Since crows are pretty smart maybe you could try placing treats in one of those puzzle pet toys (or make your own)? They'd probably enjoy the challenge and it might deter invaders, although the Jays (being corvids themselves) might figure it out, too.
Otherwise, you're limited to putting out treats to when they're nearby and can see you doing so. They're quite skittish though, so avoid looking at or towards them while doing so. Eventually they should acclimate to your putting food out and get more comfortable with you.
