Can a hidden Zonyah's be activated if a Bryndhir Thundersong was played on the turn by an opponent?

Ruling: You cannot activate Zonyah's after Bryndhir's effect resolves, but you can play it in response to Bryndhir's ability when it goes onto the chain.

Sequence:

• Opponent plays Bryndhir Thundersong
• Bryndhir's ability goes onto the chain
• You can respond by activating hidden Zonyah's at this point
• Once Bryndhir's effect resolves, you cannot activate Zonyah's

Nuances:

• You cannot respond to Bryndhir being played itself, only to the ability when it goes onto the chain