Humans are a noisy species. Think about our amplified music, our cars and trucks, construction equipment, chain saws, construction projects, aircraft, windfarms and snowmobiles. There is no doubt that humans alter the natural sonic environment in profound ways.
Our noise production is not restricted to terrestrial habitats. Boat motors represent insults to the soundscape of aquatic habitats. Interference of those sounds with whale song is well known.
This column will draw from a recently published article by a team of European ornithologists, led by Margret Engel, who reviewed the literature on human-caused sounds on birds. The authors develop a valuable framework for assessing the impact of such noises.
People have mused about the effects of noise on animals over the course of history. Aristotle wrote about the effects of human noise on aquatic organisms. Leonardo Da Vinci considered the effect of noise on marine mammals. However, it was not until 1960 when audio-cassette recorders were invented that scientists had ways to assess changes in the vocalizations of birds and other animals in response to anthropogenic noise.
The authors were guided by known effects of noise on humans. Loud noises can be a nuisance, forcing us to speak louder to be heard and straining to hear other people. But loud noises can cause more serious effects like sleep disturbance, cardiovascular and metabolic problems, and adverse effects on learning in children.
The Engel article is a synthesis of the literature on known effects of human-caused noise on birds. The 50 articles reviewed were published in the past 60 years. Of course, with over 11,000 bird species, we have no information on the effect of noise on most bird species, but the reviewed articles do include data from 12 orders of birds. Birds are the best-studied group of animals in terms of the effects of human-produced noise.
The authors divided the impacts of human-caused sounds on birds into three basic categories: Acoustic Perception, Physiological Response and Behavioral Response.
As birders, we are aware of the diversity of the vocalizations of bird species. Black-capped Chickadee males give a song with the cadence of “cheeseburger” or “hey curly”, used to attract a mate and deter competing males. Soft chip notes serve as contact calls to keep a flock together. The “chick-a-dee” call is an alarm call and the number of “dee” syllables indicates the perceived risk. “Chick-a-dee” puts other chickadees on mild alert but “chick-a-dee-dee-dee-dee” means high alert.
One can understand how human-produced sounds might interfere with the acoustic signals of a chickadee. The frequency of traffic noise is quite close to the frequencies of many bird sounds and calls. Furthermore, these low-frequency sounds can travel long distances and penetrate through vegetated areas.
Human-induced sounds can mask portions of a bird’s vocalizations. Bird sounds may not be heard by other birds, making it difficult to procure a mate or to repel an intruding male or to alert other members of a flock to a Sharp-shinned Hawk.
Physiological responses are less obvious impacts of anthropogenic noise but often dramatic. Human-produced noises can raise a bird’s heart rate. This overexcitation results in reduced fledging of Muscovy Ducks, Great Tits and Eastern Bluebirds.
Noise can produce stress responses, resulting in release of stress hormones, leading to DNA damage. Such damage causes an increase in the number of infertile eggs produced.
Other physiological effects include suppression of the immune system and a reduction in overall health of birds. Noise can produce hearing loss, particularly of higher frequencies.
Behavioral responses include forcing some birds to move out of areas subject to human-caused noise. Local bird diversity then falls. The foraging of birds may diminish in loud environments. Loud noise can affect a bird’s brain, leading to reduced vigilance against predators.
The authors conclude with a fourth type of effect, impact on fitness, that combines the first three impacts to measure the overall effect of survival and reproduction as a function of human-produced noise. Their sophisticated data analysis showed two clusters of effects. One cluster included acoustic perception and behavioral responses, usually negative, indicating an interaction of these two responses. The second cluster involved physiological responses, nearly all negative.
This work is valuable in providing a solid framework from which to study the impact of human-associated sounds on birds. Mitigating those effects is a huge challenge.