I found a young magpie sitting on a low branch. Is it in trouble?

Not necessarily. You need to tell a nestling apart from a bird that has recently fledged. Young magpies leave the nest about 26 to 31 days after hatching, and their parents keep looking after them afterwards.

A young bird on a low branch may look less capable than an adult. That alone does not mean it is in difficulty. It may just be a fledgling at a normal stage of growing up, with its parents still nearby.