I initially became interested in nocturnal flight calls (NFCs) because I wanted to add species to my yard list I wouldn’t normally get otherwise. Lately, however, my interest has expanded not just to its relation to my yard list, but to its coolness. It’s pretty cool that hundreds of warblers and thrushes migrate in the middle of the night, calling as they go.
This August, I got an Old Bird 21 c Microphone and mounted it on the roof of the house. A wire runs from the microphone into my computer, and software offered by oldbird.org (tseep-r, thrush-r, and dick-r) downloads burst of sound (high, mid, and low frequency respectively) from the recording as short clips. All I need do is let the microphone run in the middle of the night and look at the results in the morning. I put the resulting clips on Raven software (offered by Cornell) to produce spectrograms, visual representations of sound. Because flight calls of warblers and other migrants tend to be very similar, pure auditory identification proves to be difficult. However, with a spectrogram, subtle differences such as pitch and modulation can be distinguished by sight.
Throughout August, I employed this method for several months, and it yielded very good results. I mainly got lots of American redstarts, northern waterthrushes, and yellow warblers. Everything changed with the arrival of a major cold front on August 22nd.
The first birds appeared at around 11:00 PM. These were ovenbirds, with their distinctive high-pitched, highly-modulated calls. For an hour these were about the only migrants present, and then came redstarts with their smooth, bisyllabic flight calls. Then very odd things started to pop up. Famous NFC enthusiast Bill Evans lists a couple of NFC complexes – groups of similar-looking flight calls that are extremely hard to distinguish, even by spectrogram. One of these is the “zeep” complex, high zig-zags featuring some dendroica warblers and the Connecticut warbler. A couple of these “zeeps” showed up, and I managed to pick out a yellow warbler. The rest I failed to identify. Another complex arose – the “double-banded up-seeps”. This complex groups together the calls of the oreothlypis warblers and the black-throated green warbler with the vesper and white-crowned sparrows. I could only narrow down some of these calls to a few species, but no decisive identification was made.
After midnight, the predominant calls were of chestnut-sided warblers and blue-winged warblers – two species with very similar spectrograms. Blue-wingeds can generally be distinguished by a longer, finer buzz that curves. Chestnut-sideds are usually steady buzzes. However, this distinction is rather difficult to make.
Soon, I got a new yard bird: Louisiana waterthrush. The Louisiana’s call is very similar to the northern waterthrush, but can be identified by its non-ascension and spaced-out modulation. Two of these flew over the yard. Then came a mourning warbler, another new yard bird. Its spectrogram is very unique for double-banded up-seeps, because of its distinct parallel lines, its modulation, and its beginning at a very low kHz. This was a very exciting find. Below is a very low quality eBird-processed spectrogram of the call (it looks much better and distinct on Raven).
The rest of the night yielded an early northern parula (a single-banded down-seep), black-throated blue warblers (single-banded up-seeps, similar sounding to a cardinal), common yellowthroats (very distinctive low buzzes), black-and-white-warblers (highly modulated, long calls that go down and up), a magnolia warbler (very long, fine buzzes), Canada warblers (quick, downward blip rising up to sloppy modulation – very distinctive), and others. There was a total of 15 species of migrants – amazing for August.
Further information regarding NFCs can be found at the following website, one of my favorite resources: http://pjdeye.blogspot.com/2009/02/eastern-warblers-various-calls.html
Also look at the important work done by Bill Evans on his website: http://www.oldbird.org/Library.htm
The eBird checklist for the night described: http://ebird.org/ebird/view/checklist/S31290155
Preston Lust, Westport