verb agreement - "was" or "were" when there is number mismatch between subject and predicative complement

I have a question about this sentence:

The only thing he feared more than the wolves were the swirling buzzards.

I believe it to be correct, but someone suggested that the "were" should be changed to "was".

Which one is correct?

Answer

The only thing he feared more than the wolves were the swirling buzzards.

in this sentence, was would be correct auxiliary verb because the only thing is a Singular.