Mystery mystery

field recording undetermined (sex) uncertain Used an Old Bird 21c NFC microphone on top of Brush Mountain, a major migratory ridge, on a night with north winds. Calls are from a single bird in flight approaching and passing over, then fading away. Each call rises and fades in pitch (1 to 7 Khz range) and the duration of the individual part of each call (c. 9 complex notes per call) increases from 20 to 150 ms and back to 20 ms before immediately repeating the sequence. Guesses so far have been: Red-breasted Nuthatch, Franklin's Gull, Laughing Gull, Marbled Godwit, Black Tern, and American Avocet, but no call found in online repositories matches it. RBNU would be the only expected species on the list.; animal seen:no; playback used:no