Why does thoth have a head of a bird?

The question might better be phrased as 'why is the ibis represented as the god thoth?'

The Egyptian glyph that takes the form of an ibis is translated in ancient greek as 'thoth'. 'Thoth' is the phoenetic rendition of the sound of the Egyptian Word that is represented in hieroglyphic writing as an ibis. (pic)

According to the Corpus Hermeticum, Book XVI "the very sound of an Egyptian Word resonates with the thing signified by it". There are multiple levels of meaning for the Word.

'Thoth' is the phoenetic rendition of the sound that an ibis makes. It is a short sharp tympanic rumble. Usually described as a silent bird, the ibis, during its regurgitative digestion, makes the sound 'thoth'.

That sound 'thoth' is in fact our Word 'god'. It came into greek as 'theos' and into latin as 'deus' and into germanic languages as 'got'.

Hence, 'god' is simply the sound that an ibis makes.

The greek Word theos literally means 'primary reference point for ranking'. (cf LidDELL & Scott Greek Lexicon). This is the real meaning of our Word 'god'. In the greek, theos is related to the Words from which are derived our English Words 'theatre', 'thesis' and the definite article 'the'. All of them imply a focus of attention, casting light upon a subject, around which others are ordered.

'The god thoth' is represented pictorially as an ibis-headed human. Again, there are multiple levels of meaning. The ibis headdress is a mask or 'persona' as the latins have it. 'The god thoth' is therefore presented as a 'person'. From this image derives the sense of a 'personal god': the primary reference point for ranking which has an identity and existence in its own right, as 'an end in itself' (i.e. person) rather than being simply 'a means to an end' (i.e. thing), (cf. Kant's distinction between 'person' and 'thing'). We tend to identify or restrict 'person' today as 'human being', but the sense is much broader than this and is certainly not restricted to human beings.

The ibis headdress indicates the function of the image. As a wading bird, the ibis traverses both land and water and its long curved beak probes deeply below the surface to get at the real nourishment below: such as the larvae of locusts and crustaceans etc. It is an image dear to all esoteric teaching. The agricultural value of the ibis in controlling locusts enhanced the egyptians' regard for the bird. Moreover, the ibis writes upon surfaces, leaving its footprint as 3 +1; which is the image enshrined in the scandanavian world-tree yggdrasil; the Hebrew tree of the Sephiroth; the pythagorean yau; etc. (pic) This is the central numeric of ancient science.

Each of these ibis-functions emerges in the areas traditionally described as Thoth's domain: as the ancient archetype of esoteric or hermetic teaching, of writing and language.

'The god thoth' is also traditionally associated with Moon; and this reflects another significance of the ibis image. In the taxonomy of birds, ibis is classified as 'Tartarus', referring to the ancient greek story about the personage of the same name who was cut into pieces (like the Egyptian Osiris) and restored (hence the thoth-medicine connection) but with the shoulder lost, only to be replaced by a lustrous ivory substitute. The healthy young ibis is a similarly lustrous white, in body shape like a shoulder, offset by black head and neck.