Pecking At Windows:

It’s usually due to the fact that they can see their own reflection and not realizing it’s themselves they can see. Their reflection is perceived as a rival and the threat of their established nesting sites, food sources being taken away is very real or, another group of the same species has moved into the area particularly during nesting season.

What You Can Do:

Pull down your shades/close verticals/pull curtains across—this makes it difficult for birds to see their reflection.

Consider soaping your windows for a couple of weeks.

Break up the reflection by hanging something in front of the windows. You can use one inch tape or ribbon to create vertical stripes every four inches on the OUTSIDE of your windows.

Flying Into Windows:

If you’re feeding the birds—STOP! It’s inevitable that birds will fly into your windows if they’re coming into your garden to feed on what you have provided. Birds startle easily and no-one has any idea, including the birds, in which direction they’re going to go when frightened...sadly...it’s usually into windows.

What You Can Do:

All of what is explained for birds pecking at windows. You can also do the following:

Build a net frame to act as a barricade by mounting fine mesh netting in a rigid frame and use shelf brackets to hold the frame approximately two inches away from the window.

Install indoor/outdoor blinds on the OUTSIDE of your windows.

Consider This:
Birds and windows nearly always have an unhappy ending and must cause a very high mortality rate. As far as I know, no-one has carried out a survey to establish just how high the mortality rate is. As we continue to build, even more windows will create a hazard for our native birds and more will die. We can carry out the simple measures as explained above to lessen the impact windows have on our birds.

Information courtesy:
Audubon
http://www.audubon.org