This refers to the text on f.46r which says cranes keep watch in turn at night, holding a pebble in their claw to ward off sleep.

Of cranes. Cranes take their name, grues, from the sound of their own particular call. or such is the low, muttering sound they make. It is interesting to recall how cranes organise their journeys. They go to some extent in military formation, and in case the wind should be against them on their way to their chosen land, they eat sand and ballast themselves to a reasonable weight by picking up small stones. Then they fly as high as they can, so that a from higher vantage point they can look out for the lands they seek. As they fly swiftly on their way, they follow one of their number in a V-shaped formation. Confident in its navigation, it leads the flocks. It scolds the laggards and keeps the formation together with its calls. When it grows hoarse, another takes over. Cranes are united in their concern for those who tire, to such an extent that if any drop out, they all surround the exhausted birds and support them until their strength is restored by this period of rest. At night cranes keep careful watch. You can see the sentinels at their posts; while the other members of the flock sleep, others do the rounds and check lest they should be ambushed from any quarter; with their tireless energy, they ensure total vigilance. When the crane’s turn on watch is over and its duty is done, it settles down to sleep, first giving a cry to wake one of those already asleep, whose turn it is to be on duty. The new guard take up its allotted task willingly, not refusing, as we do, gracelessly, because we want to go on sleeping; instead, rousing itself readily from its resting-place, it takes its turn and repays the service it has received with equal attention to duty. Cranes do not desert the flock, because they are devoted by nature. They keep a safe watch, because they do it of their own free will. They divide the watches at night and take them in turns, according to a roster, holding small stones in their claws to ward off sleep. They give a cry when there is cause for alarm. Their colouring shows their age, for as they grow older, it grows darker.

We can take the sentinel cranes to mean those discerning brothers who provide temporal goods for their brethren in common and have a special concern for each one of the community. They watch over the obedience of their brothers, as far as they can, protecting them prudently from the assaults of devils and the incursions of this world. The cranes who are chosen to watch over the others hold a small stone in their claw, which is raised off the ground, fearing lest any of them fall asleep, in which case the stone will slip from their claw and fall; if it falls, the crane wakes up and cries out. The stone is Christ; the claw, the disposition of the mind. For as anyone goes on foot, so the mind strives with its dispositions for its desires, as if on foot. If, therefore, anyone stands guard over himself or his brethren, let him carry a stone in his claw, that is, keep Christ in his mind; or let him be very careful lest, if he sleeps in sin, the stone should fall from his claw, that is, Christ depart from his mind. If, however, the stone has fallen, let him cry out by means of confession, that he may awake those who sleep, that is, let him urge his brethren to watch out attentively as much for him as for their own faults. The colouring of the cranes reveals their age, for it grows darker as they grow older. This colour in old age refers to the elderly when they weep for their sins. For when the elderly remember their faults, they change colour in their latter years. For the old change their love of former pleasures into the sadness of repentance.

Folio 45v – the vulture, continued. De [gruibus]; Of cranes.