It is estimated that half of all the birds threatened with imminent extinction (510 species) are threatened either wholly or partly by introduced species. ³

Invasive plants affect birds in many ways. Invasive plants replace native vegetation needed for food, shelter and nesting. Invasives, such as multiflora rose, produce fruit that are of lower nutritive value to our native birds than native shrubs, like flowering dogwood. Invasive plants are likely to be unpalatable or even toxic to our native insects. What affects insects, in turn, will affect birds. Less food for insects leads to fewer insects; fewer insects may lead to fewer birds. Studies have shown that predation of certain species of birds, such as Northern robin and the wood thrush, increases when they nest in invasive, exotic shrubs, such as Amur honeysuckle or common buckthorn, as opposed to nesting in native shrubs with which they have co-evolved.⁴

Red imported fire ants, as anyone from the South knows, are extremely aggressive and have a powerful sting. Ground-nesting birds, as well as other ground-nesting animals, are greatly impacted by the presence of the red imported fire ant. Fire ants have been shown to attack bird eggs, juveniles, and adults. Newly hatched chicks can be completely engulfed by an onslaught of fire ants. Red imported fire ants have been shown to reduce bobwhite quail chick survival and, in one study, 38% of chick deaths were attributed to fire ant stings. The golden-cheeked warbler and black-capped vireo are also seriously impacted by red imported fire ants.

European starlings are native to western and central Europe and were introduced into New York in 1890. These invasive birds are extremely abundant. Population levels in North America have reached approximately 200 million. They have invaded any habitat that has suitable nesting sites, including woodlands, farmlands and cities. European starlings are cavity nesters and nest earlier than other cavity nesters. This may lead to serious competition for suitable nesting sites. They have also been shown to destroy the eggs and nestlings of tree swallows, woodpeckers, purple martins, great crested flycatchers and eastern blue birds.

West Nile virus was first detected in the United States in New York City in 1999. It has now spread across the U.S. and is present in every state except Alaska, Hawaii and Washington. The virus has been detected in over 250 species of birds in the United States. The American Crow, blue jays, and fish crow are particularly at risk. Infected birds become weak and can lose the ability to fly or walk. The virus is lethal to crows, but mortality rate among other species is not fully known. Deaths attributed to the virus have been noted in almost 60 species of birds in the northeastern United States. The disease can be spread to humans through infected mosquito bites. People that contract West Nile virus usually have mild flu-like symptoms or no symptoms at all. However, if the virus enters the brain it can cause the brain and spinal cord to swell and can lead to death. There are no specific vaccines or treatments available for the virus. So far in 2008 there have been 236 reported cases and two deaths caused by West Nile virus in the United States.