An unlikely plague has hit London and other Western European cities: rose-ringed parakeets (Psittacula krameri). Those birds, native in Asia and Sub-Saharan Africa and a well-liked pet have appeared in swarms. However, this did not come as  a surprise as scientists believe that they descended from birds that have been set free by their owners.

There numbers have since increased significantly, around 2.000 were estimated to live in Amsterdam (the data is from 2006, so the figures are likely to be higher),  10.000  in Brussels (allegedly they are descendants of 50 birds that were released in 1975 by the manager of an amusement park) and 30.000 in London.

What is the problem? People like these green birds, don't they? The have been reported to cause major crop damage in their native areas, eating fruits and cereals. In the UK they are still mostly found in urban areas, therefore besides damaging trees and eating fruits the resulting impacts are still low. However, scientists highlight that parakeets may become a pest in the future as the birds spread to rural areas.

The second problem is the competition with native species for nesting sites. This could especially hit nuthatches and starlings, the competition with woodpecker is estimated to be negligable but the number of studies to date is very low so all possible impacts are not yet understood.

What has allowed the population to expand? According to the scientists this is not clear. Many theories exist but none of them is really satisfying. More food could be available for the parakeets or predators could have disappeared. Or climate change and the resulting milder climate,  could be the reason but on the other hand the past two winters were especially cold. Or it could be that the population has reached a certain size so that it is easier for each of them to procreate. No one really knows but the phenomenon has caught the attention and will be monitored closely.

For more information:

http://www.ua.ac.be/main.aspx?c=diederik.strubbe&n=14 (A Belgian study on the spread of parakeets)

http://mudfooted.com/green-ring-necked-parakeets-amsterdam/ (parakeets in Amsterdam)

http://www.nytimes.com/2011/05/14/science/earth/14parakeet.html?_r=1&scp=1&sq=parakeet&st=cse (NY Times article on parakeets)

http://www.projectparakeet.co.uk/index.html (The UK-based project to research the impacts of the parakeets)