The Mystery of Night

Last September fall songbird migration was well underway. Almost every day I encountered a migrant in our yard, our meadow, or our forest. Many of the birds are not as colorful as the males are in spring and there are huge numbers of immature warblers that wear the drab coats of females such as the warbler I spotted as I walked up our driveway. It was foraging in a small black walnut sapling behind an old apple tree and paused long enough for me to note its grayish back and head, faint wingbars, yellow throat, breast and belly with faint streaking on its neck and breast. After studying Roger Tory Peterson’s two pages of fall warblers in his Field Guide to Birds of Eastern and Central America, I decided that the bird was an immature female prairie warbler.

Meanwhile, our birder son, Mark, was out every morning at dawn hiking eight miles up and down our property, recording bird species and numbers on his phone, the same as he had done during spring migration when he had found 101 species one mid-May day.

In both spring and fall, the migrant birds come into our property to eat and rest for the day because most songbird species—sparrows, warblers, flycatchers, thrushes, orioles, and cuckoos—migrate at night when they are safer from predators such as day-flying raptors, and it is also quieter.

Migrant birds use the Earth’s magnetic field to help them migrate, and recently researchers discovered how they see in the dark. A protein called cryptochrome, which is sensitive to blue light, allows them to do this because of an unusual cryptochrome eye protein birds have called CRY4.

So many birds migrate each fall that they can be detected by Doppler radar. And they are especially abundant when low weather systems are followed by highs that bring north winds.

Before they leave their breeding grounds they gorge on foods filled with carbohydrates and lipids like berries and other fruits and store them as fat. For instance, ruby-throated hummingbirds double their weight in four days before they embark on a 2,000 mile migration to Mexico and Central America. And here in Pennsylvania most ruby-throated hummingbirds are on their way as early as mid-August or early September.

Migratory birds sleep less, but they do sleep while flying. Swainson’s thrushes sleep for nine seconds on one side of their brain and then switch to the other side for nine seconds, keeping themselves half- awake to avoid predators or mid-air collisions with other migrating birds, researchers discovered.

In addition, migratory birds give short calls as they migrate at night, probably also to avoid colliding with other birds during massive migratory flights as well as for echolocation. Most of those calls sound nothing like their daytime calls and songs.

Back on September 14, 1896, Orin Lobby sat on a hill near Madison, Wisconsin and counted 3600 calls from night-flying birds during five hours of listening. His was the first known attempt by anyone to do this. There were only two other such studies in the next 50 years—one by Paul Howes in 1914 and the other by Stanley Ball (1952)—and both documented the night time call of thrushes, which are more distinctive than other songbird calls. Ball’s work was also the first to use such calls to time the migration of a region’s species.

The first audio recordings made of nocturnal flight calls (NFCs) used reel-to-reel tape recorders to automatically record 10-minutes out of every hour all night so recorders could sleep and later listen for bird calls on the recorders.

Then Bill Evans, in the 1980s, used commercial hi-fi video cassette recorders to record 8 to 10 hours of NFCs a night. In 1994, he joined the Cornell Lab’s Bioacoustics Research Program (BRP) working under Dr. Chris Clark to help develop computer-based automatic NFC detectors and BRP programmer Harold Mills wrote the first working NFC detector which could detect short, high-pitched flight calls of warblers and sparrows.

But Evans wanted to encourage citizen scientists to participate in NFC monitoring so he founded a nonprofit he called OldBird for NFC monitoring. Then he contracted a former BRP programmer, Steve Mitchell, to develop advanced software to be used on home computers. They began with dickcissel and thrush detection, but today anyone interested in listening to and recording the calls of NFCs on their home computer can purchase their own Autonomous Recording Unit and set it up.

That’s what our son, Mark, did last fall. On a pole near our barn, he attached an OldBird, 21c microphone enclosed in a large white bucket pointed skyward and ran a cable from the microphone into the barn where he had his laptop computer equipped with software to analyze bird calls.

The bucket, he explained, helped the microphone pick up calls as high as approximately 1,950 feet in the sky and within 975 feet of the microphone.

There is no field guide to NFCs, but there are sources online that are mostly based on Evans’s and O’Brien’s CD-Rom “Flight Calls of Migratory Birds: Eastern North America.” Mark showed me pages of NFC spectrograms and also had me listen to the short “zeeps” of several of these birds. The length of each call (in milliseconds) was at the bottom of the spectrograms and the sound—from 0 to 12 Kilohertz—on its left side. Near zero it was mostly black from insect noise, and Mark found that the din from katydids until a good freeze in mid-October made it almost impossible to hear the bird calls in September. So he did most of his recording from October 14 until November 22.

In all, he, with the help of fellow NFC experts and enthusiasts, identified 47 species. Some were easy, such as American robins that have the same call they use in the daytime, and others have distinctive spectrographs such as that of American redstarts which I identified immediately. The sparrows and warblers are all in the high kilohertz (six to 10 range) and nine warbler species have almost identical “zeep” calls lasting 60 milliseconds, while another nine warbler species with “up” calls last 30 to 50 milliseconds. But only two warbler species—northern parula and pine warblers—have the “single down sweep” calls at 50 milliseconds.

The sparrows are mostly two to a category, but Mark was able to distinguish most species. Still, even the experts are still working on 10% to 20% of bird species’ calls that can’t be identified. And he had his own mystery bird at 11:37 on November 12. No one, not even Bill Evans, is able to identify it so far.

On the other hand, the following night Mark heard the clear call of a snow bunting, a species we have never seen on our property. Another was that of a horned lark with the same call as we have heard down in Sinking Valley where they nest and even winter. He also recorded many savannah sparrows even though again we have never had one here.

As expected, the thrush calls were easy to distinguish and he heard gray-cheeked and Swainson’s thrushes, both of which I encountered on my walks. On September 19 a gray-cheeked thrush was foraging in the undergrowth beside Laurel Ridge Trail. A grayish, plump thrush, it has one of the longest migration routes—from the edge of the tundra to Brazil—of the thrush species and is the first thrush species to migrate, passing through Pennsylvania as early as the last week in August, although their primary time is from the second week in September to the first week in October.

The Swainson’s thrush I observed on October 4 in the underbrush beside our hollow access road. It breeds across Canada and Alaska and in the northern United States and migrates to Central and South America.

Wood thrushes and hermit thrushes both nest in Pennsylvania and wood thrushes, which are headed to Mexico and Central America for the winter, are mostly gone here by the first week in October, but hermit thrushes, the only thrush species to winter in the United States, including in Pennsylvania mostly in the southeastern part of the state, are the last to leave in mid-October to early November. Mark counted sometimes dozens of them a night.

As David Brown from Lycoming County writes on his excellent website about NFCs, [hearing] “the descent of thrushes in the pre-dawn hours of a fall is hard to beat…”

Flocks of geese, swans, and shorebirds also migrate at night and Mark recorded “tons of tundra swans,” and one night a greater yellowlegs call, a month later than any have been reported migrating. It is for such discoveries that Mark especially enjoys analyzing NFCs.

But he also likes hearing mammals moving about such as deer startling a bird or coyotes howling in the distance, what he calls the “mystery of night.”