It is a cool, breezy day today, and I am very happy to have been able to turn the air conditioning off (with an acknowledgment that I REALLY appreciated it during all of the recent days of high humidity and ninety degree temperatures!). At last I can open up the windows and breathe fresh air and listen to all of the noises going on outside.
On top of the steady percussive sounds of rain dripping off of the tree and shrub branches, the birds are really putting on a musical show. The sparrows (white-throated, song, and chipping) and the house finches are singing and trilling the main song with punctuations of blue jay whistles and robin cackles. The Carolina wrens are rolling out their long cadences, too, along with the buzzing of the chickadees and titmice. I can even hear some starlings chipping and fluting off my neighbor’s tree. Every moment is a new combination of sounds and textures. You can almost make out the new hatchling cardinals tucked up in their nests in the arbor vitae calling to their parents for more food.
Many nature writers have talked about the purpose and the complexity of these bird sounds. Jon Young in his book “What the Robin Knows” could tell without looking when a neighbor’s cat entered his backyard by the subtle changes in tone and tempo of the surrounding bird songs. Information was being exchanged, you just had to open up your ears and mind to hear and understand it.
An article about six weeks ago in the New York Times (May 18, 2015) took the discussion of bird songs to even higher levels. Two biologists (Erick Greene at the University of Montana and Jesse Barber at Boise State) talked about their research into the functionality and importance of bird songs. Bird songs, they agreed, were important not only for a specific bird species but for many other species of birds and even mammals that share the same forest habitat. Many of these songs had local and very immediate impacts, while others exerted their influences over great distances.
For example, chickadees seeing a raptor will begin to sing out their “chick-a-dee-dee-dee” call. This song calls other chickadees to the area to set up a “mobbing” of the potential predator. Erick Greene found, though, that was much more information in these mobbing calls than simply “get over here!” If the raptor was especially threatening to chickadees (if it was, for example, a relatively small, agile hawk like a Cooper’s hawk), then the chickadees sent out their distress call more frequently and added more “dees” to the endings! If the raptor was less of a threat (if it was a large, but fairly clumsy Goshawk, for example that a chickadee could expect to evade fairly easily) then they slowed down their calls and abbreviated the “dee” endings! Greene also found out that red squirrels picked up and mimicked the chickadees’ distress calls and even seemed to be aware of the gradations of their relative threats (red squirrels were MUCH more cautious when the Goshawk level of a warning was given!).
Chipmunks, on the other hand, seemed to be more clued into warning calls from tufted titmice. Titmice were also carefully listened to by many species of small birds including red breasted nuthatches. Titmice were called the “crossing guards” of the forest by Katie Sieving of the University of Florida for their influence in keeping other bird species from going out into open areas of the forest habitat during a predator warning event.
Further, Greene’s study on lazuli buntings clearly showed that a threatening hawk cruising several miles from a bunting inhabited site was perceived by the buntings by means of a rolling chatter song that ripped across the forest ecosystem at a speed of 100 miles per hour! This song was spread at frequencies too high for the hawk to hear, so the warning was being transmitted on a “private” channel for all of the small, potential prey birds. In response to the multispecies clamor the buntings were able to dive into protective shrubs well before the approaching hawk burst into their feeding habitat.
Barber is also testing what happens when the quality of the bird song warning system is compromised. Noise pollution is becoming a very serious problem even in remote areas of the United States. It was noted that 83% of the land area of the lower forty-eight states is now within two thirds of a mile of a road. Further, expanded oil and gas exploration not only generates substantial noise from drilling machinery and activities, but also includes extensive expansion of roads and road usage.
Noise is directly stressful to birds. Their overall health, vigor, and size declines in noisy environments. Noise also blocks group generated alarm calls causing individuals to increasingly have to spend their individual time and energies watching out for predators and other dangers. This energetically expensive individual “alert” time would have been better spent feeding or resting. Barber’s “artificial road noise” experiments showed an overall 25% decline in bird abundance in noisy areas with some areas being entirely forsaken by all native bird species!
My yard is regularly patrolled by a pair of sharp-shinned hawks. The hawks have to come into the feeder areas via all sorts of hidden routes in order to avoid detection by the chickadees, titmice and blue jays. They need to grab their prey quickly and rapidly exit the area to avoid mobbing especially by the bold jays. The sharp-shins do get an occasional bird and, obviously, secure sufficient prey to live and reproduce on, but they have to be good at the stalking, swooping, and killing or the information system of the feeder birds will thwart them. The impact of a predator is almost always the ecological and evolutionary improvement of the prey species. Instead of bigger, faster and stronger, the feeder birds are just more verbal!
Enjoy the summer!