It is estimated that half of all the birds threatened with imminent extinction are threatened either wholly or partly by introduced plant and animal species.1
Invasive plants affect birds in many ways. They replace native vegetation needed for food, shelter and nesting. Invasives, such as multiflora rose, produce fruit that may be of lower nutritional value to our native birds than native shrubs, like blackberry, blueberry and dogwood species. Invasive plants are also likely to be unpalatable or even toxic to our native insects. What negatively affects insects, will negatively affect birds. Many native song birds feed their young on insects, so fewer insects means that fewer young birds will reach maturity.
Other studies have shown that certain species of birds, such as Northern robin and the wood thrush, are lost to predation 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.2
Ground-nesting birds, as well as other ground-nesting animals, are negatively affected by the presence of the red imported fire ant. Red imported fire ants originated in South America. As anyone from the Southern U.S. knows, they are extremely aggressive and have a powerful sting. 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. 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 populations of the golden-cheeked warbler and the black-capped vireo are also seriously affected by red imported fire ants.
European starlings are native to 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. Starlings invade any habitat that has suitable nesting sites, including woodlands, farmlands and cities. European starlings nest earlier than many native cavity nesters, creating serious competition for suitable nesting sites. They also destroy the eggs and nestlings of tree swallows, woodpeckers, purple martins, great crested flycatchers and eastern blue birds.
West Nile virus has been detected in over 250 species of birds in the United States. It has now spread across the U.S. and is present in every state except Alaska, Hawaii and Washington. The virus was first detected in the United States in New York City in 1999. 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. Deaths attributed to the virus have been noted in almost 60 species of birds in the Northeastern United States. The disease is spread to humans from infected Asian tiger mosquitoes which were accidently introduced to the United States. Symptoms range from mild and flu-like to no symptoms at all. However, if the virus enters the brain it can cause swelling of the brain and spinal cord and lead to death. No specific vaccines are available for the virus at this time.
1Simberloff, D. 2007. Aliens among us. Conservation Magazine 8 (2). pp 18-19.
2Schmidt, K.A. and C.J. Whelan. 1999. Effects of exotic Lonicera and Rhamnus on songbird nest predation. Conservation Biology 13 (6). pp 1502-1506.
K.A. Rawlins, 2013. Why should I care? Series: I Am A Bird Watcher, Why should I care about invasive species? The University of Georgia Center for Invasive Species and Ecosystem Health,, Tifton GA, BW-2013-113. 2 p.
This publication based on: Midwest Invasive Plant Network. 2008. Why should I care about invasive plants? How invasive plants impact hunting, fishing, boating, gardening, hiking, biking, horseback riding, and other recreational activities in the Midwest. From mipn.org/InvasivesBrochure.pdf.