Birdwatcher details moment he found body of teen allegedly killed by MS-13.

A Queens birdwatcher described the harrowing moment he spotted the corpse of a teenager whose murder was allegedly ordered by an MS-13 gang boss — telling a Brooklyn federal jury Wednesday, “I saw a person face down.”

Eric Miller, of Bayside, was searching for birds on a trail in Queens’ Alley Pond Park on the morning of May 21, 2017, when he caught a whiff of a foul smell, he testified during the murder trial of alleged ruthless gang leader Melvi Amador-Rios.

“I encountered a smell of decomposition,” Miller said, calling it “overwhelming.”

“I figured it was a raccoon,” he said — until he walked closer and got a better glimpse through his binoculars.

“I saw a person face down” from about 20 feet away, Miller recalled. “I looked out with the binoculars and could see [the body] was covered with maggots.”

The corpse belonged to 16-year-old Julio Vasquez, whose hit was allegedly ordered by Amador-Rios, 32, the reputed boss of the Centrales Locos Salvatruchas clique of MS-13.

Vasquez was stabbed more than 30 times and nearly decapitated after prosecutors say he disobeyed an order to kill — and Amador-Rios began suspecting he was a snitch working with law enforcement.

The teen was lured to the park, where gang members Josue Leiva and Luis Rivas knifed him to death on May 16, 2017, according to prosecutors.

NYPD Officer Nicholas Kostas, of the 111th Precinct in Bayside, Queens, testified that he responded to a call about the teen’s body four days later.

“I noticed the body was lying face down with a blue hat lying next to it,” he told the jury Wednesday.

Leiva and Rivas have previously pleaded guilty to charges in connection with Vasquez’s murder.

Amador-Rios, aka “Letal” or “Pinky,” is standing trial on charges of racketeering, murder in aid of racketeering, and related crimes.

He faces a life sentence if convicted.