Of all my errors of well-intentioned parenthood, an incident involving a dear friend's son and a parrot has filled me with the most guilt.

My friend, who lived in the same apartment complex in Rome as I did, had died the year before after a long illness. We were all still reeling from her loss when one day I suggested to her son, then 13 years old, to bring their caged bird up to our terrace for a little fresh air and sunshine. Caged animals have always filled me with dread and Shaulin, as the bird was called, seemed particularly miserable in a dark corner of their flat.

Giovanni and I placed her cage on a table outside in the shade. She sprang to life instantly, flitting about the cage and bursting into song. A surge of relief ran through me.

That is, until Shaulin hopped over to the cage door and popped open the latch with her beak, fluttering off to perch on the palm tree in our condo courtyard. For a few minutes, I held out hope she might come back. Then she flew off out of sight.

It was a horrible moment. Through an act of busybody do-goodery, I'd managed to bring about one more terrible loss to a boy already overcome with grief over his mother. I apologized, repeatedly, and tried to soften the blow by suggesting Shaulin might be happier free - to which Giovanni simply said, "The seagulls will eat her."

As if to back up his point, the enormous gull perched like a sentinel on the roof of the next building let out a great, cat-like howl.

Biologist Bruno Cignini says, Rome offers ideal conditions for invasive species to settle

Rome: Tourist and invasive species magnet

A few months later while speaking with Francesco Messina, head of tree maintenance for the City of Rome, my guilt over Shaulin shifted a little.

We were standing in a small historic park called Villa Lazzaroni where the palm trees were dying. A copper-colored beetle from Asia called the red palm weevil had made it to Italy in 2004, and was devastating the country's palms.

As we talked, above us dozens of wailing green streaks flashed by: parrots, dashing from one tree to the next. After asking Messina if seagulls ate them - no (sigh of relief) - he explained that just like the red palm weevil, parrots are an invasive species, proliferating in Rome's parks after people who had kept them as pets set them free.

With global warming and rising winter temperatures, he said, thousands of the birds now comfortably live here.

But Rome's foremost expert on the city's birdlife, biologist Bruno Cignini - director of Rome's Museum of Zoology - said it's more complicated than just global warming and a few parrots let loose.

"What you have to understand about Rome is that compared to most major cities in Europe, it has a vast number of historic villas and natural parks that make it an ideal city for invasive species to set up in," he explained in his office adjacent to Rome's Bioparco zoo.

Originally from South America, monk parakeets have made a home for themselves in Rome

Cignini has some experience with this. When huge swarms of starlings had invaded the trees along Rome's Tiber River banks in the 1990s, and their slippery guano was regularly causing car and motorino accidents, he came up with a technique of recording and playing their warning cries back to them to force the birds to move out of the city center.

Friendly parrots

Cignini says not one, but two very similar species of parrots have proliferated in the past two decades: the monk parakeet from South America, and the rose-ringed parakeet from Asia Minor.

He said it's possible the Asian parakeet migrated here. However, the gray-throated monk parrot that lives primarily in cedar trees in the Caffarella Park off the ancient Appia Antica road in south Rome almost definitely arrived here in a less conventional fashion.

"There was a clothing outlet shopping center near that Caffarella that had a huge cage with dozens of parrots," said Cignini. "When it closed, they just let the birds free - and their population has taken off in the park."

While the parrots don't directly threaten other local bird species, Cignini says they're amazingly adaptable.Their gregarious habits of taking over whole trees to set up nests either inside trunk holes, as the rose-ringed parakeet does, or in complex beehive-like structures on branches built by the monk parakeet, squeeze out local species such as sparrows, making it harder for them to find living space and food.

Still, parrots play a fairly benign role in transformation of the bird-scape of Rome. Seagulls, too, have become ubiquitous - and are far less pleasant.

Combative seagulls

The arrival of seagulls in Rome can be pinpointed to the misplaced benevolence of the founder of the Italian World Wildlife Foundation, renowned environmentalist Fulco Pratesi.

Rome's abundant household garbage is an endless source of food for seagulls and other invasive bird species

In 1971, a friend of Pratesi handed him an injured female gull in a shoebox. Pratesi got permission from the head of the Rome zoo to leave the bird there to convalesce. Then one spring, a male gull passing overhead spotted her.

"They became a couple and built a nest of paper handkerchiefs, dirty rags, and other debris on concrete rocks," explains Pratesi. "Their offspring kept reproducing, and gradually they spread to the historic center."

There are now hundreds of gulls perched on church cupolas and apartment rooftops throughout Rome - some 20 kilometers (12.5 miles) from their natural habitat along the Mediterranean Coast.

"These are extremely aggressive birds," says Cignini. "If one lays an egg on your terrace, it's next to impossible to make it leave. When people have tried, the birds send out a war cry, and a whole group will attack you, one after the other."

He says an initiative to reduce their numbers by poking holes in eggs was blocked by environmental groups. And Italian legislation bans killing them.

With Rome's garbage pickup problems, the birds have a buffet of rotting food piled up in the streets below to feast upon nightly, their screeching wails piercing the quiet before dusk.

Indeed, where once stood a royal seagull on the condo roof across the way, now stand at least three. And that's not the only change in the view from my apartment terrace. The courtyard palm tree that Shaulin, Giovanni's beloved parrot, first landed on after her escape? The red palm weevil infested it, and it was dead two years later.