Traffic Noise Provokes ‘Road Rage' In Male Galápagos Warblers

Forbes

12-04-2025

Yellow warblers near busy roads become increasingly aggressive when traffic drowns out their territorial songs, and noise pollution could trigger fights.
A pair of Galapagos Yellow Warblers (Setophaga petechia aureola; Female, left; male, right). (NOTE: ... More This image has been cropped from the original. Credit: Çağlar Akçay / Anglia Ruskin University)
A recent study has found that songbirds in the Galapagos Islands are changing their behavior because of human noise. Basically, birds exposed to vehicles moving along a roadway were found to show increased territorial aggression and to sing louder songs.
'Birds use song during territorial defense as an aggressive signal,' explained the study's co-author Çağlar Akçay, a Senior Lecturer in the School of Life Sciences at Anglia Ruskin University, where he studies animal behavior and communication.
'However, if external noise, such as traffic, interferes with the signaling, effectively blocking this communication channel, increasing physical aggression would be an appropriate response,' Dr Akçay added.
Increased physical aggression has consequences. A 2020 study found that the mortality rate of wild songbirds along roads in the Galápagos Islands was much higher than in other natural areas (ref). This earlier study provided the researchers with the motivation to form an international collaboration to ask how this growing threat could be minimized.
Topographic and bathymetric map of the Galápagos Islands, Ecuador. (Credit: Eric Gaba, translated by ... More NordNordWest / CC BY-SA 3.0)
The Galápagos Islands or Islas Galápagos in Spanish, are a remote archipelago of consisting of 16 volcanic islands in the eastern Pacific Ocean located roughly 900 km (560 miles) off the coast of Ecuador and clustered around the equator. They were visited by Charles Darwin in September 1835 when he was just 22 years old. These islands are home to many species of birds and animals that are found nowhere else in the world, and are well-known for their giant tortoises, which comprise 15 subspecies (13 extant and 2 extinct). Tragically, these giant tortoises were almost all wiped out by humans (who ate them) along with their pests, particularly black and brown rats and domesticated goats (which competed for scarce food and destroyed the fragile habitat).
Today, these islands are widely viewed as a natural living laboratory by scientists, particularly evolutionary biologists, ever since its finches inspired Darwin's theory of evolution by means of natural selection. However, despite their scientific importance, recent decades have seen significant human population growth. Along with a rise in tourism, the permanent human population is increasing by over 6% per year, leading to more and more vehicle traffic on the islands' roads.
This increased vehicle traffic is leading to large increases in birds are killed by automobiles. Amongst those fatalities, Galapagos yellow warblers are far-and-away the most common victims.
'An earlier study by Garcia-Carrasco et al (2020) found that 70% of the roadkill on the main road in Santa Cruz are yellow warblers (ref),' Dr Akçay pointed out in email.
Adult male Galapagos Yellow Warbler (Setophaga petechia aureola), singing. (Credit: Alper ... More Yelimlieş.)
Galapagos yellow warblers will look familiar to most birders: they are a subspecies of the migratory yellow warbler, Setophaga petechia, which is the most widespread species of Setophaga warblers in the New World. They breed throughout almost all of North America and the Caribbean, and even in the northern parts of South America – including the Galapagos, where they are resident.
In the Galapagos, the yellow warbler, Setophaga petechia aureola, differs somewhat in appearance from its mainland cousins by having a rust-colored cap on its head. Its genetic divergence is low, probably because it colonized the archipelago relatively recently; less than 300,000 yrs ago. These warblers are insectivorous, and thus, they likely also face human pressures from growing pesticide use.
The study was co-led by both Leon Hohl and Alper Yelimlies, who were volunteers in the lab of senior author, organismal systems biologist Sonia Kleindorfer, at the University of Vienna. Professor Kleindorfer is an expert on how animal behavior shapes evolutionary dynamics in birds and parasites. (Mr Yelimlieş is now a graduate student in Professor Kleindorfer's lab.)
A yellow warbler catching flies from atop a Galápagos tortoise whilst it was munching grass. ... More (Credit: Matthew Roth / CC BY-NC 2.0)
In this study, Dr Akçay and collaborators asked whether Galápagos yellow warblers show increased aggression and flexibility in their song when confronted with a simulated intruder under conditions of experimentally increased traffic noise depending on their experience with such noise. To do this, the researchers studied the behaviors of 38 males on two islands within the Galápagos Archipelago, Floreana and Santa Cruz. Whilst 20 of the study birds lived within 50 meters of the nearest road, 18 others nested more than 100 meters away from traffic.
Dr Akçay and collaborators placed a speaker in the territory of each study male, and played two different recorded soundtracks on different days. One soundtrack featured the song of another male Galápagos yellow warbler, whilst the other featured traffic sounds in addition to warbler songs.
The males that lived near busy roadways behaved more aggressively than those living farther away by approaching the speaker closely and repeatedly swooping low over it, probably in search of the intruder.
By contrast, males that lived farther away from roads were less aggressive when traffic noises were present in addition to warbler songs. Were these birds afraid of the unfamiliar traffic noises?
Dr Akçay and collaborators also reported that males living on the more densely populated Santa Cruz Island sang for longer periods of time when traffic sounds were present, whilst the reverse was true for those warblers on the more sparsely populated Floreana Island.
And yet, the effect of living on a roadside territory was noticeable even on tiny Floreana Island, which has a total population of roughly 10 vehicles, suggesting even minimal traffic affects the birds' responses to noise.
'Our results show that the change in aggressive responses in yellow warblers occurred mainly near roads,' Dr Akçay reported. 'Birds occupying roadside territories on both islands, and therefore having regular experience of traffic noise, may have learned to increase physical aggression when the territorial intrusion was accompanied by traffic noise.'
In addition to escalating their physical aggression, Galápagos yellow warblers living on the more populous island of Santa Cruz increased the duration of their song when confronted by traffic noise.
'We also found some evidence of birds trying to cope with noise by adjusting their song, with yellow warblers in all habitats increasing the minimum frequency of their songs to help them be heard above the traffic noise,' Dr Akçay reported.
These findings indicate that an individual bird's previous experience with traffic noise motivates them to adapt and adjust relevant features of their songs.
How did the female warblers react to traffic noises?
'Unfortunately, the responses of females to anthropogenic noise have been understudied,' Dr Akçay replied in email.
'During breeding, females are quite unresponsive to territory intrusions, yet during non-breeding they are as aggressive as male warblers,' Dr Akçay pointed out in email. 'These make the same questions (whether aggression or songs are affected by noise) relevant for females as well. We are currently working on several studies on female song and duetting in this species including a follow-up study to this one investigating the influence of noise on female warblers and pair defense behavior.'
What are the most important lessons to be learned from this study?
'Our study shows the importance of considering behavioral plasticity in conservation efforts and developing strategies to mitigate the effects of noise pollution on wildlife,' Dr Akçay explained. 'It also highlights the significant impact of human activities on wildlife behavior, even in relatively remote locations such as the Galápagos Islands.'
Leon Hohl, Alper Yelimlieş, Çağlar Akçay, and Sonia Kleindorfer (2025). Galápagos yellow warblers differ in behavioural plasticity in response to traffic noise depending on proximity to road, Animal Behaviour | doi:10.1016/j.anbehav.2025.123119