Based on the data provided, the correct response is:
Higher altitudes are correlated with lower song frequencies.
This conclusion is supported by the observation that as altitude increases, the number of birds singing below 1500 Hz decreases. At low altitude, there are 80 birds below 1500 Hz, at medium altitude there are 50, and at high altitude only 20 birds sing below 1500 Hz, indicating that higher altitudes are associated with lower frequencies of bird songs.