There is no doubt these four little sets of realistic birds fit into a series.  They are all about 3'' in height with identical 3-2 pour hole pattern.  More importantly, they all have a dull/non-glazed finish and are depicted sitting on delicate leafs with flowers.

The sets are imported from Japan (some have red & gold "Made in Japan" paper labels), likely circa 1960s.

Dull / bisque finished small little realistic birds salt and pepper shakers - bluebirds

Blue Birds

Dull / bisque finished small little realistic birds salt and pepper shakers - yellow canary birds

Yellow Canaries

Dull / bisque finished small little realistic birds salt and pepper shakers - cardinal birds

Cardinals

Dull / bisque finished small little realistic birds salt and pepper shakers - robins

Robins