I would say .270 Win. Not only a true 'stop 'em in their tracks' calibre but also fairly un-fashionable so you can pick up a damned good quality second hand rifle for a song. Never had a red with mine, but would have no hesitation in taking the shot if it was offered. Every other species I've pointed it at has fallen over very quickly indeed. Lot's of 'Estate rifles' on the Scottish estates either were or are .270's and there has to be something to learn from that surely ?
Stick your decoy atop a pole or dead tree near (ish) to a crows nest and you'll see them mobbing it in no time. High up in a tree looking at a rookery works well too. The ruddy things know that they are bad news for nests and youngsters, so they know that others can do the same to their eggs and young. They hate to see any other predator near their homes. Had on permission that was plagued by rooks from a nearby rookery but couldn't get permission to 'get under the bu88ers' so rigged an owl decoy up dead tree 100 yds my side of the boundary. 3 visits later and no more problem.
I had a noticeable increase in the pulling power of my decoys when I changed from plastic finish to flocked. I now use a dozen flocked full bodies from A1 and a home-made flapper once I have a fresh kill to put on it. The slow flapper is amazing to use. I've shot no end of rooks and crows that simply didn't see me bring the gun to bear on them because they had turned their heads completely aside to stare at the flapper as they came in.
Wildcat evolution moderator any good?
Spent my money on a Wildcat Evo. Would do the same again in a heartbeat. Good kit.
Which caller works on rooks?
Whatever call you get it will take some practice to get it right. Good advice is to do this practice whilst alone over the fields and well away from mates and/or family unless you want to spend the next 10 years being reminded of all the times you managed to sound like a duck being rodgered by King Kong.
If the ruddy things are on the clamps and it's all too close to 'civilisation' for the shotgun, hit them from a good hide (perhaps inside a barn nearby ? ) with a rimmy and sub-sonic ammo. No good if they're in the air, I know, but they're not a problem up there You could even use some really quiet ammo like Z-Langs. Very nearly as quiet through a well moderated rimfire as an air rifle going off but more than enough stopping power for rooks'n' stuff.
Best tip I ever had was from the son of a famous bookie .. "Never put anything on a horse that you couldn't afford to flush down the toilet. The money in that game always flows one way and one way only in the long run".
A .22 rimmy firing sub-sonics through a good moderator usually doesn't spook the gee-gees, but if you feel you'd like to go even quieter, look at getting yourself some .22 shorts like the Dynamit Nobel Z-Lang ammo. It's no good in semi-autos as it lacks the punch to cycle the action, and you'll need to check your zero and allow a little more elevation for those longer shots (learn your new trajectory ) but that stuff is as quiet as a quiet thing that's trying to be quiet whilst providing the performance of a good FAC air rifle or better. https://rws-munition.de/en/rws-sportshooting-area/rws-sport-ammunition/rws-rimfire-cartridges-for-sport-shooters/Product/show/rws-z-lang.html#!0/278/52 I shot bunnies on the paddocks of a couple of very posh stud farms for years and some of those top-end Ferrari race horses are as skittish as a paranoid schizophrenic on speed, but I never had any problems with them spooking at the shot with Z-Langs. Mind you, if they suddenly noticed me emerge from behind a shed or from a ditch they often acted like I'd whalloped them with a electrified carpet beater Ruddy things are mad. One end bites and t'other end kicks.
Extra loud sub-sonic ammo usually isn't subsonic ... that's the problem. Manufacturers want you to get the best from their ammo so you'll buy it again, so they'll load subsonics to within an inch (metaphorically) of being supersonic, through their test rifle on a certain day under the prevailing conditions at the time. Trouble is, the speed of sound is NOT a constant. It varies with humidity, altitude, and a host of other factors. Ammo that is nicely sub-sonic on a warm dry day in, say, Nevada where it's made and tested, may well go super-sonic on a cold, damp evening in Blighty when you're out bunny bashing. It only needs to be 1/2 ft/sec more than the speed of sound there and then and you'll get a 'sonic crack' as it leaves the rifle. I've found that Winchester Super X (40gr) performs the best for both consistency and accuracy in both my .22RF rifles, but even that produces a 'cracker' every now and then. Usually on the aforementioned cold damp evening.
Hoping I can make it again Mick.