As Marcha points out, birds hit windows because they either see through to another window, or because they see the reflection of the trees and sky in the window. Either way, they think it's a way 'through'.
Around here the worst time seems to be after the year's broods have fledged and before they migrate. We've had good luck with two strategies--we tape the outside of the windows--just an x across the pane of glass or criss-crossing done in masking tape will do (remember to take it down after the birds have migrated or the adhesive will leave a hard-to-remove residue). This breaks up the reflection (which is why it has to be on the outside of the pane since anything inside is masked by the reflection) and warns that there's something solid there.
The other thing we do is to hang something shiny from the eaves that will sway in the wind. Currently, there are spinning Christmas icicle decorations along the eaves of problem areas. They stay up all year. They don't entirely eliminate window hits, but seem to slow the birds down as they approach--fatalities went way down.
I wish they'd figure out a way to make TV and radio towers safer for birds, too. In Milwaukee, they sometimes have thousands (even tens of thousands) of migrating birds die each night as they try to make it past the TV towers. :sad: The fatalities are staggering in number if you add them up year after year.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."