In the Daytime
During the day, reflected light poses a severe threat to birds. Birds can see through glass and what is reflected on glass, but they cannot see the glass itself.
Attracted to the reflection of a landscape that is actually behind them, or to a plant that is on the other side of a window, many birds fly straight into windows and reflective building exteriors.
Some birds have even been observed attacking their own reflection, believing it to be a competing bird intruding on its territory.