A Busy Night Sky: Things that go "tseep" in the dark

"First Fall Swainson's Thrushes heard overhead this morning at 5:15AM. Right on time."

This welcome post was shared by Portlander Wink Gross on Oregon Birders Online (OBOL) on 24 August 2011.

It may sound counter-intuitive, but one of the best ways to appreciate the raw numbers of songbirds migrating through your area is to go outside at night. While you may be able to see lots of migrants during daylight hours, the number of migrants that one can hear during their nocturnal flights is often exponentially greater. I can safely say that I've never seen more than 500 Swainson's Thrushes during any single fall migration season, but there have been many nights when I've heard more than 500 nocturnal Swainson's Thrush call notes in the span of 15 minutes...or less. Although the number of actual birds these calls translate to remains a bit of a mystery, I've always assumed that I'm not hearing any single thrush call more than 4-5 times as it flies over.

When conditions have been ideal--generally dense overcast, which results in low-flying birds--I've spent up to an hour listening to the non-stop (at least 20 calls/minute) nocturnal vocalizations of Swainson's Thrushes. When I start doing mental extrapolations, the potential number of birds passing overhead on such nights becomes staggering.

In the grand scheme of things, I live in a rather depauperate nocturnal soundscape. First, I'm stuck in the American West, which offers neither the variety nor the shear numbers of migrant passerines that one might experience living in the eastern one-third of North America. Secondly, I live in the middle of Portland, Oregon's "urban surf" zone, thus just hearing the muted flight calls of many species presents a challenge. Despite these shortcomings, Portland does get good nocturnal flights of Swainson's Thrushes. During peak movements the fairly loud flight calls of low-flying Swainson's Thrushes may be heard at rates of 50+ calls/minute and, for brief bursts, the flood of airy "weep" calls (http://pjdeye.blogspot.com/2009/02/thrush-calls.html) can be so frequent that they cannot be easily counted.

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This image shows the sonogram of a Swainson's Thrush flight call.  (Source: http://pjdeye.blogspot.com/2009/02/thrush-calls.html)

Last October I made my first pilgrimage to Cape May, New Jersey. Cape May is considered by many to be the best place in all of North America to experience the fall migration of songbirds. Many of the birders who call Cape May home have devoted much effort to the study of songbird flight calls. They and others have worked with the Cornell Laboratory of Ornithology to create  libraries of recordings of flight calls that are coupled with visual sonograms for these vocalizations. It's surprising how helpful it is to see the "shape" of each flight call expressed in a sonogram as you listen to and attempt to learn their differences. If you are blessed with good hearing, over time it is possible to start recognizing the unique subtleties of these vocalizations and use them to identify birds that you can't see in the night sky.

During our visit to Cape May, Shawneen Finnegan and I stayed with Michael O'Brien and Louise Zemaitis. On one particular evening I decided to go outside at about 11PM, just before we were all set to retire for the night. Though I hoped to hear some birds overhead, Michael, who is an authority on the flight calls of North American passerines, suggested that even on good flight nights he and Louise rarely hear more than a few calling birds directly over their house. Thus, I ventured outside with tempered optimism. To my surprise, within the first 30 seconds I heard no fewer than 15 unidentified call notes. I went inside and told the others that there seemed to be lots of birds passing overhead. Once outside, it took less than 15 seconds for Michael to announce, "we need to go to the Convention Center!"

The Cape May Convention Center is located along the beachfront boardwalk in the "downtown" section of historic Cape May on the southern tip of a long peninsula. The boardwalk and adjacent street are well-lit and parallel the Atlantic Ocean side of the peninsula just east of the mouth of Delaware Bay. During autumn flights, birds that are generally moving south or southeasterly tend to follow the coastline once they reach the Atlantic, putting them on a southwesterly heading as they move done the Atlantic side of Cape May. The Cape May boardwalk and Convention Center lie right below this nocturnal flight line at the southeastern corner of the cape.

After making the short drive downtown, the four of us spent the next hour sitting along the boardwalk as Michael called out one species after another, "Blackpoll, Palm, Black-throated Blue, Palm, Gray-cheeked Thrush, Swamp Sparrow, Indigo Bunting...and on it went. In all, we heard the flight calls of about 25 species of passerines, including those of Seaside Sparrow and Bicknell's Thrush, which only Michael recognized. In addition to the array of passerine vocalizations we heard, ambient light from the boardwalk illuminated the ghostly shapes of dozens of herons and egrets as they passed overhead in small groups. It was a spectacular experience, particularly for a Westerner unaccustomed to such flights.  

Back home in Portland, most of the nocturnal calls I hear are those of the Swainson's Thrush. It is a loud and fairly distinctive note that can be learned readily, so don't dismay if your hearing isn't perfect. Even if you don't hear or recognize any other flight calls, there will often be enough thrushes sounding off to keep you more than entertained. Over the next several weeks, look for nights with low overcast and venture out into the cool night air, or set an alarm to wake you about an hour before sunrise. If you happen to hit a good flight, it will surely change your notion of what is happening in the night sky as you sleep.

Here are some additional links to online resources if you are interested in learning more about nocturnal flight calls:

http://www.birds.cornell.edu/birdcalls/species/

http://www.birds.cornell.edu/brp/research/nocturnal-migrant-flight-call-research

http://www.oldbird.org/

http://www.pbs.org/wgbh/nova/kalahari/migrating.html

1

That is a whole new world of birding I have not yet ventured into due to my intermountain west location, but it sure sounds fascinating.

2

Good on you, Dave, for pointing out early in your piece that what we count are call-notes, not known numbers of Swainson’s Thrushes (or other species). I would nod to your thought that the same bird might be heard a few times. Birders hearing these calls, and keeping count of them, should bear in mind that the call density, or total, can be used as an index to flight intensity, but that it’s unsupportable to claim X or Y number of individuals. That said, keeping track of the number of calls heard across a period of days or weeks—or consecutive fall seasons—is certainly worthwhile. It may not contribute to perception of population health in the manner of the Breeding Bird Survey data set, but if nothing else, it gets people away from the screen and standing outside in the real world. // The concept of “urban surf” is real, and it can be vexing. Sometimes it is sobering. I well remember standing at the end of Royal Avenue at Fern Ridge Reservoir in early afternoon one year on “Black Friday,” the day after Thanksgiving, and being aware as never before of the cumulative sound of thousands of automobiles—although most of them were miles away, closer to the Eugene urban center. It was the silly shopping rush, of course. The subdued roar of all the consumers helping spur the GDP made it an experience quite different than any other time I’ve birded there.

3

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