Clarification about taxon (plural taxa)
Definition of taxon (From ICZN glossary):


  A taxonomic unit, whether named or not: i.e. a population, or group of
  populations of organisms which are usually inferred to be
  phylogenetically related and which have characters in common which
  differentiate (q.v.) the unit (e.g. a geographic population, a genus,
  a family, an order) from other such units. A taxon encompasses all
  included taxa of lower rank (q.v.) and individual organisms. The Code
  fully regulates the names of taxa only between and including the ranks
  of superfamily and subspecies.


Birds are called Aves and that is a taxon. Cats come under Felidae. If you are asking - "Is a colloquial/vernacular name considered a taxon". Perhaps the group of organisms can be referred to by a colloquial name when they are not given any scientific name but in this case these taxa are well defined by a standard scientific name (according to the nomenclature codes such as ICBN, ICZN, etc).

So "cat" is not a taxon, but cats fall under the taxon called "Felidae".

In some cases certain groups are not even actual clades. A clade is a monophyletic group i.e. all its members have a common ancestor. The group &mdash; reptiles, is not a clade.