I’m trying to find the title of a movie my dad used to watch when I was a kid.

My dad used to watch this suspense/horror movie when I was a kid. I want to say the movie is from the 80’s, but I could be wrong. It was about a man who was having strange things happen to him. It turns out he was supposed to have a twin, but the twin died before they were born. Now the twin is trying to come back and take over his life. The man accidentally finds out about the twin from his old family doctor, and he goes to the courthouse to look through documents and see for himself if it’s true or not. I *think* the twin can temporarily take over the man’s body, so sometimes he just wakes up in random places not knowing how he got there. Whatever alternate world the twin exists in, is connected to the real world. Of the twin is injured, the man is injured in the same spot on his body and vice versa etc.. at one point in the movie the man stabs himself in the hand with a pencil to harm the twin. There are also a significant amount of birds in the movie. It starts out small and grows and the plot progresses.