Fear the squirrels.

My story, from one summer in college:
The fire escape outside the window was easily accessible from a nearby tree, and the sliding screen in the window had an ominous squirrel-sized hole in it covered with duct tape when I moved in, which I unfortunately disregarded. I left the window open when I went home over the July 4th weekend, and came back to find a new squirrel-sized hole in the screen, and bits of food and poop that was larger than mouse poop EVERYWHERE. My roommate's computer was a file-sharing server that would overheat if we left the window closed, and the squirrels were not to be deterred, so I tried putting in a window fan and filling the rest of the space in the window with a wooden board. They chewed through the fan grille and got in again. So, I left the fan on all the time, and that was effective, though wasteful. But, I forgot one day, and they entered again, made a mess again, and chewed through the fan's power cord before leaving. Brilliant scheming little bastards. Finally replaced the power cord, fixed holes in the fan with wire, remembered to leave it on all the time, and made big-scary-monster noises when squirrels approached, and managed not to have any more problems. Good times. (I also only once saw a very brave one get its nose whacked by the fan; it didn't seem there was any permanent damage done.)