You get some trigger time.
They save some money.
It’s a win win.
Well, in the name of full disclosure:
It's a win win loose.
Really.
▪You get some trigger time.
WIN ▪The farmers save some money.
WIN ▪The pigeons, rats, and squirrels die.
LOOSE And
loosing pretty seriously, if you think about it. A loosing of
life.

But like Dizzums called it above:
You're looking for something to kill. And you could do a lot worse in the pursuit of a kill. Instead of a
win win loose, it could be a
win loose loose.... (like what some nut jobs do, killing the
neighbor's cat with a pellet gun, stupid nut jobs!).

So, you go right ahead and
kill some pests!

(win win loose).

On finding a "permission": Even better than cold calling on farmers is have someone who knows a farmer present you to them, or at least tell the farmer you'd be coming.
That happened to me.
Now, if you treat your "permission" nicely, you'll be getting more permissions.....:
Treat them politely, be nice, thank them for the opportunity, announce your coming with advance notice, don't bring others along unless you got permission, fess up to any damages you did and offer to pay in full, possibly show (or at least verbally report on) your kills — because
more kills mean more money savings for the farmer,.....
And after you have established a good relationship — you can just ask for referrals to other farmers (because farmers know farmers....).

Eventually,
if you go down far enough the all too well known AG rabbit hole — you'll find quarry where you didn't think there was any.... I call this the
"life in permanent hunting mode." Once I reached that life mode
I began looking at everything from a potential hunting perspective: What animals to kill, from what positions, ranges and angles, potential safety concerns, etc.
To illustrate the "life in permanent hunting mode": Yesterday I was sitting at a table in my house having an engaged conversation. My cellphone was off, laying around there somewhere on the table.
As I was in the middle of a sentence I SAW a PEST PIGEON — on my dining room table! WHAT?!? Well, the rat with wings had decided to settle on a post just outside my house — and the dim reflection of the pigeon on the cellphone glass was all I needed to awaken my hunter's instinct — that's life in permanent hunting mode.
Good times. Killing times. Go get'em!!
Matthias