Whether local motion particularly agrees with birds.
Whether this local motion is for the sake of the nutriment or the disposition.
"And some people," etc. First one inquires with which animals local motion from place to place agrees.
1. And it seems first that it agrees with fish. For everything is received in a recipient according to the mode of the recipient. Since, then, the sea is especially fluid and movable from place to place, such motion must especially suit fish; therefore, etc.
2. It seems to agree with the walkers. Because motion such as this agrees with an animal that has very good estimative power, because they feel [aestimant] it is better to be in one place than in another. But a good estimative power only suits walkers, and therefore, etc.
The Philosopher says the opposite. For he says that it especially suits birds.
After this one inquires into the cause of this motion, whether it is for the sake of nutriment or for the sake of the disposition in one that possesses it.
3. It seems that it is not for the sake of the nutriment. Because this would agree equally with the large and the small, with the birds of prey and those that are not birds of prey, and this is not true. For the large birds migrate and not the small, and similarly the birds of prey do not migrate but others do.
Nor does it seem to be for the sake of the disposition in one possessing it, for the same reason. And in addition to this, all those that are in the same region migrate this way.
One must reply first to the second argument that there is a twofold reason why animals migrate. One stems from the nutriment, and the other from the one possessing it or from that one's complexion. It stems from the nutriment, because animals with a good estimative power change to another location when the aliment appropriate for them is lacking in one place, moving to where it is more abundant. And this is why many fish move from the bottom of the sea to the shore, because the earthborn things on which they are nourished are more abundant there.
Another reason stems from complexion. For those, like the crane, that greatly fear the cold, following a natural instinct, move to warm places before winter, and those that fear the heat move to cold places, and this is why fish that fear cold move from the bottom of the sea to the shore, whereas those that fear heat move in the opposite way.
And for the same reason some animals remain on land during the day but spend the night in the water, because the land is warmer during the day owing to the surrounding warm air and the appearance of the sun, and the water is warmer at night for a period of time because just as the air quickly receives heat in the presence of the sun, so it loses it in its absence. But water receives heat with more difficulty, and for this reason it retains it longer.
Moreover, at night in the absence of the sun, the cold of the middle interstice flees the heat that was first received in the air at the surface of the water. And this is one reason why water is warmer at night than it is during the day, in comparison to the land. Thus both the nutriment and, similarly, the animals' individual complexion cause migration among them. This change, however, principally suits birds and fish next, because birds are better disposed to movement than are walkers and have better estimative power than swimmers. And similarly birds exist in an element, namely, air, that is especially suited for motion. Thus, on account of these many reasons, this is more appropriate to birds than to other animals.
1. On to the arguments. To the first, one must reply that although the sea or the water is movable, air is nevertheless more movable and more divisible, and although water sometimes contributes more to a swirling motion, air nevertheless contributes more to straight movement.
2. To the second argument one must reply that although the estimative power is better in walking animals, still they possess more weight. Because of this the argument lacks force.
We have already spoken to the other question.
3. To the first argument one must reply that in order for animals to move from region to region it is necessary for them to be good-natured and strong, because they must pass through many dangers. Moreover, it is necessary for them to have good estimative power, so that they can better take precautions. These things are not found in small birds as much as they are in large birds. For large birdslike cranes, geese, swans, and the like migrate more than do small birds. Still, small birds like sparrows, which are confident in their wings, do this.
Moreover, birds of prey are not as good-natured, nor do they remain in a flock, because they fear the loss or theft of their prey, which they acquire only with great effort. Similarly, they know how to live in places known to them better than in ones unknown.
And moreover the change of seasons has a greater impact on things born of the earth than on animals because things born of the earth grow for half the year and for the other half they wither. But birds of prey do not live on things born of the earth and this is why it is not as necessary for them to migrate, as do birds like cranes and ones like them that live off things born of the earth.
-  On the middle interstice or interval of the air, see QDA 7.9-11. Recall that in this Aristotelian cosmos the earth, the heaviest of the elements, is surrounded by water, which is by nature cold and somewhat lighter than earth, and then by air, and finally by the lightest of all the elements, fire.
-  "Things born of the earth," terranascentia, would seem mostly to be plantlike, but may also include things thought to emerge from the ground, often by spontaneous generation. The crane was much admired for its migration.