On rules for calling a bird as birb – When Is a Bird a ‘Birb’? An Extremely Important Guide

1. Rule 1: Birbs are often (though not conclusively) small.
2. Rule 2: Birbs are often (though not always) round.
3. Rule 3: Birbs appear cute.

Source: When Is a Bird a ‘Birb’? An Extremely Important Guide | Audubon