Sharp-tailed Sparrow flight call

Birds sometimes produce two sounds simultaneously when calling or singing.

These calls were made by a sharp-tailed sparrow (a Nelson's and a Saltmarsh were in the same bush and I couldn't determine which one called) and show simultaneous higher and lower calls on the sonagram. The left side of the syrinx apparently generates the lower sound. One of these calls is made with only one half of the syrinx, presumably the right side.

(Nov, NJ).