Colorado - (NBC) - The sound of robins singing in the spring is familiar to most, but they also make another sound, a sound like someone knocking on your door or window.
"This is the time of year when they're setting up their territories," Tom Parchman with the Audubon Society of Greater Denver said.
Most birds are very territorial and will compete for a good nesting area.
In some cases, the birds will catch their reflection and think it is another bird.
"He sees the reflection of himself and thinks it's an intruder so he's trying to get that intruder away from his territory," Parchman said.
Robins will spend three to four hours flying into a window thinking it is another bird.
While many people find dead birds that have flown into windows, it does not come from this behavior.
"The way to stop it is to break up that reflection," Parchman said.
He recommends putting down your shades and says that as the angle of the sun changes, a lot of those reflections may go away.