Grackles will likely have all their fledglings out now, as well (our great tailed grackles are long-fledged down here in the south), and protein/fat is more the food the growing birds need, so they are probably wandering more widely in search of food. But the crows could definitely be a factor.
Blue jays, crows, and roadrunners (in the southwest) will all regularly predate squirrel babies, as well as nestlings and fledglings of other bird species, but I'm not sure exactly how that plays into post-fledging behaviors. Squirrels will also eat nestling birds. Incidentally, I assume grackles do as well. I caught one harassing a baby box turtle in my backyard recently, and I bet it was testing its luck at eating it (it gave up, eventually).