This week I've recorded several nocturnal calls that match those of Bicknell's Thrush. The calls have an initial vertical rise to a high point around 5kHz or higher, and then taper downwards, looking a bit like a flag. Other calls by gray-cheeked thrushes usually peak at around 4kHz and are highest at the "buffalo hump". More research is needed to separate the two species by nocturnal call, but for now these are probably Bicknell's Thrushes calling. In the previous two weeks no calls like these were present among the hundreds of gray-cheeked flight calls that I recorded during the night, and in past years I've only recorded these calls in the last week of September and first week of October, so possibly Bicknell's Thrushes have a fairly short peak migration period. The calls are higher and purer-toned than most other gray-cheeked thrush calls, but difficult to tell apart from higher frequency gray-cheeked calls without looking at a spectrogram.
(4.27am, 26 Sep, PA).
(6.20am, 26 Sep, PA)
(5.46am, 25 Sep, PA)
Typical gray-cheeked thrush call for comparison (6.00am, 24 Sep, PA)