Bird mortality is a natural and ongoing process within ecosystems, influenced by various factors. While some bird deaths are expected, others can signal broader environmental issues or human impacts. This article explores the diverse reasons why birds die, from everyday occurrences to more unusual and widespread events.
Everyday Bird Mortality
Many bird deaths stem from natural causes. Birds often die of old age, with lifespans varying greatly by species; smaller passerine birds live only a few years, while larger raptors can survive for decades.
Predation is a constant natural pressure. Predators like hawks, owls, snakes, and raccoons regularly hunt birds for sustenance.
Birds can also die from common diseases. Environmental stressors like starvation or harsh weather conditions, such as cold snaps, heatwaves, or storms, cause death. Collisions with natural objects like trees are also common.
Human Activities and Bird Deaths
Collisions with human-made structures are a pervasive cause of bird deaths, with an estimated 365 million to 988 million birds dying annually from striking buildings and windows in the United States alone. Vehicles also pose a substantial threat, accounting for an estimated 89 million to 340 million bird fatalities each year.
Power lines and communication towers cause electrocutions or collisions. Wind turbines, while a source of renewable energy, can also cause bird fatalities through collisions, especially for raptors and migratory species. Their placement in migratory pathways exacerbates this impact.
Pesticides and other toxins in agriculture and urban areas can directly or indirectly harm birds. Rodenticides can cause secondary poisoning in birds of prey that consume affected rodents. Insecticides can harm birds by reducing insect populations, a primary food source, or by direct ingestion of treated seeds.
Habitat loss and degradation, driven by urbanization, agricultural expansion, and deforestation, diminish available nesting, foraging, and resting areas, making bird populations more vulnerable. Domestic animals, particularly outdoor cats, are a significant source of bird mortality, with estimates suggesting they kill 1.3 billion to 4 billion birds annually in North America.
Light pollution from urban centers disorients migrating birds, causing them to deviate from their paths and become more susceptible to collisions. Fishing gear, including discarded lines and nets, can entangle birds, leading to injury, drowning, or starvation.
Environmental Changes and Diseases
Environmental shifts and widespread disease outbreaks increase bird mortality. Climate change impacts, such as prolonged droughts, intensify water scarcity and reduce food availability, stressing bird populations. More frequent and severe storms can destroy nesting sites and displace birds from their habitats.
Unseasonal temperature shifts can disrupt breeding cycles or lead to hypothermia or heat stress. Such events alter ecosystems and place immense pressure on avian species. Large-scale pollution events, like oil spills, coat birds’ feathers, impairing their ability to fly and regulate body temperature, often leading to death.
Industrial waste and plastic pollution pose additional threats. Birds can ingest plastic debris, causing internal injuries or blockages. Entanglement in plastic or discarded fishing gear also leads to injury or death.
Emerging or widespread diseases cause significant bird die-offs. Highly pathogenic avian influenza (HPAI), commonly known as bird flu, has caused mass mortality events in wild bird populations globally, spreading rapidly among waterfowl and other species. West Nile virus, transmitted by mosquitoes, can lead to neurological disease and death in many bird species, particularly corvids and raptors.
Botulism, caused by bacteria, produces toxins that paralyze birds, especially waterfowl, leading to large die-offs. Salmonella outbreaks can occur in birds, particularly around feeders, leading to illness and death from bacterial infection. These diseases can spread quickly through dense bird populations.
Identifying and Reporting Unusual Events
Distinguishing normal mortality from a mass die-off is important. A mass die-off typically involves an unusual number of deaths, affecting multiple individuals or species within a concentrated area and short timeframe. For example, finding dozens of dead birds in a small park over a few days would be more unusual than finding a single dead bird occasionally.
Reporting unusual bird mortality events is important for scientific monitoring, public health, and conservation efforts. Wildlife agencies rely on public reports to track disease outbreaks, identify environmental hazards, and understand population trends.
If you encounter an unusual number of dead birds, do not handle them directly. Contacting local wildlife rehabilitation centers, state or federal wildlife agencies, or university extension offices is the appropriate action. In the United States, agencies like the U.S. Fish and Wildlife Service or state departments of natural resources have specific reporting protocols.
The public can contribute to bird conservation by creating bird-friendly environments. Actions like adding window decals, keeping cats indoors, and avoiding yard pesticides can significantly reduce human-caused bird mortality. These measures mitigate common risks.