According to the Roman author Pliny the Elder, the crane was the most vigilant of birds. One crane would always stand guard while the others slept, holding a stone in its claws; when it fell asleep, the sound of the dropping stone woke the other cranes, and the next guard took up its post. By the sixteenth century, because of its reputation for vigilance, the crane had become an emblem of papal authority - just as a crane guarded its flock, so the Pope watched over his congregation.
Raphael presents the cranes as fishermen watching over the water, echoing the actions of the fishermen in the boats. This emphasises Peter's role as a fisher of men, and because the cranes are a symbol of papal authority, their presence provides another link between Peter and Pope Leo X.
The cranes are thought to be the work of Giovanni da Udine, a painter in Raphael's workshop who was noted for his skill in depicting animals and fish.
Ravens were thought to be particularly unpleasant birds: they fed on corpses ('going for the eye first'), and were said to refuse to feed their young properly until their first feathers grew in. Their bad reputation originates in the raven which Noah sent out from the Ark and which - unlike the dove - did not return (Genesis 8:6-7). Because the Ark was generally taken by theologians to represent the Church, ravens became symbols of sin and apostasy. At the time the cartoons were painted, ravens were also used in allegories to represent corrupt priests.
The ravens may also have been included to represent the bounty of God. Christ said to his disciples 'Consider the ravens: for they neither sow nor reap; which neither have storehouse nor barn; and God feedeth them: how much more are ye better than the fowls?' (Luke 12:24). And Job is reminded 'Who provides food for the raven when its young cry out to God and wander about for lack of food?') (Job 38:41).
In this scene, ravens appear near swans. This may refer to a contemporary proverb about the difficulty of converting sinners: 'one may as well try to make ravens white or swans black.
In the Sistine Chapel tapestry made from the cartoon, the birds have been depicted with white plumage and may represent seagulls.
Swans were noted for their beautiful song; it was even claimed that they sang so beautifully that lute players sometimes invited swans to join them in making music. They were also said to bring good luck to sailors.
However, swans were also common symbols of pride and deceit. They appear here with ravens, perhaps as an illustration of a contemporary proverb about the difficulty of converting sinners: 'one may as well try to make ravens white or swans black.'
Fishes and eels
Fishes were sometimes seen as emblems of compassion and piety, apparently because some species were known to shelter their young in their mouths. Here, they may represent souls that have been saved (taken up in Peter's nets), in contrast to the shellfish on shore which have been tossed aside and picked over by the cranes.
Christ tells Peter and Andrew to let down their nets into deep water, and many of the fishes depicted here are recognisable deepwater varieties. Raphael has included eels, lampreys, a small shark, flounder, and a small fish native to Israel, the Saint Peter fish.
Eels were thought to be relatives of snakes and other serpents. They were said to be born from mud, which explained why they were slippery. If eels were drowned in wine and someone drank the wine in which they had been drowned, that person would acquire a distaste for alcohol.
The crab was a symbol of greed, cunning, and covetousness (one of the seven deadly sins), and was said to feed upon the distress of others. According to legend, they hunted their favourite prey, oysters, by deceit, waiting until the oysters opened their shells before inserting a pebble so that the oyster could not close its shell again. The crab then feasted on the hapless oyster.
In contrast with the 'good' fishes in the boats, who may represent the souls of the saved, the crabs and other crustaceans are symbols of vice, picked over by other fishermen - the vigilant cranes.