But we have a few remarks to make, out of a larger number, in answer to these statements of Celsus, that we may show the ingratitude towards his Maker which is involved in his holding these false opinions. For Celsus, although a man, and “being in honour,” does not possess understanding, and therefore he did not compare himself with the birds and the other irrational animals, which he regards as capable of divining; but yielding to them the foremost place, he lowered himself, and as far as he could the whole human race with him (as entertaining lower and inferior views of God than the irrational animals), beneath the Egyptians, who worship irrational animals as divinities.
Let the principal point of investigation, however, be this: whether there actually is or not an art of divination, by means of birds and other living things believed to have such power. For the arguments which tend to establish either view are not to be despised. On the one hand, it is pressed upon us not to admit such an art, lest the rational being should abandon the divine oracles, and betake himself to birds; and on the other, there is the energetic testimony of many, that numerous individuals have been saved from the greatest dangers by putting their trust in divination by birds.
For the present, however, let it be granted that an art of divination does exist, in order that I may in this way show to those who are prejudiced on the subject, that if this be admitted, the superiority of man over irrational animals, even over those that are endowed with power of divination, is great, and beyond all reach of comparison with the latter. We have then to say, that if there was in them any divine nature capable of foretelling future events, and so rich (in that knowledge) as out of its superabundance to make them known to any man who wished to know them, it is manifest that they would know what concerned themselves far sooner (than what concerned others); and had they possessed this knowledge, they would have been upon their guard against flying to any particular place where men had planted snares and nets to catch them, or where archers took aim and shot at them in their flight.
And especially, were eagles aware beforehand of the designs formed against their young, either by serpents crawling up to their nests and destroying them, or by men who take them for their amusement, or for any other useful purpose or service, they would not have placed their young in a spot where they were to be attacked; and, in general, not one of these animals would have been captured by men, because they were more divine and intelligent than they.
Source: Against Celsus (New Advent)