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Many species of birds make flight calls while they migrate during the night, and I have been recording these calls to follow the fall migration. This page presents some of the data I have obtained during the last ten years.
The high-frequency calls (red-brown bars) are usually made by warblers and sparrows.
The mid-frequency calls (orange bars) are made by thrushes and a number of other mid-size birds such as Rose-breasted Grosbeak. Due to time constraints I discontinued following mid-frequency calls in 2016.
How the recordings are made:
Basically, I have put up microphones pointed at the sky. The mic outputs are recorded with Song Meter autonomous digital recorders. In 2015 I installed Oldbird 21c microphones at two sites. The Oldbird microphones are more sensitive than my old microphones, so the total number of calls detected is greater than in previous years.
I record the calls from civil-twilight after sunset to civil twilight before sunrise at three sites in Albany County, New York. The first is in a partially developed suburban area in Slingerlands about 8 miles (13km) west of the Hudson River at an altitude of 320 feet (98m). The second is at the Emma Treadwell Thacher Nature Center at Thacher State Park. It is located 7 miles (11.3 km) directly west of and 1000 ft (305 m) higher than my site in Slingerlands. In 2014 I added the Albany Pine Bush Discovery Center. It is located 4.4 miles (7.1km) northeast of the Slingerlands site at an altitude of 350 ft (107 m).
The bird calls are extracted from the sound file by using two free software programs, Tseep-x and Thrush-x, which were obtained from Oldbird.org. Tseep-x extracts short high frequency calls (6,000 - 10,000 Hz) which are produced by warblers and sparrows. Thrush-x extracts mid frequency calls (2,800 - 5000 Hz) which are produced by thrushes, rose-breasted grosbeak and other species. Each of these programs produces many false positives (which result mainly from insect and amphibian calls). I separate the bird calls from the false positives by using GlassOFire (free from Oldbird.org). To identify species, I examine each call's sonogram by using audio editing software (principally Raven Pro), and I listen to some of the calls.