Birds “are not the same after they go through quarantine,” said one man, whose name was redacted in a report by the federal investigators.

Getting caught can result in a fine of several thousand dollars, as well as criminal charges. But enthusiasts and career smugglers said it was worth the risk. And demand for these featherweight champions has apparently not diminished.

For decades, the epicenter of New York’s racing scene has been Phil “Scooter” Rizzuto Park, which the men still refer to by its old name, Smokey Oval. In between the handball court and the baseball diamond, the men erect a patchwork of wood and metal cages perched on car tops, benches, hanging on telephone poles or wooden stakes planted in the ground. In recent years, the mood there has become tense with a pervasive fear that undercover agents and informants lurk with binoculars and hidden cameras.

MR. HARINARAIN, KNOWN AS “BUSH,” wears a gold, flat-braided chain with a bird charm. He presides as an unofficial spokesman for the birdmen, at times to the chagrin of his peers who are reluctant to expose their passion to criticism. After emigrating from Berbice, Guyana, in 1987, he began tending to the birds belonging to the elder statesmen in the park. When he could afford his own finches, he began organizing competitions, eventually succeeding the older generation.

“I control the birds now,” he said.

Federal agents have visited Mr. Harinarain’s home in Cypress Hills, where he lives with his wife and 43 birds that chirp from almost every corner, including the bathroom. The more valuable ones are monitored with webcams streamed to his iPhone. And each week, he spends about $100 and about an hour every other day inspecting their feathers and toes, scooping 10 varieties of seed into their trays, dropping liquid vitamins into feeding tubes and filling plastic cups with water so they can “take a shower.”

His garage is crammed with bird trophies and dozens of dangling empty cages. He was among the first in New York City to breed the finches, called “towa towa” in Guyana. But it was a tedious and difficult process, so now he imports them from Brazil, which has an organized industry of breeders. From a filing cabinet, he pulled out the birth certificate of one of the birds, a blue sheet of paper with a diagram resembling a family tree. Like racehorses, the male songbirds are bred for their pedigrees, he explained.

“Most of the guys don’t want to talk because they have illegal birds,” he said. “Mine are legit.”