I had a pigeon reported from within an army barracks once. Arranged with Amtrak to pick up the pigeon from there. The pigeon was left at the front gates awaiting pick up. Amtrak failed to pick up the pigeon, their excuse...........................they knocked but nobody answered.
Think there's many reasons, one not mentioned yet is racing y/birds on systems. Inexperienced y/birds are sent on eggs, w/hood or being split all week and allowed to run together on Fridays. They just get over excited and cant handle it early doors.
I wouldn't go to every Tom, Dick and Harry for pigeons.
I would study the results in the fancy press, not the adverts.
Buy from one source to cut down the chances of inheriting health problems into your loft.
Dont know about squirels Samantha but could be magpies/crows if they can find a way into your lofts. Funny was only talking about this last night............the culprit was a crow.