Basic info: found a baby bird already fledged. It is a common linnet. It was found on the ground with no means to hide. It was unable to fly up, so based on information from some literature it was about 16 days old. I found the baby bird on the 19 June. So it might be about 45 days old now (it's 16 July when I write this)
Question 1) How do I know that the bird is ready to be completely self-sufficient? I don't have to feed it anymore, but I still have to help it, for example by holding the apple (it can use its beak to tear little "beakfuls" but e.g. cannot get through the tougher skin of an apple/berry, etc.) It knows how to get grains from a spike. It can snap and "chew up" soft seeds like sunflower seeds and similarly soft/fragile/brittle ones. It also loves eating the mealworms, it actually demands them (I hear that baby linnets are fed with insects, thes source text sounded a bit like the adult linnets don't - may I expect a kind of signal moment when the bird stops demanding the mealworms?). All in all, if the bird is hungry, it nibbles and eats seeds and tiny pieces of other stuff but cannot cope with food like apples and berries, though I read and saw on videos linnets eat that too.
Question 2) I also read that these birds spend the winter in Italy, so another problem is: What happens if I set the bird too late for the bird to find its kin and socialize with the flock so that it can move to south with them?
Important note: I come from central Europe, I live in the neighbourhood of the bird's nesting place, it is its home around here.