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Causes of Bird Die-Offs

What Can Kill Flocks of Birds?


Bird die-offs can be disturbing and may seem like harbingers of greater environmental problems, but die-offs are more common than many birders realize and there are many reasons why dozens, hundreds or even thousands of birds may die at once. Understanding the causes of bird die-offs can help put these events into perspective and encourage everyone to take steps to prevent die-offs from occurring.


Many bird die-offs are caused by massive trauma to birds' heads, wings and bodies from powerful collisions. This can occur when flocks are startled and panicked and their natural agility is compromised, which leads to collisions with other birds in the air or with obstacles such as buildings, trees, electrical wires or radio towers.

Decrease the Risk: Avoid startling roosting flocks of birds with poorly timed fireworks or other loud noises. During migration seasons, encourage buildings to turn off lights that may attract bird flocks to window collisions.


Inappropriate use of pesticides or insecticides on agricultural crops can poison flocks of birds that feed on the contaminated grain. Similarly, chemicals that have contaminated popular water supplies can result in mass poisonings of bird flocks. Landscaping chemicals such as fertilizers and herbicides can also be poisonous if used improperly.

Decrease the Risk: Always use outdoor chemicals appropriately and discard unused chemicals safely. Consider supporting local farmers who use organic techniques and minimize the use of added chemicals on fields and lawns.


A sudden cold snap can kill winter birds, despite the many ways birds have to keep warm in winter. This hazard generally affects small birds more easily but can impact any bird species depending on the degree of the temperature change and the availability of suitable shelter.

Decrease the Risk: Provide winter bird shelters in a bird-friendly backyard, and keep feeders stocked with high-oil foods to allow birds to build up energy reserves that can help them maintain the necessary body heat to survive.


Migrating birds that cover long distances without rest can succumb to exhaustion, and even if they do not die immediately, they may become disoriented and crash into the ground or obstacles in the air and die from the resulting trauma. While many birds are equipped for long distance flights, if they become disoriented and fly longer than necessary, exhaustion can be fatal.

Decrease the Risk: Preserve suitable habitats along migration flyways to give migrating birds familiar oases to rest and recuperate. These habitats can be as large as a nature preserve or as small as a bird-friendly backyard.


Diseases such as avian botulism, West Nile Virus and avian influenza can have devastating effects on flocks of birds. If conditions are deadly, the disease can spread quickly and may overcome dozens or hundreds of birds in a very short time.

Decrease the Risk: Keep bird feeders clean to avoid spreading diseases in the backyard, and stay alert for warnings about disease outbreaks. Avoid feeding ducks bread that can lead to avian botulism in local ponds, and report infected birds to appropriate officials so suitable action can be taken.


Ticks, mites and other parasites can quickly spread diseases or other harm to a flock of birds. Whether the disease is fatal or the birds suffer other consequences, such as debilitating feather damage, the results can be catastrophic.

Decrease the Risk: Clean bird houses before and after nesting seasons to minimize parasites, and do the same for any bird roost boxes in the winter.


Pollution kills hundreds of thousands of birds every year, either through small scale poisonings or large events such as oil spills or other toxic hazards. Pollution can also impact birds' food supplies, nesting areas and migration routes, all of which can increase bird mortality.

Decrease the Risk: Always dispose of toxic chemicals appropriately, and offer assistance to wildlife cleanup and recovery efforts either through financial or material donations or volunteering.


A violent storm can kill dozens or hundreds of birds quickly, either through impacts with brutal hail, disorientation from whipping winds or lightning strikes that can decimate a bird roost. Larger storms such as hurricanes can destroy shorebird and waterfowl nesting habitats as well.

Decrease the Risk: Provide safe roosting areas in your backyard and support habitat conservation and restoration in storm-damaged areas. Recycle Christmas trees to minimize beach erosion, and support zoos that provide captive breeding programs for birds impacted by storm damage.


Invasive predators can quickly wipe out nesting colonies of birds, and even small scale predators such as feral cats or out of control dogs can kill dozens or hundreds of birds. Rats, snakes, raccoons and other predators can also kill birds and have an impact on local bird populations.

Decrease the Risk: Keep pets controlled and take steps to protect backyard birds from cats. Avoid supporting feral cat colonies, and provide safe bird houses for nesting birds.

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