Birds face mortality risks from various sources, both natural and human-induced. While death is an inherent part of ecological cycles, human activities increasingly contribute to bird mortality, often at accelerating rates. Understanding these factors is important for conservation efforts.
Natural Causes of Bird Mortality
Natural causes of bird mortality include predation, disease, starvation, and extreme weather events. These factors are part of the natural balance of ecosystems.
Predation by other wildlife is a common cause of bird death. Birds are prey for various animals, including other birds, mammals, and reptiles. For example, raptors like hawks and owls hunt smaller birds, while snakes and raccoons may raid nests for eggs and chicks.
Diseases and parasites also contribute to bird mortality. Avian diseases such as avian influenza, West Nile virus, and salmonellosis can spread rapidly through bird populations, particularly in dense congregations. Parasites, both internal and external, can weaken birds, making them more susceptible to other threats or directly causing death.
Starvation can occur when food resources become scarce due to environmental changes or competition. This is particularly evident during harsh winters or in habitats with insufficient foraging opportunities. Young birds are especially vulnerable if parents cannot find enough food to support them.
Extreme weather conditions, such as severe storms, prolonged cold spells, or intense heatwaves, can lead to bird mortality. Birds may succumb to hypothermia, hyperthermia, or be displaced from their habitats. Weather patterns can also disrupt migration, leading to exhaustion and death.
Direct Human-Induced Hazards
Human activities and infrastructure create immediate dangers for birds. Collisions with structures, vehicles, and predation by domestic animals represent direct threats.
Bird collisions with buildings, particularly windows, are a widespread issue. Birds often perceive reflective or transparent glass as open sky, leading them to fly directly into the surface. Hundreds of millions to over a billion birds die from window collisions in the United States annually. Most fatal collisions occur at homes and buildings shorter than four stories, but large structures with extensive glass also pose a threat. Artificial light at night can exacerbate this issue, attracting and disorienting nocturnal migratory birds, causing them to collide with lit buildings.
Vehicle strikes also account for bird mortality. Birds flying low or foraging near roads are susceptible to collisions with cars and trucks.
Electrocution and collisions with power lines and communication towers are direct hazards. Power lines can electrocute birds if they simultaneously touch two energized parts or an energized part and a grounded component. This is particularly risky for large birds like raptors, contributing to millions of bird deaths annually.
Communication towers are responsible for millions of bird deaths each year, particularly night-migrating songbirds. Birds are attracted to and disoriented by non-flashing obstruction warning lights on towers, especially during poor visibility, leading to collisions with the tower structure or guy wires.
Domestic cats are a major cause of direct bird mortality. Free-ranging domestic cats, both pets and feral populations, hunt and kill billions of birds annually. This predation pressure can significantly impact local bird populations, especially in suburban and rural areas.
Broader Human Environmental Impacts
Beyond direct hazards, broader human-driven environmental changes indirectly contribute to bird mortality by degrading their habitats and affecting their survival. These systemic issues include habitat loss, pollution, and the impacts of climate change.
Habitat loss and degradation are significant threats to bird populations globally. Urbanization, agricultural expansion, and deforestation convert natural landscapes into human-dominated areas, removing essential breeding, foraging, and resting sites for birds. Habitat fragmentation, where large natural areas are broken into smaller, isolated patches, also reduces habitat quality and connectivity, making it harder for birds to find resources and successfully reproduce.
Pollution affects birds in multiple ways. Pesticides can directly poison birds or reduce their food sources. When birds consume pesticide-coated seeds or insects that have ingested these chemicals, it can harm their development, reproduction, and even lead to death.
Oil spills are another pollutant, coating birds’ feathers, causing hypothermia, loss of buoyancy, and poisoning when birds attempt to preen. Plastic pollution, particularly in marine environments, causes millions of bird deaths annually through ingestion and entanglement. Birds mistake plastic debris for food, leading to starvation due to blocked digestive systems, or become entangled in plastic waste like fishing lines.
Climate change alters bird survival by changing environmental conditions. Warmer temperatures can lead to mismatches in food availability, where the peak emergence of insects, a crucial food source for breeding birds, occurs before chicks hatch. This temporal mismatch can reduce nestling growth and survival.
Climate change also impacts migration patterns, with many species arriving at their spring nesting grounds earlier than in previous decades, sometimes by several days per decade. Extreme weather events, such as increased frequency and intensity of heatwaves, droughts, and severe storms, directly threaten bird survival and can disrupt their habitats and food sources.