Question about the songbirds in Fellknight queen

[friendly sauce]

how many hp do they have? The tactics say she uses vampiric touch on them but I can't seem to find anything on how much hp they have even though it references they retain these hit die after being polymorphed.

[The Human Diversion]

Baleful Polymorph specifically states that the creature still retains its class level and/or hit dice. So if you took a 10th level barbarian that had all 7's for hit dice (using the standard "half plus one" rule) and baleful polymorphed it into a tiny bird with a con of 8, you'd have a creature with 62 hit points