This is my living room window at home. I recommend window clings a lot for clients when we are working on changing a behavioral response of excessive barking at stimulus (and other related responses such as increased heart rate and breathing, lunging, pacing, hyper-vigilance).
It is very difficult to effectively modify behaviors when they are practiced a lot (and a history of association between the stimulus/behavior/consequence). When dogs can see out windows, just as dogs will do who are left outside in yards), they will notice and react to what they see outside. It can make for a stressful day, as there may very well be trigger stacking occurring – your dog has not had time to full destress with its hormone levels back to baseline before the next stimulus/trigger, and the intermittency of the stimulus can cause your dog to be on a heightened state of alert in anticipation for a possible ‘sighting’.
Time and again I hear from those who put the clings up that they have seen a dramatic difference in their dog.
Clings are easy to install and inexpensive. (They let light in, however, you cannot see shapes -like the frosted windows for showers.) Simply clean and wet your window, peel the backing off the cling, and adhere it to your window. They can be taken off and reapplied easily too. I got mine off of Amazon.