As life returns to normal from the COVID-19 pandemic, the dichotomy in city life between the weekend and weekday has become increasingly obvious. On weekdays, swarms of commuters travel to and from their jobs around the city, creating the phenomenon known as rush hour. The honks of cars, shouts of pedestrians, and more all combine to create high levels of ambient noise. But on the weekends, many workers sleep in, relax, and recover from the work week, creating a quieter, calmer environment with dimmed city noise and a minimized rush hour.
To examine if the dichotomy between busy, bustling weeks and slow weekends impacts animal behavior, scientists developed the Weekend Effect Hypothesis. This hypothesis predicts that weekly human activity cycles cause animals to interact with their environment differently, based on the levels of ambient noise and human activity (Figure 1). Several studies have utilized birdsong as a lens to explore these behavioral impacts, as scientists noticed that higher noise conditions may force birds to sing more or less frequently (for instance, reed buntings living next to busy roads sang fewer songs on days with heavy traffic and road noise) or modify the characteristics of their songs to deal with the changing conditions and higher noise levels (Nemeth et al. 2013).
Diniz et al. (2021) were curious as to how these noise patterns impacted an urban population of Rufous Hornero (Furnarius rufus) living on Universidade de Brasília’s city campus. Rufous horneros inhabit urban spaces in South America and appear to have acclimated well: in the study area, several territories inhabited spanned roads and paths while continuing to enjoy breeding success. Each pair maintains a territory by attacking horneros infringing on their space, keeps watch for rivals and potential predators, and sings individually and in duets where one partner initiates and the other responds. As duets require complicated signal sending and receiving, where one bird listens to another and adjusts its behavior accordingly, the researchers used this vocal trait to explore if varying noise levels between weekdays and weekends impacted how horneros communicated with one another.
Before examining the effect of changing urban noise levels, Diniz et al. (2021) first needed to determine if the levels of pedestrian, cyclist, and vehicle noise in hornero territories actually varied between weekdays and the weekend. If no difference existed, the campus population would not be subject to the weekend effect. Thankfully for the study group, the numbers of pedestrians, cyclists, and vehicles were significantly higher on weekdays, and this activity elevated ambient noise levels. This effect was seen across all territories, and thus, the experiment could proceed towards exploring if horneros responded to these changing conditions as predicted.
The researchers then measured changes in hornero vocal behavior. They counted the number of times horneros initiated or responded to their partner’s duet, and the length of each duet, in addition to measuring several features of the hornero song. Ultimately, increases in noise levels and human activity on weekdays did little to impact the vocalizations of the horneros. With no change in duet characteristics, the researchers concluded that the horneros were still able to effectively communicate in high noise conditions. The horneros duetted loudly near one another, so seemingly they could hear each other well over distracting background noise.
The horneros did increase the mean and maximum frequencies of their songs on weekends. Higher frequency songs carry higher energy in their sounds and most organisms perceive them as louder. However, urban birds sometimes sing at lower frequencies because these sounds are less likely to be absorbed by buildings or surfaces common in an urban environment. On weekends, when some physical barriers (like cars or other vehicles) leave the urban jungle, the horneros sang loudly and at higher frequency without these additional surfaces dampening their songs’ sound waves and making them difficult to hear.
The study also categorized hornero behavior into sentinel behavior – where the birds keep watch for predators or other potential threats – or foraging behavior. Between the weekend and the week, horneros did not vary the proportion or amount of time spent as sentinels or foraging for food, again indicating that horneros generally fail to respond to changing urban conditions; the activity of humans during the week failed to distract or scare them. The lack of difference in this variable resembled the similarity of hornero duets between weekends and weekdays.
At first glance, the results of the study seem promising, yielding taglines of “birds fail to behave differently in cities, no matter the amount of background noise and distracting humans!” or perhaps “urban birds perfectly acclimated to a human environment!” However, upon closer look, the study leaves more questions than answers. The horneros did respond to the change in noise levels between weekdays and weekdays by adjusting frequency – but were they perfectly acclimated to the city and thus did not modify other behavioral aspects, or were the smaller levels of weekend noise still enough to change their natural patterns of activity? Would rural-dwelling birds respond dramatically differently to city conditions? Perhaps their evolutionary history has even locked horneros into permanent patterns of vocal signaling and they are unable to change their vocalizations in response to elevated human activity. Ultimately, are these rufous horneros adapted city slickers, or do they struggle on the streets?
References:
Diniz, P., Silva-Jr, E. F., Macedo, R. H. 2021. Urban cycles of human activity do not significantly alter the behavior of a duetting bird. IBIS: International Journal of Avian Science, 163(4): 1394-1408. https://onlinelibrary.wiley.com/doi/full/10.1111/ibi.12956.
Nemeth, E., Pieretti, N., Zollinger S. A., Geberzahn, N., Partecke, J., Miranda, A. C., Brumm, H. 2013. Bird song and anthropogenic noise: vocal constraints may explain why birds sing higher-frequency songs in cities. Proceedings Of The Royal Society B: Biological Sciences, 280(1754). https://doi.org/10.1098/rspb.2012.2798.
