That actually might be a fairly good strategy. Sure, the hawk can still see them, but to get them, it would have to land and then walk under the hammock - something it would prefer NOT to do.
it is wise they try to hide from a hawk. maybe they were just looking for bugs stirred up from last time you were on the hammock? my birds do that all day long. but i dont have hammock. i have other stuff they look around. nothing abnormal about that.