Hello, @2122-jakob-be and a hardy, if premature, welcome to the world of PCP air guns!
Pigeons, rats, and other small vermin at 15-25 meters can be handily dispatched with a .177 Dreamline. Although, personally, when going after rabbits at 40+ meters I'd prefer the .22 caliber for 1-shot/1-kill. The .177 (especially in a powerful rifle such as the Dreamline) would probably do the job with a head-shot. Can you can hit a bottle cap at that yardage consistently? That is probably the determining factor there.
Something else to consider is your target's backdrop. If the pigeons are inside (barn, silo, etc.) you have to be wary of pass-through or misses (nobody's perfect, despite what you see on YouTube) and the possibility of punching holes in the rafters/roof. In that case, a .177 might be your best bet.
Enjoy,
Mondo