As the loopy title suggests, Regretfully, So the Birds Are is theater of the absurd. Julia Izumi’s play concerns three New Jersey siblings adopted from Southeast Asia by a Caucasian couple (Gibson Frazier and the incomparable Kristine Nielsen) who’ve refused to tell the children where they were born. The parents’ rationale is that, if their adoptive offspring don’t know where they come from, they’ll feel their “origins are the Whistler family” rather than separate, far-off countries.