I have Battlefield 1 and Battlefield 2. Say I have a bird token at Battlefield 1. Then I want to play Stalker Wolf. Can I kill the bird at Battlefield 1 and play the wolf to Battlefield 2?

Yes, you can kill the Bird token at Battlefield 1 and play Stalking Wolf to Battlefield 2.

According to the rules and FAQs for Stalking Wolf:

1. Paying the Cost: You must kill a Bird, Cat, Dog, or Poro you control as an additional cost to play Stalking Wolf. Killing your Bird token at Battlefield 1 satisfies this requirement.
2. Choosing the Destination: While the card text says "You may play me to its battlefield" (referring to the battlefield where the sacrificed unit was), this is an optional choice. You are not restricted to playing Stalking Wolf to that specific location.
3. Legal Destinations: As per FAQ #9954, you can choose to play Stalking Wolf to any other legal location, such as your base or another battlefield (like Battlefield 2), provided you have a valid reason to play it there (such as having control of a unit at the target battlefield to satisfy the Ambush keyword's requirement).

Sequence:

1. Initiate the play of Stalking Wolf.
2. Pay the additional cost by killing your Bird token at Battlefield 1.
3. Choose the destination for Stalking Wolf; in this case, Battlefield 2. As long as you have units at Battlefield 2 to satisfy the Ambush requirement, it is a valid destination.