|A Eurasian collared dove (Streptopelia decaocto) in Polland|
The main source of introduced birds has been through the pet trade. For example, birds are believed to have escaped from a breeder in Bahamas in the 1970's. These escapes are believed to be the sources of the birds now seen in the Bahamas, as well as the Caribbean and the North America.
There are several major concerns involved with having these birds become an invasive species. For one, they compete with native species such as the turtledove and mourning dove. They are also known for fouling and eating grain crops and products, and are thus an agricultural nuisance. They are also known to carry West Nile virus, and are believed to be possible amplifiers that, as carriers, could cause outbreaks of the disease. They also carry another virus, pigeon circovirus, which has the potential to kill dove and pigeon species that the collared dove may come in contact with.
This is a species that I have seen in my area. It's hard to miss that black collar. As with any invasive, it concerns me greatly whenever I spot one. In certain parts of the bird's introduced range, they are popular with hunters. Hunting can be one of the most effective methods of controlling an invasive species, so I see this as a positive thing.
Source is the Global Invasive Species Database. Image is from Wikimedia Commons under a Creative Commons license.