There is a particular quality of light in late March — thin, uncertain, not yet committed to the idea of spring — that makes everything in the Loop look provisional. The buildings seem to be waiting. The lake holds its breath. And now, in the skeletal honey locust trees along Michigan Avenue, something green has arrived that does not belong and yet has decided, with the conviction of all great settlers, to stay.

A colony of monk parakeets, numbering somewhere between forty and sixty individuals depending on who is counting and how still they are willing to stand in the cold, has been building communal nests in the trees bordering Millennium Park since late February. The birds, native to South America and roughly the size of a robust fist, are neither quiet nor subtle. Their calls — a sharp, ascending screech that one birder described as “a smoke alarm having feelings” — have become the dominant audio texture of the east Loop morning.

Dr. Nina Vasquez, an urban ornithologist at the Field Museum, has been monitoring the colony since its earliest sightings. “Monk parakeets are remarkable engineers,” she said, crouching beneath a nest the size of a laundry basket. “Each pair builds their own chamber within a larger communal structure. It’s essentially a condominium.” She paused, looked up at the nest, and added: “With better governance than most actual condominiums.”

The birds are not new to Chicago. Colonies have persisted in Hyde Park and on the South Side for decades, descendants of pets released or escaped in the 1960s and ’70s. What is new is their expansion into the Loop — a migration that Vasquez attributes to a combination of milder winters, abundant food waste, and what she called “sheer audacity.” Monk parakeets, she explained, are among the few parrot species that build stick nests rather than nesting in cavities, which allows them to colonize virtually any structure with a horizontal surface. “They’ve nested on cell towers, stadium lights, highway signs. A honey locust is practically a luxury development.”

The pigeons, for their part, have not taken the news well. While anthropomorphizing avian behavior is a fool’s errand — and we are, admittedly, in the right publication for that — multiple observers have noted a distinct territorial tension in the Millennium Park area. Pigeons that previously roosted in the affected trees have been displaced to nearby ledges and awnings, where they gather in what can only be described as committees. “I’ve never seen pigeons look offended before,” said pedestrian and amateur birder Mark Tanaka. “But there’s no other word for it.”

City officials have taken a cautiously neutral stance. The parakeets are not protected under federal migratory bird law — they are classified as an invasive species — but neither are they causing the kind of infrastructure damage that has prompted removal efforts in other cities. A spokesperson for the Department of Streets and Sanitation confirmed that the city is “aware of the parakeet situation” and is “monitoring it,” which is municipal language for “we have not yet decided whose problem this is.”

For now, the parakeets seem content. They chatter through the mornings, shred bark with industrious beaks, and occasionally swoop low over the Bean in formations that startle tourists and delight absolutely everyone else. There is something defiant about their presence — bright green against gray steel, loud in a city that respects loudness, building homes in a place that was never meant for them but suits them anyway. In this, perhaps, they are more Chicago than they know.