I'm off to Islay later this year and have half a mind to snap some wildlife.
I don't think wildlife are very amenable to being snapped, unless your trip is just to take photos, I'd just take what you have on the odd chance that you'll get lucky, or buy a lens that's likely to get plenty of use afterwards. I wouldn't go to the expense of hiring unless you're going to be sat all day, everyday with it and then accept that you may be giving it back without a photo worth keeping.
Those wildlife photos that we'd all have loved to have taken tend to have been the result of a bigger investment in time than money, and I say that even after looking at lens prices
When I started working with a keen wildlife photographer, I made the mistake of thinking the dozen photos he was showing me were from their latest trip, not from the last decade. They were good, one of them was sold for a postcard, but that was it, all of them, they had nothing else they deemed good enough to show.
OTOH, I wouldn't go to Scotland without a decent pair of binoculars, I've seen the Sea Eagles, Puffins, Arctic Skua, Great Terns, then bought the postcards. Also heard but didn't see a Corncrake, the couple at the same hostel had made the trip to see it and despite all the kit hadn't got a photo in a weekend.