nest but remains dependent on its parents for food and protection. This high rate of loss means that a wide variety of animals rely on young birds as a readily available food source.
Avian Predators: Hunters from Above
Birds themselves are significant predators of other birds’ young, accessing nests at height without needing to climb. Corvids—including crows, ravens, magpies, and jays—are common and effective avian nest raiders.
These intelligent, opportunistic birds typically raid nests during the day, sometimes working in small groups to distract parent birds. Their omnivorous diet and keen eyesight make them threats to open-cup nests. Corvids often have an increased impact in urban and suburban areas where human-provided food boosts their numbers.
Raptors also target baby birds. Hawks and various owl species seize nestlings and newly fledged young. Owls, being nocturnal hunters, often strike under the cover of darkness, while diurnal raptors take advantage of vulnerable chicks during the day.
Mammalian Threats: Ground and Climbing Raiders
Mammals represent the most diverse group of nest predators, employing a variety of tactics, from climbing to relying on scent trails. The domestic and feral cat is the most impactful predator in human-dominated environments. Cats hunt with stealth and persistence, posing a constant danger to nestlings and young fledglings that cannot yet fly well.
Raccoons and squirrels are highly prevalent in urban and suburban landscapes and are adept at accessing elevated nests. Raccoons, known for their dexterity, are opportunistic raiders that often destroy an entire clutch of eggs or nestlings. Squirrels are documented predators of eggs and young, particularly in cavity nests, using their climbing skills to reach high-up locations.
Other wild mammals, including foxes, coyotes, skunks, and opossums, primarily target nests on or near the ground. These generalist predators often rely on a strong sense of smell to locate nests hidden in shrubs or leaf litter. Smaller rodents, such as rats and mice, can also be significant local predators, consuming eggs and small hatchlings, especially in confined spaces like nest boxes.
Silent and Specialized Nest Predators
Reptiles, particularly snakes, are highly specialized and effective predators of bird nests due to their unique anatomy and hunting methods. Species like the rat snake are renowned climbers, able to ascend sheer surfaces and tree trunks to reach nests at significant heights.
Unlike mammals, snakes typically consume the prey whole, leaving behind a clean nest that can sometimes make predation difficult to confirm. Snakes locate nests using scent and heat, and their limbless bodies allow them to enter small openings, making them a threat even to birds nesting in cavities or birdhouses. Studies in North America suggest that snakes are one of the most frequent nest predators, especially in lower latitudes.
In addition to reptiles, other specialized threats exist, such as weasels, which are slender enough to enter the small entrance holes of cavity nests and are known to kill nestlings. Certain predatory insects, like fire ants in the southern United States, can swarm and kill fresh hatchlings that are unable to defend themselves.