Sometimes birds fly into windows. Other times, they might perch on the outside of your windowsill and peck a little. It might look like they’re trying to get your attention, and if you’ve been stuck inside for the past year, you might even be tempted to let it in for a quick chat. (But don’t.)
source https://lifehacker.com/how-to-stop-birds-from-pecking-at-your-windows-1846372723