She can't catch the bird through the glass, of course, but that hasn't stopped her. She ought to be embarrassed that she and the jay are about evenly matched when it comes to understanding how glass works, but she gives no sign that she is. She just wants to smack that bird, and the bird just wants to peck her, and neither of them has any intention of letting a little thing like reality thwart their will.
It's a bit like watching one of those political debate shows on television.
I think I'll close the drapes for a while. The bird will think the cat is gone and go away. Then I can open the drapes again and let Portia look outside in peace, and I'll be able to use the computer without having a cat leaping hither and yon across my desk.