House sparrows, also known as Passer domesticus, are well-adapted to living in cities and other urban areas. So, it’s surprising when their population has decreased over the past couple of decades. A study published in Frontiers in Ecology and Evolution, an open access journal, indicates that the sparrows living in cities show signs of stress associated with the toxic impacts of an unhealthy diet and air pollution.
According to Amparo Herrera-Dueñas of the Complutense University of Madrid, these noisy and gregarious birds that live in cities are more vulnerable to the impact of air pollution during the breeding season. This is because they are torn between allocating the limited resources to fighting toxic environment or laying health eggs, and both cannot be supported by the bird’s poor diet.
Stress measured in rural, suburban, and urban areas
The researchers took blood samples from hundreds of sparrows from suburban, rural, and urban areas in the Iberian Peninsula. These samples were analysed for oxidative stress. This is a measure of how much, an environmental factor such as air pollution, might be weakening the sparrows’ natural defence.
Scientists warn that an unhealthy diet or air pollutants can catalyse the process of formation of free radicals. These free radicals tend to accelerate the ageing process of cells. In humans, this process has been linked to various respiratory diseases, including asthma, cancer, and cardiovascular disorders.
Air pollution is one of the primary factors in the catastrophic demise of house sparrows populations
The study findings revealed that house sparrows in urban areas suffered a higher level of free radical damage compared to their country cousins. Besides, further analysis showed that the city-dwelling birds were attempting to fight these damaging molecules. However, compared to the rural birds, their natural defence had somehow lower capacity to do so.
Amparo Herrera-Dueñas asserts that urban habitats are polluted with toxic chemicals such as poisonous gases, heavy metal particles, and other substances. These pollutants are the primary causes of oxidative damage. They also cause both short-term and long-term stress imposing a deleterious impact on sparrows and other wild bird populations. Sparrows from urban populations have a higher telomere shortening rate and seemingly a higher expression of genes associated with immune, oxidative stress, and inflammation responses than those living in the rural regions.
The sparrow fad
The house sparrow is native to Eurasia. However, beginning in the mid-1800s, these birds spread around the world. Thanks to international releases by people, sparrows are now found in all continents except Antarctica. Studies show that Sparrow is the most widespread wild bird globally.
As with many elements of conservation history, many details regarding sparrow introductions are not well-documented. The first introductions to the American continent were to New York City in 1851. The first eight pairs released didn’t fare as expected. But this set of a wave of sparrow introductions throughout the United States and the entire continent.
Some sources associated the ‘sparrow fad’ with people breeding these birds, and others catching and releasing them into new regions and continents. Nest boxes were set up in most cities to help increase the sparrow populations. While ornithologists raised concerns over the benefits of sparrows, their arguments proved very futile against the growing number of sparrow enthusiasts who were releasing cages full of these birds.
The primary reason for the increased desire to increase sparrow populations was for pest control. For example, the introduction of these birds to Philadelphia in 1868 was apparently an attempt to control inchworms. Unfortunately, the cure proved worse than the disease as these birds can thrive on a variety of foods such as spilled gains and garbage.
Besides, the house sparrow is an aggressive bird. Often, it nests in cavities. These birds pushed out some native species such as the Eastern bluebirds. Public sentiments changed quickly against sparrows and by the late 1880s, about three decades after the first sparrow was introduced, different United States cities paid bounties for these birds. However, by then, sparrows had firmly established and spread quickly.
A recent study indicated that these birds underwent genetic changes such as modified skull development and a specific gene that helps in the creation of enzyme amylase that helps in the breakdown of starch. According to the researchers, these changes helped sparrows adapt to varying human settlements dominated by livestock, agricultural fields, and other forms of agriculture.
There are still sparrows around
Well, a recent report reveals that there are still more than 540 million house sparrows flying around. However, due to the impact of air pollution and other human activities, the population of these birds has been declining over the last few decades.
All ecosystems change, but our environment tends to change rapidly. If we want to protect biodiversity in our cities and on a planet that will have 9.8 billion people by 2050, we should think about our actions and their impact on wildlife.