The pecking birds, possibly doves, are an iconographic motif that frequently appears in the cloister and in medieval sculpture in general.
It could be a reference to the Roman author Pliny the Elder, who describes in his Natural History the ability of ash wood to drive away snakes.
At the end of Antiquity, this ability was attributed to a mythical tree, the peridéxion, where doves could shelter from the snakes.
Christian theologians made this mythical tree a symbol: the tree represents the Christian faith, where the doves, symbols of the Holy Spirit (one of the forms of divine power), come to feed on the fruits of the tree, which symbolize heavenly wisdom.
To stray from this tree is to risk falling victim to the snakes, an animal associated with evil and the Devil.