Threats to Native Birds
Major threats affecting bird populations
Habitat Loss and Degradation
By far the largest threat to birds is loss and/or degradation of
habitat due to human development and agriculture. In some regions the
loss of habitat is extreme. For example, more than 95% of Tallgrass
Prairie habitat in the U.S. has been destroyed since the 1800s. Aquatic
habitats are drained, plowed, filled in, and channelized, while
terrestrial habitats are clearcut, overbrowsed, and fire-suppressed.
Natural disturbance regimes are changed by water and forest management
practices. Habitat is also degraded by the introduction of invasive
plant and animal species that can alter the nesting, foraging, and
roosting habitats for birds.
Increased Numbers of Competitors
Competition with abundant and exotic species makes it harder for native species to survive.
Example: Snow Goose populations tripled in size between 1969 and
1993. Their foraging behaviors leave the arctic tundra denuded of
vegetation, which will probably take hundreds of years to regrow. As a
result, shorebirds that breed on the tundra are now declining.
Birds are targeted for activities such as hunting and capture for
pets. Each year, tens of thousands of migratory birds, including
Baltimore Orioles and Painted Buntings, are captured for the caged bird
trade on their wintering grounds in Mexico, Cuba, and Central America.
Painted Buntings have declined 60% during the last 30 years, according
to Breeding Bird Survey data.
Introduced predators are especially problematic on islands where, in
the historic absence of predators, birds have evolved to nest on the
ground. Hawaii and Guam are well-known examples where many of the
native birds have gone extinct due to predation by introduced cats,
dogs, rats, mongoose, ferrets, brown tree snake, monitor lizards,
sheep, goats, pigs, and more.
Birds can become sick or die from eating toxins, or absorbing them through their skin. Examples of such toxins include DDT, pesticides, metals, oil spills, and bilge discharges. A recent connection has been made between herbicide spraying on lawns and bird kills (see the Audubon website).
Indirect Chemical Pollution
Acid rain has recently been linked to population declines in forest birds. Acid rain washes calcium out of the soil and decreases the amount of calcium-rich prey for birds like thrushes, which require a calcium source to produce eggs. This interaction is being studied in more detail by the Lab's Birds In Forested Landscapes citizen-science project.
Eutrophication, excessive plant growth resulting from an influx of nutrients, changes the water quality and species composition of lakes and ponds. This can deprive birds of their food source, or even make the water body unsuitable as bird habitat.
Avian diseases, including avian malaria, pox, House Finch disease, and West Nile virus, are leading causes of death among some bird populations. Many of Hawaii's native birds suffered drastic population declines once introduced mosquitoes began transmitting avian malaria between birds. The Cornell Lab of Ornithology is studying the mechanisms behind the House Finch eye disease (conjunctivitis) and West Nile virus.
- Human disturbance of nesting, feeding, and roosting areas. Disturbance can come from kayaks, jet skis, low flying aircraft, pets and feral animals, off-road vehicles, and other outdoor activities.
- Many seabirds suffer from the longline and gill net fisheries
when they become entangled and drown.
- Diminished food supply through fisheries, eutrophication (see above), habitat alteration that reduces prey (salinity changes in estuaries, forest structure), and replacement of food sources with invasive species.
Example: Shorebirds migrating along the Atlantic coast rely on horseshoe crab eggs to increase their body weight before continuing their flight to their Arctic breeding grounds. Horseshoe crabs are harvested for bait, and their populations take many years to recover. Therefore, shorebirds that rely on horseshoe crab eggs have declined in numbers and often have insufficient weight for breeding.
- Degraded quality of foraging habitat by the invasion of introduced species or by fisheries that trawl the sea bottom.
- Public dislike for some species, such as colonial waterbirds that nest or roost in urban and suburban areas, may be a barrier to long-term conservation. Examples include Double-crested Cormorant, Ring-billed Gull, or vultures.
- Increased human populations bring communication towers, wind power development, domestic cats, lighted buildings along migration corridors, nest parasitism, and competition with exotic species such as European Starlings and House Sparrows.