Invasive species are the main cause of recent bird extinctions

Humans have been transporting animals and plants from one part of the world to another for thousands of years, sometimes deliberately as in the case of livestock, and  companion animals such as cats and dogs and sometimes accidentally (for example rats escaping from boats). Often introduced species fail to establish themselves in the wild in new locations. However, a proportion thrives and spread. Such 'invasive alien species' may have catastrophic impacts on local fauna and flora. Invasive species can affect native ones by eating them, competing with them, hybridising with them, disrupting or destroying their habitat or, in the case of pathogens or parasites, by sickening or killing them. Over the last five hundred years, invasive alien species have been partly or wholly responsible for the extinction of at least 65 bird species, making this the most common contributory factor in recent losses to the world’s avifauna ().

• Invasive alien species have been implicated in nearly half of recent bird extinctions

Invasive species are a particular threat on islands

Overall, invasive species affect around half of currently threatened bird species. Island species are particularly susceptible because of their isolated evolutionary history, with three-quarters of threatened birds on oceanic islands affected by invasive species (). Rats and cats have had far and away the greatest effect, threatening the survival of hundreds of different bird species worldwide, but other species can also have devastating, if more local, impacts ().

• Small island birds are most at risk from invasive alien species
• Native birds on Gough Island are being devastated by house mice

Invasive diseases are a growing problem

Diseases carried by invasive pathogens and parasites are already implicated in the decline and extinction of many bird species (). Some diseases, such as West Nile Virus, appear to be spreading to populations previously unaffected, including to species already threatened by other factors (, ). ‘Bird flu’ is of particular concern, not because of direct impacts on wild birds, rather because it is leading to negative public perceptions towards birds ().

• Native forest birds in the Hawaiian Islands are limited by introduced avian diseases
• Avian diseases are spreading to impact hitherto unaffected populations
• West Nile Virus is spreading throughout the Western Hemisphere
• The H5N1 avian influenza virus: a threat to bird conservation—but indirectly

Global travel, trade and a changing climate encourage the spread of invasive species

Increases in human mobility and expansion of global trade encourage the spread of invasive species. Global climate change creates conditions suitable for new invasives. For example, increased temperatures potentially enable disease-carrying mosquitoes to expand their ranges. These factors, together with the degradation and fragmentation of natural habitats which make it easier for invasives to establish themselves, mean that invasive species are likely to become an increasing threat.