I do not know, moreover, how Celsus could hear of the elephants' (fidelity to) oaths, and of their great devotedness to our God, and of the knowledge which they possess of Him. For I know many wonderful things which are related of the nature of this animal, and of its gentle disposition. But I am not aware that any one has spoken of its observance of oaths; unless indeed to its gentle disposition, and its observance of compacts, so to speak, when once concluded between it and man, he give the name of keeping its oath, which statement also in itself is false.
For although rarely, yet sometimes it has been recorded that, after their apparent tameness, they have broken out against men in the most savage manner, and have committed murder, and have been on that account condemned to death, because no longer of any use. And seeing that after this, in order to establish (as he thinks he does) that the stork is more pious than any human being, he adduces the accounts which are narrated regarding that creature's display of filial affection in bringing food to its parents for their support, we have to say in reply, that this is done by the storks, not from a regard to what is proper, nor from reflection, but from a natural instinct; the nature which formed them being desirous to show an instance among the irrational animals which might put men to shame, in the matter of exhibiting their gratitude to their parents.
And if Celsus had known how great the difference is between acting in this way from reason, and from an irrational natural impulse, he would not have said that storks are more pious than human beings. But further, Celsus, as still contending for the piety of the irrational creation, quotes the instance of the Arabian bird the phœnix, which after many years repairs to Egypt, and bears there its parent, when dead and buried in a ball of myrrh, and deposits its body in the Temple of the Sun.
Now this story is indeed recorded, and, if it be true, it is possible that it may occur in consequence of some provision of nature; divine providence freely displaying to human beings, by the differences which exist among living things, the variety of constitution which prevails in the world, and which extends even to birds, and in harmony with which He has brought into existence one creature, the only one of its kind, in order that by it men may be led to admire, not the creature, but Him who created it.
Source: Against Celsus (New Advent)