Why COVID-19 Made Sparrows Sexier

Birds adapted speedily to reduced traffic noise during the lockdown.

Posted Oct 01, 2020

Adaptation is much faster than the glacial pace of gene selection. Animals must exploit current opportunities to survive, to compete with rivals, or to evade predators. Researchers report that male birds altered their songs during the quiet of the pandemic to wow females.

The Time Scale of Evolution

The time scale of genetic adaptation is painfully slow with mutations occurring only about once per 100,000 years.

With behavioral adaptation, we do not need so long to react to change. Evolution operates at three different time scales as developed in my new book, Evolution in the Here and Now. First, there is gene selection. Second, genes are expressed differently depending on early experiences (epigenetics). Third, there is learning, individual and social.

Moose That Do Not Fear Wolves

Moose are not born afraid of wolves—as many adaptationists had assumed. They acquire adaptive fear from observing their mother's response to various species (1). By the same token, the food an individual chooses to eat is largely a function of the foods preferred by their mothers.

Behavioral adaptation is a highly flexible process with many adaptations occurring within the lifetime of the individual.

This brings us to California's sparrows and their responses to the quieter hum of traffic with the COVID-19 slowdown. Many people reported that birds seemed to be singing more loudly. In reality, birds were singing at a lower volume, but with better delivery, as described in a recent Science paper.

Male songbirds sing both to repel rivals from their territory and to attract females. The song of the male conveys a great deal of information to females about his health and physical condition. Birds may sing in local dialects and females prefer males whose songs resemble those of their own fathers. This indicates that the birds are of local origin and therefore familiar with the local ecology (2). The virtuosity of individual singers is affected by what young males hear from accomplished singers.

Large populations of Dufour's larks have more accomplished singers (3). When these populations were splintered in Europe by plowing up land and leaving small islands of meadow, the complexity of their songs suffered because local populations contained fewer virtuoso singers.

If cultivation had adverse effects on the song attractiveness of Dufour's larks in Europe, the pandemic had the opposite effect on white-crowned sparrow songs by reducing the ambient traffic noise over which they had to sing with noise levels reverting to those characteristic of three-quarters of a century ago.

Sexier Sparrow Songs

In some species, males develop highly ornate songs that are attractive to females. The quality of a male's song conveys a great deal of information about the health and vitality of the singer.

Ornithologists have long known that birds adjust their song to compensate for background conditions. For example, birds living in woodlands must sing louder to defeat the muffling effect of trees and leaves.

Similarly, birds reproducing in noisy areas with a lot of traffic noise must adjust their song so that its active space remains big enough to attract females and repel rivals.

With the reduction in traffic noises, the white-crowned sparrows in California could reduce the effort of projecting the song over an adequate distance. Song volume was reduced by a third. With reduced volume, singers could pay more attention to the quality of the song and were able to hit lower notes.

By analogy, human songs suffer if the vocalist struggles for volume and seems to shout, or scream, rather than to sing. With a relaxed effort to project the voice, a person may sing more sweetly and the same is true of songbirds.

Ironically, human observers reported that birds seemed to be singing more loudly. In reality, their volume was lower but the song quality was better so that it could be heard more distinctly against a relatively quiet background. Indeed, the song could be heard from twice as far away.

Just as the human audience appreciated the clarity of sparrow song, the intended audience, the females, likely found the songs more attractive also.

Conclusions

The flexibility of sparrow songs is just one more example of adaptive changes occurring in the life of the individual. Such flexibility has been downplayed evolutionary studies as scientists focused exclusively on genetic determination.

While most genetic changes are glacially slow, we now know that gene expression can be altered by environmental conditions with important implications for a variety of human problems from child abuse being perpetuated in families to vulnerability to obesity, depression and suicide (4).

The bottom line is that behavioral adaptation is a highly flexible process for humans as well as for sparrows. Whether it is a good song or defensive responses to a difficult early environment, many adaptations occur within the lifetime of an individual.

References

1 Berger, J., Swenson, J. E., & Persson, I. L. (2001). Recolonizing carnivores and naïve prey: Conservation lessons from Pleistocene extinctions. Science, 291, 1036-1039.

2 Grant, P. R., and Grant, B. R. (1997). Hybridization, sexual imprinting, and mate choice, American Naturalist, 149, 1-28.

3 Laiola, P., and Tella, J. L. (2007). Erosion of animal cultures in fragmented landscapes. Frontiers in Ecology and Environment,5, 68-72.

4 Franklin, T. B., and Mansuy, I. M. (2010). Epigenetic inheritance in mammals: Evidence for the impact of adverse environmental events. Neurobiology of Disease, 39, 61-65.