Early this morning I woke with a start, thinking that the neighbor kids had to be killing eachother from the screams and screeches I was hearing. Turns out it was my male cat Charley sitting at our back door with one of the neighbor's cats (female, I'm pretty sure) on the other side. Neither of them were acting very aggressive, but they were both projecting these closed-mouth siren-like sounds of varying pitch, and when one would start the other would join in. These two have sniffed eachother over before, but aren't exactly friends (though definitely not enemies, we let this cat inside our house once and had no problems and it trips through our yard a few times a day without notice from our sometimes territorial cats). We cracked the door to see what their reaction would be, and to hopefully quiet them down if they could hear eachother better, and they didn't seem all that interested in sniffing at eachother and just sat still howling at eachother. Does anyone have any insight into this new behavior? My boyfriend suggested that the other cat was in heat. Charley's been neutered, though not until after he had matured, so maybe he was responding? I've never seen anything like this and none of our other cats seemed all that interested in what was going on...