There were tears in the public gallery as 17-year-old Cairo Parker's manslaughter verdict was read out as someone cried out, "He's only a baby!"
The judge Mr Justice John Griffith Williams lifted restrictions on naming Parker and said he would hear mitigation tomorrow afternoon before sentencing the defendants on Wednesday morning.
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The Gambling Commission said: "Integrity in sport is not a joke and we have opened an investigation to establish exactly what happened."
The zoo had pest controllers bury pellets in the bird enclosure in order to get rid of rats, but they brought the pellets to the surface.