Bird mortality stems from a complex interplay of various factors, ranging from inherent biological processes to pressures exerted by the natural environment and dangers introduced by human activities. Understanding these diverse causes is important for public awareness and for informing conservation efforts aimed at protecting avian populations.
Biological Processes and Predation
Birds, like all living organisms, have natural lifespans that eventually conclude due to aging. As birds grow older, their bodies become less efficient, making them more susceptible to environmental challenges.
Diseases and parasites also contribute to bird mortality. Avian Influenza, a viral infection, can spread among bird populations, with highly pathogenic strains capable of causing sudden death. West Nile Virus (WNV), transmitted by mosquitoes, infects over 250 bird species, with corvids like crows and jays being particularly susceptible to severe illness and death, often displaying neurological signs. Salmonella bacteria can cause salmonellosis, leading to ruffled feathers, diarrhea, and lethargy, especially in songbirds during winter months through contaminated feeders.
Natural predation is an inherent part of the ecosystem, where birds serve as a food source for various animals. Raptors, snakes, foxes, and raccoons play their role in the food web, maintaining ecological balance.
A lack of sufficient food and water resources directly impacts a bird’s ability to sustain itself, often resulting in starvation and dehydration. These conditions can arise from changes in environmental conditions or scarcity of resources, weakening birds and making them vulnerable to other threats.
Environmental Pressures
Extreme weather conditions represent a significant threat to avian survival. Events such as severe blizzards and prolonged cold snaps can lead to hypothermia and a scarcity of accessible food. Conversely, intense heatwaves and droughts can cause dehydration and deplete vital water sources. Storms can physically harm birds, destroy nests, and scatter food supplies.
Natural habitats provide birds with essential resources, including food, shelter, and safe nesting sites. The destruction or alteration of these habitats, often through human development or agricultural expansion, removes these necessities. This directly impacts bird populations by reducing their ability to find sustenance and raise their young.
Large-scale natural disasters also have profound effects on bird populations. Wildfires can incinerate vast areas of habitat, displacing and killing birds. Floods can inundate nesting grounds and foraging areas, leading to widespread mortality. Volcanic eruptions can release ash and toxic gases that contaminate the air and water, impacting birds over broad regions.
Human-Related Dangers
Human infrastructure and activities introduce numerous dangers to birds. Collisions with human-made structures are a pervasive problem, with reflective or transparent glass on buildings often being invisible to birds, leading to an estimated 100 million to over 1 billion bird deaths annually in the United States. Vehicle collisions are another major cause of mortality, with estimates ranging from 89 million to 340 million bird deaths per year on U.S. roads. Power lines and communication towers also pose collision risks, particularly for migratory birds.
Pesticides and various poisons pose chemical threats to birds. Herbicides, used to control weeds, can reduce food resources and contaminate habitats, while insecticides, especially neonicotinoids, can directly poison birds that consume treated seeds or contaminated insects. Rodenticides, particularly anticoagulant types, lead to secondary poisoning in birds of prey that consume poisoned rodents. Lead poisoning from discarded fishing tackle or ammunition fragments is a concern for many species, including loons, swans, and raptors, as birds may ingest these items. Oil spills, even small ones, can coat feathers, causing hypothermia and loss of buoyancy, and birds attempting to preen can ingest toxic chemicals.
Birds can also become entangled in human-made debris. Discarded fishing lines, plastic bags, and other synthetic materials can trap birds, leading to injury, starvation, or drowning.
Domestic animals, particularly cats and dogs, have a substantial impact on bird populations. Free-roaming domestic cats are significant predators, estimated to kill between 1.2 billion and 4 billion birds annually in the United States. Dogs, even when leashed, can disturb birds, leading to nest abandonment, increased stress, and reduced foraging efficiency.