They have two chances really! They either will or won't make it. If a fox, stoat, crow, magpie comes across them then its curtains. How big were they? They will keep pretty still and quiet whilst the parents are off foraging. Also as mentioned above skylarks do a pretty convincing 'rat run' to attract predators away. If they are nearly fledged they will hide wherever they happen to be. Survival is a matter of chance- nature in the raw. Best bet for you is to steer clear of the nest as much as possible- you trace will lead others there.