Like a bird that strays from its nest is a man who strays from his home.
As I considered this verse, I thought of a young bird wandering off from its nest. Away from the protection of home and family, it would be vulnerable to all sorts of dangers. It might be injured, a predator could come and snatch the unprotected bird, using it for food for its own young ones. At the very least, the straying bird would be cold, lonely and hungry. Consider a man or woman who leaves the comfort of home and family, to search for adventure or to seek comfort in the arms of someone not their spouse. This person is away from the security of home an family, lost and unprotected.