Identifying nocturnal flight calls is fraught with the possibility of error, as the vocalizing birds are not visualized. In the majority of cases typical calls are going to be correctly identified, but many nocturnal flight calls are aberrant and could be assigned to the wrong species. A case in point is this recently landed Hermit Thrush that was calling before sunrise. The first few calls were hoarse and to me sounded like a possible late Rose-breasted Grosbeak. Soon though, the calls became the purer-toned typical vocalizations of a Hermit Thrush (Oct, PA).
The doubled line indicates that the sound is polyphonic, both syrinxes creating sound simultaneously, giving it the off-key or hoarse sound
The purer-toned third, fourth and fifth calls are represented by a single line