Human created noise interferes with birds’ lives

Anthropogenic (human-created) noise affects avian safety and cooperation, as well as selection of nesting sites and success in rearing their young. Creative adaptation helps birds survive.

The specific sounds birds use to alert members of their species to potential danger, known as alarm calls, can be useful to other species that live nearby and are at risk from similar predators. More eyes and ears mean more safety. Anthropogenic noise makes this more difficult, but our avian friends are using their brains to adjust to these changes.

Alarm calls alert other birds to a nearby predator. They are a signal to take cover or flee. Most birds have different calls for different types of predators, which is important because aerial predators such as hawks and owls require a different response than do ground predators. Aerial predator alarm calls send birds into bushes or trees, deep under cover. When ground predators such as domestic or wild felines, foxes, raccoons or weasels, are nearby the alarm call tells other birds to fly to escape.

Most often avian species respond to other birds’ distress calls when those calls are similar in structure. This discovery was made during a study to determine why some birds respond to another species’ call while others don’t. For example, a white-throated sparrow (Zonotrichia albicollis) call isn’t recognized by either swamp (Melospiza georgiana) and song sparrows (Melospiza melodia) despite white-throated sparrows sharing habitats and regions with both these other sparrows. In this case, the calls are too dissimilar to their own for them to recognize them as warnings.

Conversely, species separated by thousands of miles responded to another species’ calls played by researchers. For example, the apostle bird (Struthea cinerea) in Australia recognizes the mobbing calls, which are essentially a call to arms, of the Carolina wren (Thryothorus ludovicianus), found in the Americas. Clearly this does the apostle birds little good, but they respond because the call is similar to their own.

Birds use those mobbing calls when certain predators are nearby. These predator species, usually hawks or owls, fly in quickly and try to snag one bird, then fly off to consume it in a more secluded area. Upon hearing these calls, multiple species, now aware of the danger, will band together to dive bomb the predators. Mobbing destroys the element of surprise, often deterring the hawk or owl from their hunt, at least temporarily.

Chickadees rely on mobbing calls for help Photo by Peter Lewis on Unsplash

Today anthropogenic noise often masks communication between birds, making cooperative efforts such as these less likely to be successful. Even worse, anthropogenic noise also has a negative impact on nesting success, which is defined as birds raising their chicks to fledging, the time they leave the nest. In one major study, researchers used data, collected by Nest Watch volunteers for the Cornell Lab of Ornithology, from nearly 60,000 nests to arrive at this conclusion.

Birds’ response to other birds distress calls varies with the sound heard

In order to protect their chicks from the time the eggs are laid to the time the chicks leave the nest, or fledge, bird rely on vital information in calls from other birds to be aware of potential danger. Looking at everything from the time the first egg was laid in each nest, to the number of nest failures (no chicks hatched), partial hatch (not all eggs hatched) and nesting success (all chicks hatched), the researchers found human-created sounds caused acoustic masking. In other words, anthropogenic noise prevented birds from hearing the vital information in the calls of other birds.

Interestingly, this was most common for birds who communicate in lower frequency ranges, the same ranges as many sounds produced by the human environment. Never slackers, a few avian species are learning to compensate by singing at a higher frequency.

Another problem for birds is that anthropogenic noise hides predator movement, allowing those animals to sneak in and hunt more successfully. There seems to be an evolutionary game of chess going on here, requiring each species to figure out a way to up their game against their predators. Or their prey.

Interestingly, one common nest predator, the western scrub-jay (Aphelocoma californica), is also unable to function well in noisy habitats, suggesting that their ability to find and predate on nests may be reduced due to acoustic masking. While this is a positive for their avian prey, it has potential for a less positive knock-on effect, because the impact on the jays has an impact on a tree species. Jays are important dispersers of pinon pine seeds, another of their food sources. If jay populations decline, fewer seeds are spread, resulting infewer pinon pine trees.

For me, the most interesting finding is that these adaptations are not just the result of evolution. Some birds, notably songbirds and parrots, have brains as big as primates, allowing them to problem solve. How do they do pack so much information into such small heads? They concentrate a lot of neurons in their telencephalon.

Never heard of a telencephalon? I hadn’t either until I started looking into the science of bird intelligence. Turns out it’s what we call the cerebrum in humans, which includes both the left and right hemispheres and the corpus callosum. Bird telencephalons are separated into four lobes: frontal, parietal, temporal and occipital. Lots of places for deep avian thinking about ways to avoid or escape predators.

I didn’t doubt that birds were bright but now I know why. Join me in exploring other aspects of avian intelligence in future posts.