Birdball is an elusive game supposedly created by William, Dom, NiceTrySean, and Grant Kirkhope.
At one point Dom mentioned that Birdball is in fact real, but he said "i can’t tell you more without getting in trouble. there’s been a dark cloud hanging over it since we did that exhibit in 2014"
Details about the game are sketchy and mysterious. To date, no working links have been found to download Birdball.
It may have some connection to the mysterious Kevin Patterson.
According to William Pugh, it was realized exclusively for the PS1 several years ago.