i have always paid attention to the birds while hunting. Blue Jays and crows will often fuss when a deer approaches. Squirrels also can let you know that there's something moving. When the woods is quite,you have probably been detected. If it's noisy then you are probably in the clear.
Yesterday I saw something unusual. A hawk was sitting on my chain link fence watching a squirrel on the ground maybe 30 feet away. After a minute or so he went after the squirrel, picking him up, but then dropping him back on the ground. After about 10 seconds he went after him again, and did the same thing, dropping him back to the ground. After that he just sit on the fence and watched the squirrel, who seemed unafraid of the hawk, staying out in the open for the next 3-4 minutes. Only after I went outside did they both leave. Does anyone have an explanation of what that was about?