The growing number of alien bird species is harmful for native birds. A study suggests that at least 1,000 alien species of birds were introduced to areas with human activity in the past 500 years -- half of them are exotic birds. These invasive species threaten the bird biodiversity by competing with local species and carrying diseases. They can also cause serious damage by destroying local crops.

It is difficult to predict where invasive species will survive. If the species become fully invasive, it will be difficult to control or eradicate them. The study mapped the global distribution of invasive bird species and identified the factors for their which may became a guide to determine where possible invasion would arise next.

Investigating about 1,000 bird species from the 1500-2000, the researchers found out that the invasion of alien species are caused mainly by human activities. "It has given us valuable insights into the different stages of species invasion - humans play a key role, but so too do environmental factors that allow alien bird species to thrive in new locations," Dr Ellie Dyer, lead researcher said according to The BBC.

The results were published in the journal, PLOS Biology. The first wave of alien species was introduced in the mid-19th Century in Europe. Meanwhile, the second wave started after the World War II where wild birds were introduced in pet trade. Dr. Dyer added that they mapped the richness of the alien species and located their population in detail.  

Three of the worst invasive alien bird species -- the Indian myna, the European starling and the red-vented bulbul -- have caused extinction of other species. Dr David Orme from the Department of Life Sciences at Imperial and co-author of the explained that the myna eats the eggs of other birds. On the other hand, the startling and bulbul cause damage of up to hundreds of millions of dollars in US agriculture by eating crops.