Storm discovered the "intruder", so I opened up the window. They just stared at each other un-moving for literally an hour. Than, Ambercat wondered what was up, and jumped up on the window frame. Instantly, he began a string of cat profanities, which was answered by the neighbor cat, and this quickly escalated, and Ambercat started trying to find a way through the screen. I closed the window, and the worked up neighbor cat retreated to the edge of my lawn, and stayed there for about 1 more hour before melting away.