If I have a bird token at a battlefield and the opponent ambush react plays orange rengar and I ambush react play stalking wolf killing my bird token and playing stalking wolf to base. Would there rengar go back to hand because the battlefield is now open

No — Rengar, Trophy Hunter stays on the battlefield; it does not go back to your opponent's hand.

Ambush's location requirement is only checked as the unit is played, before its own finalization completes (Rule 822.3) — it isn't a continuous condition and isn't re-checked afterward. On top of that, a unit doesn't linger on the chain: it resolves and enters as soon as it finalizes, and the whole play process finishes before the opponent regains priority (Rule 337.2). So by the time your Stalking Wolf kills the Bird, Rengar has already entered play, and killing the Bird afterward can't undo that play or eject it.

The "returned to hand" outcome only happens when the Ambush condition fails at the card's own check‑legality step — e.g. Stalking Wolf killing the very Bird its own Ambush play depended on (FAQ #10375). That's the same card, not a separate one, so it doesn't apply here. And even if you picture the two plays linked on a chain, finalization runs in the order cards were added (Rule 337.1.b), so Rengar finalizes while the Bird is still alive.

Note the battlefield also isn't "open" after this — Rengar is sitting on it.