I quote from the Tzaangor entry in the Silver Tower book:

"These savage avian beastkin have been warped into shapes more pleasing to Tzeentch."

So I guess the cat's out of the bag. It's official: Tzeentch is into birds.

However looking at his range it was kind of an open secret. The Lord of Change has always had a distinct avian shape. Recently the Horrors have started picking up the trend, sporting (fake-looking) beaks and feathers. The Gaunt Summoner has a feather skirt any Skink Priest would be jealous of. Though birdified Screamers and Flamers have not been spotted (yet), the Kairic Acolytes too wear all sorts of birdlike accesories.

So Tzeentch likes birds. Why not, aviculture is a fun hobby after all.

Except it makes no sense in this case.

One has to keep in mind that Daemons are, by and large, just facets of their creator. One has to think of them as tiny god clones really. So we know that Khorne is obsessed with dogs, and that's fine because he is described as having dog-like traits and features himself. Nurgle is just a pile of rotting dross, and naturally his Daemons resembe just that. Slaanesh favors lithe and adrogynous Daemons because, again, this matches his own nature.

Problem with Tzeentch is, he's just an amorphous shapechanging blob. The older Horror models seemed to emulate this aspect after a fashion, but no more. There is really no fixed shape associated with Tzeentch, so logically there shouldn't be one for his Daemons, either. Especially not birds.

Why you ask? Well, does anyone remember which Symbol/animal Nurgle is mostly associated with among the northern tribes? It's the Crow. Yup, exactly. The Crow, or in other words, a bird, is the sacred animal of Nurgle. So why on earth would Tzeentch form his Daemons into a shape resembling the symbol of his archenemy? This is completely beyond reason and logic and can probably only be explained with "Chaos is illogical".

If anyone has ever come acrooss a piece of background clearing up this matter I would deeply appreciate it.