If the birds have regular, clean fresh grit then being without any for a couple of days while maybe shut in a box with the cockbird isn't going to cause them a defficiency!
Do the birds in the transporter during holdover get grit, NO, do they all fall out of the transporter after a 5 day holdover, NO and some of the cockbirds in the bakets may have been chasing others around almost continualy!
Phil