From the long tradition of bird lore and from what we have gathered of Shakespeare's attitude towards birds it is easy to understand why he made them one of his favourite subjects for metaphors.
These metaphors created with relish by Shakespeare would be understood by the greatest number, especially since they are often constructed on an intricate pattern that provides many levels of understanding. This pattern fits each category or personal liking within his audience.
Traditional bird lore provides Shakespeare with a web of commonplaces but at the same time with a hierarchic view on the organization of the world. As everything that was known was graded in value, so were birds. Shakespeare, by playing on these various scales and these common beliefs produces many diverse rhetorical effects.
Nature represented the general order of things: an eagle is a "feathered king" (PHT 11), but a king is the embodiment of the noble spirit and characteristics of the eagle: Posthumus, the"noble lord" can behold the sun with firm eyes. In this case, the whole play, Cymbeline, takes on an Æsopic flavour:
[Iachimo] terms [Fidele] "alone th' Arabian bird" and, in view of her seeming destruction and restoration, this Phoenix symbol is the appropriate one. For the rest, the Princes are the poor unfledg'd' who make their cage a "quire", the lack of definition corresponding with their unrevealed identity. Cloten is, fittingly enough, "a puttock"; Belarius who is wise, is "like a crow"; Posthumus' supposed seductress is "some jay of Italy"; and Iachimo is, apparently, a raven. The inference, I think, is that Shakespeare tended to see his characters simultaneously as human beings and as birds, so that they are given the kind of double signification that belongs to animal fables of the Æsop kind - or rather the double signification that had recently been brilliantly exploited by Ben Jonson in Volpone, where Volpone is the Fox, Corbaccio the Raven, Voltore the Vulture, Corvino the Crow.
Fables are used to stage human behaviour and Shakespeare is interested in the disruptions of human life and society. In order to illustrate them he uses an ordered disruption of language, that is rhetoric.
Obviously it is important to establish that Shakespeare acquired, probably at Stratford grammar school, a much more comprehensive grounding in rhetorical theory than most of us have today, and that the plays contain abundant evidence of his competence to deploy all the tropes and schemes identified by contemporary rhetoricians.
Indeed we will see how Shakespeare makes use of every possible contrast between two or more bird species, such as the colour, the song or call, or their value in falconry. Allegories are therefore mixed with scientific notions of the time or everyday life knowledge to produce powerful images. Ideas about flight or movement are usually not associated directly with bird names and need to be expressed otherwise.
Many of these birds (and the values associated with them) have long passed into popular language, to give someone bird names is evidence of that. Therefore not only does Shakespeare use commonplaces, but he often makes a commonplace use of them. This is not pejorative at all, images (either verbal or visual) must be recognized at once in order to produce the maximum effect. What is important is the way these images are organized. Shakespeare's genius lies in the way he introduces them in the context and adorns them. This can explain why Shakespeare never ceases to be popular and why his highly metaphorical style is not an obstacle to this enduring popularity.
If "birds of a feather flock together" it is also true of Shakespeare's birds, he often associates them, as a painter would add colours - when Shakespeare groups corvidae (crows, ravens and daws), he adds blackness, literally and figuratively. When he groups songbirds in a song, he is in the same way a musician.
Shakespeare's ideas "flock together" through the mechanism of association of ideas which ARMSTRONG studied in his book Shakespeare's Imagination - A Study of the Psychology of Association and Inspiration . If some of his conclusions are sometimes dubious, the notion he develops about "Image Clusters" is very interesting. A good example is that one of the "Pistol-Turkey Image Cluster". He uses a small chart and a text to present the results of his analysis (p. 84), but they can be grouped in a larger chart with some modifications and additions (see chart p. 17 and PEACOCK and TURKEY).
Again we see how birds prove to be interesting material as far as imagination is concerned; many birds are at the core of "Image-Clusters" such as those of the Kite or Goose. This particular stress on birds may arise from the fact that ARMSTRONG was an ornithologist.