Birds face many threats throughout their lives that can lead to premature death. Understanding the most common causes of bird mortality can help us better protect avian populations and support conservation efforts.
Predation
Predation is one of the leading causes of bird deaths worldwide. Birds are preyed upon by a wide range of predators including mammals, reptiles, other birds, and even some fish and arthropods. Predation rates vary by location, habitat type, bird species, and population of predators. However, researchers estimate that predation accounts for up to 80% of annual bird mortality in some environments.
Birds are frequent prey items for many predators because they are abundant, accessible, and nutritious. Common predatory birds that feed on other avian species include hawks, eagles, owls, falcons, and vultures. Mammalian predators of birds include cats, foxes, raccoons, weasels, rats, and snakes. Nestling and fledgling birds are especially vulnerable to predators.
Habitat loss and fragmentation can increase predation rates by altering predator-prey dynamics. The removal of vegetation cover and nesting sites leaves birds more exposed. Predators may also preferentially hunt along habitat edges. Providing ample habitat cover and reducing fragmentation are important in decreasing predation risks for many bird populations.
Nest Predation
A major component of avian predation involves the loss of eggs and nestlings to predators. Nest predation typically accounts for the highest source of nest failure across most bird groups. Rates of nest predation vary widely based on habitat, nest location, parental behavior, and predator population densities in a given area.
Ground-nesting and cavity-nesting birds are particularly susceptible to nest predation. Common nest predators include snakes, squirrels, raccoons, crows, jays, owls, hawks, weasels, rats, mice, and domestic cats. Nest predation risk decreases in the interior of large forest habitats compared to patchy, fragmented, or edge habitats.
Birds have evolved various nesting strategies to reduce predation, such as concealing nests, nesting in colonies, mobbing predators, and choosing relatively safe nest sites. However, nest predation remains a major obstacle for successful reproduction and fledging in most bird species.
Starvation
Starvation is another leading cause of mortality especially in juvenile and migrating birds. Lack of adequate food resources can directly lead to starvation or compound other mortality factors. Harsh weather, habitat loss, injury, and disease may prevent birds from foraging effectively enough to meet their high metabolic needs.
The first year of life is particularly risky for many bird species. One study estimated a daily mortality rate from starvation of 1.8% for juvenile passerines. Fledglings must learn to forage independently while avoiding predators and competing for limited food supplies. Expanding agricultural areas and climate change threaten insect food sources for many young insectivorous birds.
Similarly, migrating birds are vulnerable to starvation during their long seasonal journeys. Inclement weather, geographic barriers, or depleted food resources along their migration routes can prevent refueling. Exhausted migrants may die directly from lack of food or become more susceptible to predators, weather, or accidents.
Harsh winters increase starvation risk as snow cover and frozen waterways reduce avian food supplies and foraging opportunities. Supplemental feeding and preserving natural food resources through habitat protection are key in reducing starvation pressures.
Nestling Starvation
Dependent baby birds rely completely on their parents for food delivery. However, various factors may prevent adequate provisioning leading to partial or complete brood loss from nestling starvation. Insufficient food supply, nest distance, sibling rivalry, or loss of a parent are some key risks.
Nestlings require immense amounts of food to fuel rapid growth. Nesting birds are highly vulnerable to decreases in food availability from changes in insect populations, agriculture practices, droughts, floods, or habitat loss. One missed feeding can be fatal for tiny hatchlings. Nest predation also indirectly leads to starvation when parents are lost.
Asynchronous hatching and size disparities between siblings creates competitive hierarchies with smaller chicks being outcompeted. The distance between nest and foraging areas also impacts provisioning rates and nestling starvation risk. Conservation efforts aim to improve foraging areas and food availability during critical breeding periods.
Collisions
Collisions with man-made structures and vehicles account for hundreds of millions of bird deaths annually. Most collision deaths are preventable, highlighting the need for improved regulations and mitigation strategies.
Window strikes at low-rise residential and urban buildings make up the bulk of collision-related mortality. Birds either fail to perceive window glass or are unable to correct after initial impact. Collision risks increase with larger window areas, surrounding vegetation, and feeders attracting birds.
Tall towers and buildings present another major hazard, especially for night-migrating songbirds which rely on celestial light cues. Light pollution disorients birds leading to collisions with high-rise buildings. Population declines linked to building collisions have been documented for species such as wood thrushes, painted buntings, and Canada warblers.
Vehicle strikes also contribute to substantial bird mortality each year. Roads interrupt and fragment habitat while speeding vehicles leave little reaction time. Scavengers attracted to roadkill become additional casualties. Regulations on roadway lighting and streetside vegetation can help reduce collision risks for birds.
Wind Turbines
The growing number of wind turbines has also increased collision-related bird and bat deaths. Although the percentage is small relative to other impacts, certain sites have seen large fatalities of raptors, seabirds, and migratory songbirds.
Factors influencing turbine mortality include tower height, lighting, landscape location, weather patterns, and migration routes. Field studies help predict and minimize impacts through strategic placement, selectively lowering operations during key migration periods, and deterrents to make blades more visible.
While not completely eliminateable, developers follow guidelines to reduce mortality. Overall, wind energy lessens the even greater threats from climate change that imperil avian populations and habitats on a much larger scale.
Electrocution
Utility lines and electrical equipment expose birds, especially large raptors, to electrocution hazards. Electrocution deaths typically occur when a bird simultaneously contacts two energized parts or an energized and grounded conductor.
Raptors such as eagles, hawks, and owls are particularly susceptible given their large wingspans and propensity to perch on power poles and transmission towers while hunting. However, other large birds including herons, cranes, and ravens are also at risk.
Electrocution risk increases with closer spacing between energized lines and equipment. Older infrastructure generally poses a higher threat. Nearly all electrocutions are avoidable through proper electrical insulation and pole retrofitting. Power companies can also help by disabling equipment during peak seasons for at-risk species.
Power line collisions may additionally injure or kill some birds, especially larger less maneuverable ones. Marking lines reduces this risk although ongoing research aims to further improve detections and decrease mortalities from power infrastructure.
Power Line Collisions
While electrocution accounts for most power line-related deaths, collisions with power lines also contribute to mortality for some bird groups. Birds colliding with power lines may suffer direct trauma or become entangled and die from exertion trying to escape.
Vulnerable groups include larger birds with reduced maneuverability such as swans, cranes, herons, and grouse. Fast-moving waterfowl and upland gamebirds in flight are at particular risk. One study of Louisana power lines estimated an average of over 25,000 bird collisions per year, mostly ibis and ducks.
Unlike glass collisions, birds can generally detect power lines but sustained injuries occur from attempted evasive movements. Line marking devices help improve visibility and reduce collisions. Priority areas include lines near wetlands, crossing migratory corridors, spans over canyons, and areas with documented collisions.
Disease
Infectious diseases pose an additional widespread threat to wild birds. Outbreaks of diseases such as avian influenza, West Nile virus, salmonellosis, aspergillosis, and avian pox cause both annual cycles and episodic large-scale mortalities.
Waterfowl, raptors, and vultures appear among the groups suffering the largest disease-related losses. Both endemic pathogens and introduced exotic diseases impact naïve bird populations. Legal and illegal poultry trades have facilitated new disease introductions.
Birds in poor condition from habitat loss, weather events, or contaminants may be immunocompromised and suffer higher disease morbidity and mortality. Climate change may expand pathogen distributions and increase disease susceptibility for some species.
Disease monitoring, wildlife rehabilitation, and wetland conservation provide proactive approaches to mitigate disease threats. Public education also promotes practices to prevent anthropogenic disease spread at backyard bird feeders and poultry facilities.
West Nile Virus
West Nile virus is a mosquito-borne disease causing large die-offs of American crows and other corvid species across North America. First detected in 1999, human cases and bird mortality peaked between 2002-2003 but seasonal outbreaks continue.
Transmission requires both competent reservoir hosts and vector mosquitos. Climate change impacts on mosquito populations contribute to geographical spread. Wild corvids suffer high mortality with documented average annual losses of over 40% in American crows.
Over 300 bird species show West Nile susceptibility to varying degrees. Surveillance relies on reporting dead corvids which provides an early warning signal for human outbreaks. Vaccines in development offer future hopes of containing economic and wildlife impacts.
Severe Weather Events
Extreme weather events directly or indirectly cause substantial bird mortality, especially for species with restricted ranges or populations. Hurricanes, tornadoes, hailstorms, ice storms, windstorms, floods, heat waves, cold snaps, droughts, and wildfires all negatively impact birds.
Direct mortality occurs from wind turbulence, structural collapse, fire, temperature extremes, and flooding that kill both young and old birds. Indirect losses stem from injury, displacement, critical resource limitation, and reduced nesting success in severe weather aftermath.
Historic droughts have dried wetlands causing duck, shorebird, and waterbird die-offs. Heat stress kills birds unable to thermoregulate like swifts, swallows, and poultry. Late cold snaps create food shortages reducing survival.
Climate projections forecast increases in extreme events further jeopardizing sensitive species. Mitigation through greenhouse gas reductions, habitat buffers, anti-perching devices, emergency water supplies, and artificial shelters helps counteract some effects.
Hurricanes
Hurricanes devastate regional nesting colonies and food resources for coastal birds. Strong winds directly blow unfledged chicks from nests. Storm surges drown both young and adults and damage critical coastal marshes.
Studies on Least Terns found entire colonies destroyed by individual hurricanes. Foraging and nesting habitat loss persists long after storms pass. Gulls, terns, herons, egrets, pelicans, shorebirds, and some raptors are among the most vulnerable groups.
Increased hurricane intensity from climate change threatens recovering species like the Piping Plover and Roseate Tern. Post-storm monitoring helps quantify losses and identify rehabilitation priorities. Habitat restoration and artificial barrier islands provide protective buffers in hurricane-prone regions.
Other Causes
While predation, starvation, collisions, electrocutions, disease, and weather events account for most wild bird mortality, additional threats contribute to losses.
Pollution
Oil spills represent high-profile pollution events with detrimental impacts for seabirds and waterbirds. The 2010 Deepwater Horizon oil spill affected over 8,500 bird species with estimated mortality up to 800,000. Daily ingestion of oil from preening causes organ failure.
Chronic oiling still threatens wildlife. Pesticides also poison birds through direct contact or food chain contamination. Heavy metals, rodenticides, fertilizers, pharmaceuticals, and plastic pollution all elevate mortality factors.
Pollution reduction requires improved regulation, disposal practices, rehabilitation response, habitat buffers, and public education. Decreased reliance on toxic chemicals benefits avian health.
Illegal Hunting
Migratory bird poaching and black-market trapping contributes to endangerment and extinction risk for highly coveted species. Trophy collections of raptor species persist in some regions despite protections.
Unsustainable harvest for food and feathers threaten declining seabirds like murre and albatross in some countries. Stronger international enforcement of poaching and collection bans helps curb unnecessary hunting pressures.
Entanglement
Discarded fishing gear entanglement causes debilitating injuries and death for hundreds of thousands of seabirds annually. Seabird species most impacted include albatrosses, petrels, gannets, cormorants, and puffins.
Ingestion of plastic debris also kills birds through gut impaction and toxic chemicals. Monofilament fishing line poses another entanglement hazard to terrestrial and wetland birds. Proper disposal help reduces risks.
Habitat Loss
Widespread habitat destruction indirectly increases mortality by limiting food, cover, and nest sites while also elevating other threats. Habitat loss is linked to higher predation, increased disease spread, more collisions, and greater competition.
Restoring bird habitats to support sustainable populations is a fundamental conservation objective. Beyond moderating mortality factors, healthy habitat provides shelter, foraging, migration rest stops, and breeding territories.
Conclusion
Predation, starvation, collision, electrocution, disease, weather, pollution, illegal hunting, entanglement, and habitat loss represent the primary causes of mortality for wild bird populations. The leading threats can vary across different groups, ages, locations, and seasons.
Anthropogenic factors exacerbate many key hazards birds face. Climate change will likely increase extreme weather, alter predator-prey dynamics, facilitate disease spread, and degrade habitat over coming decades.
Understanding the most substantial mortality risks allows targeted conservation actions through threat mitigation and habitat management. Continued research and monitoring helps track and respond to emerging issues. Reducing preventable avian deaths remains imperative for successful bird conservation worldwide.