does a deathknell that plays a bird token count as a card played for legion?

No — a Deathknell that plays a Bird token does not count as a card played for Legion.

Legion requires a card different from the one with Legion to have been Finalized by you that turn (Rule 812.1.c). "Card" in card effects is shorthand for Main Deck card (Rule 052), and tokens are explicitly not cards (Rule 185) — even though they can be played (Rule 185.2.a). So Carrion Dredger's Deathknell playing a 1 Might Bird token doesn't satisfy Legion for any of your other cards. Only actual Main Deck cards (including cards played from Hidden, or cards that get countered after finalizing) count toward Legion.