There was not a cloud in the sky as the sun peeked over the dense canopy in the Alajuela Province, Costa Rica. The backdrop of the Arenal Volcano loomed in the distance, and the ashen drift from previous nights no longer fell. We were functioning on no breakfast and minimal sleep, but all was forgotten when the songs of the avian symphony reached our ears. Erick—our guide and seasoned bird expert—brought along his elbowed Swarovski spotting scope, rounded hat, and a keen eye. Not even two steps out of the parking lot, Erick pivoted his scope across the gravel lot and adjusted the focus of his spotting scope. Eagerly, we all stepped up to the scope, excited to view the unrevealed beauty. Looking through the clarity of the glass, we saw that Erick had pinpointed the location where a keel-billed toucan perched. The toucan displayed its vibrant colors in the ocean of greenery, the scope as a spotlight which highlighted the detail in each and every feather. Its bill was an artist’s canvas, heavenly blue blended with lime green, blurred orange strokes lining the top beak, finished with a curved crimson tip. Its ebony back relaxed, as it proudly puffed out its bright yellow breast. Suddenly, its quiescent elegance was broken, and the toucan unleashed a repertoire of calls, leading with a deep guttural croak. I listened with wonderment as the call rebounded across the dense landscape.
(Keel-billed Toucan, Ramphastos sulfuratus, image source)
Most birds, like the keel-billed toucan, perform a vast collection of songs; a result of sexual selection, where attractive songs point to strong mates. The quality of a song, as well as diversity of both repertoire and vocalization, conveys the overall fitness of the bird. Communication occurs at different frequencies, depending on the habitat and surrounding environmental factors of the species. In the rainforest, the sheer quantity of calls overwhelmed my senses. I was absorbing more acoustic diversity every minute than I would in an hour back home. Birds in dense habitats—like the rainforests of Costa Rica—often have lower frequency calls and narrower bandwidth than those who live in open habitats. The scientific theory used to describe this is called the acoustic adaptation hypothesis, which states that animals will adapt their vocalization patterns based on the acoustic properties of their environment. We, humans, build echo chambers to amplify sound, but over time birds have adapted their own vocal systems to complement existing acoustic structures. This would be like developing an adapted megaphone to amplify their messages.
Scientists determined that dense vegetation absorbs and muffles sound, so a lower frequency call would reverberate around the plants and even get amplified. The keel-billed toucan uses the thick forest to its advantage, as its low call will reach potential mates near and far. In an open habitat, such as a grassland or plains, birds have calls with high frequencies and rapid modulations, or variation in the pitch. This adaptation is beneficial because high-pitched songs and calls will degrade less over longer distances. The deep croak of the toucan would not fare well traveling over a flatland.
The black feathered and yellow-breasted bird curiously cocked its head before turning into a flurry of feathers and rising off into the depths of the rainforest. We continued walking up the shifty gravel path and spotted a duet of hummingbirds dancing around the brush and a pair of courting parrots flying along the horizon. As the parrots disappeared behind a thicket of high rising trees, a shrill whistle pierced through the rainforest. “Hear that sound?” he paused. “That is the long-tailed manakin. Scientific name: Chiroxiphia linearis.” Erick tightened his lips and procured the same noise. The rainforest sang back. “There it is again, you can hear its call,” he pointed.
(Long-tailed Manakin, Chiroxiphia linearis, image source)
A distinct song like that of the manakin serves two main purposes: mating and territorial defense. The quality of a song gauges the strength and fitness of a bird to determine which mates are adequate breeding partners. When a bird challenges a territory, responding songs indicate different levels of aggression. For instance, if a particular song is matched, as Erick matched that of the long-tailed manakin, it can be seen as an act of aggression. Meeting a song with an echo is challenging the rival bird, as it becomes a competition for strength. The quality of the song gauges if encroaching on a territory will be a fair fight, and a strong song can deter rivals. Additionally, more complexity in a song garners a higher level of threat to a territory.
The performance of competing birds was not by any means the sole source of music in the cacophony of the rainforest. As we returned to the classroom, we would hear surges of high pitched vibrations. The buzz would come in waves, seemingly as loud as a power saw, and certainly as ear-piercing. This was nature’s percussion, the sound of a bustling woodshop: the chirping of active cicadas. We were able to look at the remanence of the insects in the classroom, as we passed around the shells of the reverberant species. The shell was hollow and delicate as an eggshell, but also eerily beautiful, a perfect ghost of a living creature. It was so perfectly balanced that you could forget that the weightless crust laid on the tip of your fingers. It amazed me how such a small creature could amount to so much deafening noise, and with this deafening noise, how a bird like the beautiful keel-billed toucan or the elegant long-tailed manakin could convey their messages of territorial dominance or commence a mating ritual.
This is where we return to the acoustic adaptation hypothesis, the one that explained that animals will adapt their vocalizations to suit their habitats. The cicadas in the Costa Rican rainforest had taken a vast majority of the acoustic real estate with their shrill chirps. Birds or other animals who contest this acoustic space will have a difficult time communicating, as the overlap of signals will alter the information or disrupt mating songs. Over time, cicadas have affected the signaling and timing of birds. To adapt to this, avian species have developed a sense of when the cicadas signal, and often stop vocalizations during periods of high noise traffic, which can last up to three hours. For birds that do vocalize at the same time as cicadas, their songs occur at non-overlapping frequencies, so the information is not altered. This explains why most of the avian acoustic activity is early in the morning, the leading cause for our early and untimely pre-breakfast bird walk. In the early morning, the cicadas do not occupy the acoustic space, an opening which birds have learned to take full advantage of. Now, birds face a more grave threat than cicadas saturating the acoustic space. Noise pollution from industrial regions is making it increasingly difficult for birds to relay messages and sing mating songs. Unlike the cicadas, the noise traffic often does not cease after a few hours, which negatively impacts the birds.
References
Ey, E., & Fischer, J. (2009). The “Acoustic Adaptation Hypothesis”—A Review Of The
Evidence From Birds, Anurans And Mammals. Bioacoustics, 19(1-2), 21-48. doi:10.1080/09524622.2009.9753613
Boncoraglio, G., & Saino, N. (2006, October 20). Habitat structure and the evolution of bird
song: A meta‐analysis of the evidence for the acoustic adaptation hypothesis. Retrieved from https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2435.2006.01207.x
Morton, E. S. (n.d.). Ecological Sources of Selection on Avian Sounds. Retrieved from
Hart, P. J., Hall, R., Ray, W., Beck, A., & Zook, J. (2015). Cicadas impact bird communication in
a noisy tropical rainforest. Behavioral Ecology, 26(3), 839-842. doi:10.1093/beheco/arv018