Barking at the window

I have a 7 year old Lab/Border Collie mix who is getting worse and worse lately about barking. Not only does she bark when someone comes to the door, she will run to the window to bark and growl at anyone (human, canine or feline) walking down the street. Sometimes I can't even tell what is upsetting her - I don't see anything out there.

I have tried distracting her by giving easy commands to follow and giving her treats when she obeys. That usually stops the barking temporarily, but it all starts over again the next time she notices something outside. Sometimes she gets so worked up she doesn't seem to hear me, until I physically remove her from the window. Her hackles are raised all down her back and she will almost bark herself hoarse. Apart from the noise problem, I hate to see her get so upset. I would appreciate any tips on how I can help her.

Grizzley is still a puppy but he was/does bark at the window when nothing is there, we finally realized he saw reflections from inside the house in the window and thats what he was barking at. He was getting the same way...hackles raised, he was actually scaring me thinking there must be someone outside somewhere. Maybe thats what your dog sees when he is barking at nothing?