Which species on Réunion island is part of the procellariiformes order and is part of the hydrobatidae family and has less than 21 species worldwide?
CREATE TABLE "birds" (
  "species" text,
  "order" text,
  "family" text,
  "species_worldwide" real,
  "species_on_r_union" real
);
SELECT SUM("species_on_r_union") FROM "birds" WHERE "order"='procellariiformes' AND "family"='hydrobatidae' AND "species_worldwide"<21;