How big of a branch compared to the rooster? Can you see where he took the impact- on his head or on his back? Sleeping can be a normal post-stress reaction, not necessarily an indicator that lasting damage was incurred. If he got clocked in the head, I'd watch for neurological signs of swelling like head twisting/ stargazing. If he took the impact across his back, I'd watch for bruising and listen for any strange sounds when breathing that would indicate he may have punctured an air sac. Poor dude! Depending on his age and size, he may be able to have aspirin diluted in water as pain and inflammation relief- search this forum for dosage info.
