It's autumn in Germany and barges ply the romantic Rhine and Main rivers on a chilly December day as a flock of graceful birds fly overhead -- screeching, bright-green tropical parrots.
They are Indian ring-necked parrots, descendants of household pets that have gone forth and multiplied to such an extent in central Germany over the past 40 years that authorities now consider them an "indigenous" species.
And they are winging their way across Western Europe and even into temperate southern England, where they are flourishing, according to European ornithologists.
"They are definitely on the move," says Dieter Zingel, head of the Hesse State Ornithology Society in Wiesbaden.
"Until a few years ago, the ring-necks were isolated in and around one park in Wiesbaden and their population was no more than about 300," he explains.
"I've been monitoring that population for the past 30 years," he adds, "and I had almost concluded that the feral population was remaining constant. But now they have multiplied and are definitely on the move."
The 60cm long parrots, noted for their bright coloring and loud screams, have been sighted in Mainz and other cities along the Rhine-Main valley.
"They also appear to be nesting in these new regions, which means their young will be native to new areas," Zingel says.
As their name suggests, the Indian ring-necks are native to the Asian Sub-Continent. Large numbers were imported to Europe from Sri Lanka in the 1960s until the Convention on International Trade in Endangered Species (CITES) severely limited their import.
But by that time enough birds had flown their coops to establish viable populations in the Rhine-Main valley, where warm summers and relatively mild winters are not only ideal for wine-makers -- but also for Sri Lankan parrots.
The Indian ring-necked parrot was first recorded in a wild state in Germany in the late 1960s when a few imported birds escaped captivity and settled in a Wiesbaden park.
These feathered immigrants found the Rhine-Main valley much to their liking. In the wild, Indian ringnecks eat a variety of seeds, berries, fruits, nuts, blossoms, and nectar -- and all these foods are abundant along this valley.
In the wild, these hardy birds can cope with sultry summer temperatures as well as with chilly winters -- just like many of the wine grape varieties that thrive in this same region.
Above all, these birds are highly intelligent and adaptive when it comes to living in close contact with humans.
In captivity, besides learning to talk, these parrots are known to be great at learning tricks. Some have been taught to string beads on a rope, twirl sticks about their head, ring a bell, and pick up selected objects.
"In the wild, they are nobody's fool when it comes to carving out an ecological niche for themselves," Zingel says.
"A further advantage is the fact that they do not mate for life," says the veteran ornithologist. "That means a male can mate with a number of females and thus enhance the chances for the species' survival in a new habitat."
The incubation time is between 22 days and 24 days and the young will leave the nest about six to seven weeks after they hatch, meaning the skies over the Rhineland will be full of bright green birds all summer.
"They have been sighted in Cologne on the Rhine, and as far as Heidelberg and Worms and a few have even been seen in Stuttgart down in the Black Forest region," Zingel says.