"There are also seven cats in the tree."
I'm not a native Chinese speaker, but I've never seen/heard the word 也 in any position other than right before the verb. For example, I've heard sentences like 我也有一只狗, but never sentences like 我有一只狗也 or 也我有一只狗. I'm almost certain those latter two sentences are ungrammatical.
I think this sentence can't be considered alone. Therefore 也must come first. 柯上有七只鸟。也树上有七只猫。