Around the world, during the warmer months, the beginning of the day is heralded by bird melodies. But in addition to being wonderful alarm clocks, the dawn chorus reflects how birds have adapted their singing to both ambient conditions and interspecific competition. Most bird song is for territory defense and attracting mates (female birds often being attracted to the mate with the best territory), so they want their song to carry as far as possible. They do that by taking advantage of temperatures, avoiding wind noise, and sharing acoustic space with other species.
Effects of temperature
The falling temperatures of nighttime usually create a temperature inversion, as cooler air settles on top of warmer air. Sounds bounce off of this temperature boundary and thus propagate further. As most birds are active in the daytime and not at night, they try to get much of their singing in before temperatures start to warm and remove the inversion. Some birds not only sing at dawn, but also dusk as the temperatures start to cool. Many birds will also sing during the day, but at a lower rate than at dawn. The wind also tends to be lower at dawn and dusk than during the middle of the day, and singing into the wind takes a lot more energy to convey the same message.
Frequency constraints
Low frequency (pitch) sounds travel farther. But wind passing through leaves and branches creates a lot of low-frequency noise. Most birds sing in the 2-8 kHz frequency range, which is above much of the wind noise, but still low enough to carry well. And lucky for us, it’s also the range that humans hear quite well. In contrast, bats communicate with each other in the 10-30 kHz range, and echolocate up to 200 kHz.
Sharing acoustic space
If all birds sang at once, they wouldn’t be able to hear each other! They maximize their singing effort by dividing up the narrow band of frequencies by using different patterns of vocalizations, slightly different frequencies, and different timing. Birds that sing at nearly the same frequency will often alternate, with one bird waiting until the other is finished singing before he starts. And much like us, some birds are early risers and some aren’t. So the dawn chorus changes from start to finish, with some birds chiming in at the beginning and others waiting until the end.
On my recent trip to the Chiricahuas (see After the fire, the flowers bloom), I was able to record a lovely dawn chorus that illustrates these factors well. The second morning of our campout was almost windless, creating a wonderful recording opportunity.
The chorus starts at first light with Hermit Thrushes and Western Tanagers piping up as the last of the whippoorwills fade away (listen closely for them). They are joined by a Gould’s Turkey.
By the time the sunshine is actually visible on the canyon walls, an American Robin, and some warblers and towhees have joined in.
Forty-five minutes into the dawn, things have really quieted down. Most of the birds have either moved to other areas to sing or have just quit singing. A Red-faced warbler has started singing, and he’ll sing off and on throughout the day, whereas the Hermit Thrush will soon quite down and wait until dusk before starting up his beautiful song, which my camping buddy aptly described as “angelic.”
A compilation of these recordings appears in the album, Crossroads.
Recording notes: Recorded with Sony PCM-M10 with AT2022 and FEL SK3.5 preamp. Recording subject to normalization and some noise removal.
Photo credits: Western Tanager, www.naturepicsonline.com; Hermit Thrush, Almiyi. Both licensed through Creative Commons.
References:
Bradbury, J.W. and S.L. Vehrencamp. 2012. Principles of Animal Communication, 2nd Ed. Sinauer & Assoc.
Krause, B. 2012. The Great Animal Orchestra. Little, Brown, and Co.
Pijanowski, B.C. et al. 2011. Soundscape ecology: the science of sound in the landscape. BioScience 61:203-216.