Typically, when using the Ready action to cast a spell, the following rule applies (emphasis mine):
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration.
Now, suppose I have a Wand of Polymorph. On my turn, I take the following course of action:
I take the Ready action to use the Wand of Polymorph on the Roc if it flies within 15 feet of me.
If I were just using the wand on my turn to cast polymorph, I would have to concentrate as usual, since the rules for Activating an Item state:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell and caster level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
Since the action I am readying is not the Cast a Spell action, it is the "activate the Wand of Polymorph" action, do I have to concentrate to "hold its energy" until I release the spell on the Roc?