Walking through your nearby shopping center or downtown commercial district, you might notice that birds have made themselves quite at home on the rooftops, ledges, and benches – not to mention the trees. Indeed, to the passing observer, it would seem that birds of all species have adapted quite well to cohabitate with human settlement and all the commotion it brings; while some species hide deep in the woods avoiding any trace of humans’ ever-growing ecological footprint, birds make their right among us.
However, a recent study by researchers at the University of California Davis in the scientific journal Behavioral Ecology suggests that the incessant noise of traffic actually alters bird settlement patterns and has a negative effect on reproductive success. The researchers wanted to study how anthropogenic noise, or man-made sound, may be contributing to declines in populations of the tree swallow (Tachycineta bicolor).
Previous studies have discovered that birds nesting near roadways produce fewer eggs and have fewer healthy offspring among them that survive. Nonetheless, it was not clear whether these effects were due to a pattern of lower-fitness (less healthy) individuals selecting these nesting sites or whether there is a direct negative effect of roadways on the breeding birds.
The authors of the study set out to answer this question near their campus in Davis, California. Taking advantage of the fact that tree swallows like to nest in man-made “nest boxes”, the researchers created a breeding ground for tree swallows by creating 35 clusters of up to 4 boxes. As the swallows arrived back at their breeding ground, they settled these boxes to use as nests to raise their offspring. The clusters of boxes were divided into 2 categories – experimental and control groups. For the clusters in the experimental group, an amplified speaker was placed in the center of the cluster to play traffic noises during the period in which the birds were settling the sites. The speakers’ placement created a continuous range of amplitude of the sound as experienced by each box. On the other hand, clusters in the control category received no additional noises besides the ambient sound of the surrounding environment. The researchers observed the order in which the boxes were settled relative to the amount of traffic noise at each to study the settlement pattern.
In order to ensure that the possible effects of the noise only influenced the parent birds, the sounds were stopped after all the nests had been settled, but still two weeks before the first eggs were laid. Throughout the breeding season, the researchers visited each box every other day to record the number of eggs or nestlings (adolescent birds) and measure the health of the offspring using body condition index (BCI), a measure of mass and wing size.
The authors found their results supported the idea that anthropogenic noise exposure affects settlement patterns in addition to decreasing reproductive success. The birds tended to settle quieter nesting sites before the noisy ones. This alone might suggest the explanation that lower-quality individuals end up with the noisiest nests. However, in measuring the relative fitness of the mothers at each site, there was no significant difference on average between those at noisy sites and those at control sites. Nonetheless, birds that settled noisier sites laid fewer eggs, and their offspring had lower BCI on average than those that settled control sites.
These results suggest that the noise has a direct effect on the number and fitness of offspring. While the mechanism of this effect needs yet to be explored, the authors suggested that it could be the result of changes in stress physiology in response to the stressful sounds of traffic, decreased ability to detect predators, or decreased efficiency in finding food.
While more research is needed to understand the way in which traffic noise affects tree swallows as they observed and the effects later into the birds’ life, the authors have already begun to look at how their findings can help the conservation effort for these birds. Even adapting pavement to make less noise could have a profound effect in protecting them while they breed. In the meantime, rest assured that any birds you see on your morning commute are just as unhappy about the traffic as you are.
Injaian AS, Poon LY, Patricelli GL. 2018. Effects of experimental anthropogenic noise on avian settlement patterns and reproductive success. Behav Ecol. 29(5):1181-1189.
