Yly's answer of avian is probably best. However, ornithoid is another, rarer option that ends in -oid, like arachnoid and ichthyoid. Merriam-Webster defines it as follows:
resembling a bird : birdlike
Avis is the Latin word for "bird" (aves is the plural). The adjective ending -(i)an is from Latin, and usually comes after a Latin root, although not always. The suffix -oid is from Greek, and usually comes after a Greek root. The Greek root for "bird" that is most commonly used in English is ornitho-.
A side note: as far as I know, the combining form associated with avis is in fact avi-, not just av-. So I think "avioid" would be preferable to "avoid" as an adjective derived from avis and the suffix -oid.
I haven't found any dictionaries that list avioid, but I did come across an example online:
I was wondering, how prevalent are avioids in science fiction,
especially in modern science fiction.
– Kehaar, "rec.arts.sf-lovers" Google Group