Every year, during late summer nights (usually around 9:00 pm) in Maine, I hear a noise coming from high up in the trees. The trees I hear it from are mostly maple or conifer, but where I live there are few trees that are not. It sounds like a cardinal's chirp (fourth sound down in link), but longer, scratchier, and lower pitch. The sound starts and ends abruptly. There are three to six chirps in a row from the same tree, and then silence for a few minutes. Sometimes I will hear it again after a few minutes from the same tree and sometimes from a different tree. The skies are usually clear when I hear it. No one else in my family can hear it. I live in a suburban area, but when I go to a more densely forested area I hear it more frequently. Does anyone know what this is?
My initial guess (knowing almost no information) is that it's a Northern saw-whet owl (Aegolius acadicus). These owls live year round in Maine but also increase in number as some migrate from Canada through Maine for winter. You can hear their advertising song here.
Northern Saw-whet Owls have a distinctive too-too-too call -- an insistent series of whistled notes on roughly the same pitch, given at a rate of about 2 notes per second. Males calling to advertise their territory can be heard up to half a mile away. Other males respond with a softer, faster, lower version of the call.