The Cuckoo Flock looks like a swarm of tiny clocks of many designs, all flying on feathered wings. Individually, a given Cuckoo is a harmless pest, and will flee anything much larger than a house cat. It is when they gather together into a Flock that they become dangerous.
A Flock is a Cuckoo mating swarm, which gathers together on some arcane yet precisely-kept schedule. Much as salmon must swim upstream to their original spawning grounds, each Swarm keeps its parents' schedule. A small Flock in full mating frenzy is of little danger, while larger Flocks are a much greater threat to anyone in the surroundings. This is because of how they raise their young.
Which is to say, they don't. Cuckoo Flocks are psychic brood parasites, like their avian namesakes. Once the mating is complete, the Flock will lay their eggs in the minds of everyone in the immediate proximity, and then die off. A small Flock may use one or two people as a nest, while the largest Flocks can nest their eggs in the minds of dozens of people. Few know they've been compromised until the eggs shortly hatch. When the eggs hatch, the fledgling Cuckoos unseat the host's consciousness and take over the body, killing the original personality.
For a time that may last weeks or even months, the host becomes as a local in the Mad City, living only to fill its role. They transform into gray-suited, gray-faced businessmen that wander the City streets to and fro, going from place to place on precise schedules. If blocked, they walk around the obstacle. If constrained, they fight fiercely to get away and resume their schedules. They don't stop to talk, eat, or drink unless it's part of the schedule, and even then they'll do the exact same thing day after day — even if there's nobody to talk to or nothing to consume, they say what they have to or go through the motions of eating the same foods over and over again.
The cycle ends when the Cuckoos inside have finished their growth, having fed off such pure, dedicated precision in their host. They violently depart from their host's mind and scatter, to come together when next the mating cycle begins anew.