What's happening in the scene we can see in the Tondo?

The infant figure of St John the Baptist stands to the left with his baptismal bowl, the "attribute" with which he's often recognised in art history.

He presents a bird to the infant Christ, who momentarily turns towards his mother, symbolically anticipating his future destiny; the bird is widely believed to be a goldfinch, which represents the Christ’s Passion, the time of suffering before his Crucifixion, because a goldfinch is said to have removed a thorn from Christ’s crown when he was carrying the cross.

While it's often thought that Christ’s reaching away from the bird and towards his mother expresses his fear, more recently other scholars have argued that Christ’s pose is playful as the goldfinch was a common pet in this period.

This marble is believed to have been carved from 1504 to 1505 during Michelangelo’s first period in Florence. It's unfinished, probably because he left it when he travelled to Rome in early 1505 to work on Pope Julius II’s tomb. Giorgio Vasari, well-known for his Lives of the Artists published in 1550, suggested that Michelangelo did not complete some works out of creative frustration – an idea which has crystallised into the notion of the artist as troubled genius.