The half-mile bike ride to my garden is a pleasure in both the early morning and early evening. Soft light illuminates fluffy clouds, cool breezes bring relief from the heat of the day, and the ethereal song of the Hermit Thrush drifts through the quiet forest. Most birds are done singing for the year, their chicks are fledged and males no longer need to defend a territory or attract a mate. The cacophony of early summer is replaced by a few vocal marathoners.
Hermit Thrushes are one of the earliest birds to return to the Northwoods, and their beautiful song is a welcome contrast to the gray days of late March. With almost equal vigor, they continue to serenade us throughout the summer. You may have listened as one began its song with a thin whistle, and followed with a flutey jumble of notes. The flutey quality, and often the impression of a duet, comes from the thrush’s elaborate sound system.
Amphibians, reptiles, and mammals (including humans) sing using a larynx. We generate sound by forcing air through the trachea, causing thin membranes to vibrate. Our larynxes only use about 2% of the air we exhale. Birds, in contrast, have a syrinx. This set of membranes restricts the airflow through the trachea, right where it splits into the two bronchial tubes.
The syrinx is much more efficient than our larynx, and can turn almost all of the passing air into sound. This is why tiny birds can make gigantic sounds! With the syrinx straddling two separate tubes, each with separate sound-producing membranes, control muscles, and neural connections, thrushes and other birds can produce two sounds at once. Songbirds (including thrushes), can even do their own version of circular breathing by singing out of one side of the syrinx while inhaling quick breaths through the other side.
Not all birds have the same sound system. Both the structure of the syrinx and the training to use it can differ. Thrushes and some other “oscine” (from the Latin word “to sing”) songbirds need to learn their songs. In the “suboscine” group that includes flycatchers and Eastern Phoebes, songs are innate. Other birds, like vultures, can hardly make sounds at all.
Many herons only make harsh squawks, but the American Bittern makes a loud, guttural, booming, "oog-ka-chuk.” Males fill their gullets with air and then release it through a specialized esophagus. The bittern’s unique call has led to many other common names, including water-belcher, mire drum, and thunder pumper.
The low sound travels well in the evening air of wetlands, and can be heard at a distance of several miles. This enables scientists to count several types of marsh birds using song surveys.
Visual surveys of the bittern would be difficult. Long brown and white vertical stripes on their bellies provide them with excellent camouflage in tall marsh grasses, and they even sway with the wind to make their disguise complete. The several I have seen have been spooked into flight by plant biologists slogging through wetlands that rarely have human visitors.
Unlike thrushes, bitterns only sing during mating season in the spring. That is why I felt very fortunate on a recent evening to see two bitterns in a friend’s bur oak tree, where they looked awkwardly out of place as they went mutely about their insect catching.
The silence of summer adds to birds’ camouflage as they raise their fledglings and molt feathers. The birds that do keep singing, like the Hermit Thrush, may be conducting singing lessons with their young. Remember, they are oscine songbirds who must learn the proper melody for their species.
Both Hermit Thrushes and American Bitterns will soon be preparing for migration, and the woods will be still quieter. I may have already heard the last thrush concert for this year. Luckily, we have several hardy birds that stay throughout the year, and the cheerful calls of chickadees and nuthatches will keep me and the garden company through the winter.
For over 44 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M. The new exhibit, STAR POWER: Energy from the Sun, opened in May 2012 and will remain open until April, 2013. Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/