It may weigh just a few grams and fit into a human hand, but the European goldfinch (Carduelis carduelis) is mounting an invasion worthy of Genghis Khan. As the name suggests, the finch is indigenous to Europe but now the species can be found in large numbers in northwestern China and is moving rapidly east. Along with birdwatchers from Hong Kong, Ma Ming, an ornithologist at the Chinese Academy of Science's Xinjiang Institute of Ecology and Geography, spotted the finch for the first time in Ili Kazak Autonomous Prefecture on the China-Kazakhstan border in 1996. Mr Ma said it appeared to be a lone lost bird and he had not expected to spot the species again. 'Birds have their own national borders,' he said. 'Sometimes they might get lost and cross the line but they don't stay. Local birds immediately drive them out.' But he said reports of sightings had become increasingly frequent in the region in the decade since. Mr Ma said he saw a flock near a small village on the outskirts of Urumqi in December. 'A Xinjiang birder can detect them easily by sound,' he said. 'Their song is rather [distinctive], or, I should say, alien. They sound like a French orchestra in the Chinese countryside. 'When I saw the gold-feathered finches twittering on top of a barren tree, I was astonished by the speed of their expansion inland. In a decade, they had moved 800km southeast, and after close observation of their habitat and population we concluded with confidence that they were thriving.' Mr Ma said the explosion in European goldfinches was one of the hottest topics among members of the Xinjiang Bird Watching Society. Some members had volunteered to draw a distribution map of the species, some had studied its multiplication rate and some had analysed the bird's diet. One of the initial findings was that they seemed to outnumber native finches. Another worrying finding was that the European goldfinch was just one of more than 20 bird species that had advanced into Chinese territory in about a decade. According to Mr Ma, the most likely explanation was China's rapidly deteriorating environment. 'Government officials like to trumpet that whenever a new foreign species is found in China it is an example of our improving environment. 'See, even the birds like it here,' they always tell me,' he said. 'But I see only the opposite. Because of industrial pollution and the widespread use of fertilisers and pesticides in agriculture, most of the large predators - eagles, foxes and snakes - have been killed off. 'So when the European goldfinch arrived - they like scavenging on rubbish dumps, by the way - they probably said: 'Hey, it's dirty but much safer here! All of our enemies are gone. Let's make it home.' 'There is limited space and food for birds, so the success of an alien species means that some of our native birds will go into oblivion.' Mr Ma said global warming was likely to be another driving factor. But not everyone agrees that the growing population of European goldfinches is a bad thing. Zhang Zhengwang, secretary general of the China Ornithological Society, said that in recent years some indigenous birds had also ventured into new territory. Blackbirds and the Chinese bulbul (Pycnonotus sinensis), for instance, had moved from southern China to northeast provinces such as Liaoning. 'Climate change could be a factor, but there are also more birdwatchers in China than ever before, an important reason so many birds that were not known to have been in China have been found in recent years,' Professor Zhang said. 'The mainland is still dominated by native bird species.' Little troublemaker The goldfinch or European goldfinch (Carduelis carduelis) breeds across Europe, North Africa and western and central Asia. It lives in open, partially wooded lowlands. The average goldfinch is 12-13cm long with a wingspan of 21-25cm and weighs 14-19 grams. Goldfinches are commonly kept and bred in captivity because of their distinctive appearance and pleasant song.