How Urbanization is Causing a Decline in House Sparrow Population

Urbanization is a shift from rural to urban areas by forming towns or cities that are centralized by more jobs and people living. Urbanization has shown to have real impacts on biodiversity. This week I am exploring a recent study that exposed the decline of House Sparrows in European cities. The questions I am seeking to answer are: 1) Why are we observing a decline in sparrows in Europe over the past decade? And 2) What urban habitat characteristics are unfavorable for house sparrows?

Urbanization Problem

In 2015, 54% of the human population was urban. It is projected that by 2050, 66% of the world population will be in urban areas. This means more development in rural areas leading to greater habitat fragmentation. Urbanization causes air, noise, and light pollution. It causes a shift in what food sources are abundant and available. A scarcity in recourses can lead to competition pressures higher than previous populations have endured. Lastly, among many more, urbanization causes a change in predators. Like I mentioned last week, domestic cats are a problem for many garden birds. Having more people moving to what once was a rural area, means there will also be more domestic cats.

Some History

As the horse-drawn transport was replaced by the automobile and the cleanliness of streets has improved, food resources became more scarce for House Sparrows. Thus leading to the first observed decline in sparrow populations in the beginning half of the twentieth century.

The terminology used to describe and classify species who are affected by urbanization are: “avoiders”, “adaptors”, and “exploiters”. My first guess was that House Sparrows would be considered urban adaptors, but this study described them as urban exploiters. Urban exploiters are dependent on human resources and reach their highest abundances in urbanized areas.

So then why are we finding a decline in the population within the past decade? Maybe modern construction trends and better sanitation are responsible for reduced nesting locations and food resources.  While there are many different hypotheses about why populations are declining, I think it is most likely a combination of urban pressures that are collectively leading to the decline seen in this study.

About the Study

This research used a 15-year bird census that covered urban diversity in Paris, France. This census documented an 89% decline of species in Paris within 15 years. That is an alarming change for only a decade and a half. It is important to note that this decline also appeared globally and widespread throughout Europe. France, Belgium, Germany, and Spain all observed declining sparrow numbers. The United-Kingdom however, displayed stabilization of their small sparrow population. This suggested to the researchers that there are specific environmental changes that result in the decline observed in affected areas. This study’s focus was on exploring habitat determinants of sparrows in Paris. 

One explanation for the decline was increased predation by Sparrowhawks (House Sparrow’s predator). Over the observed 15 years, Sparrowhawk numbers increased. This correlation aligned with other studies that claimed the same phenomenon had occurred. Increased predation is an obvious cause, but why were there suddenly more Sparrowhawks in urban areas? The study briefly mentioned that sparrows adapted to urban living before Sparrowhawks did. Leaving them with a predator-free behavior that would lead to a problem upon the arrival of Sparrowhawks.

A decline in food availability was described as the most probable cause. Invertebrates are crucial for sparrow reproduction and development. Fewer grassy-green environments, where favorable prey resides, are a result of urbanization. Without the proper nutrients for development, fewer sparrows could survive to adulthood.

Contrary to what the researchers expected, the census showed that areas with newer buildings had more sparrows than those with older buildings. This suggested that House Sparrows are not lacking nesting sites in urban areas. I wish I had more evidence as to why this was the case.

Global data was used to monitor the environmental changes Paris experienced year to year. These changes included weather fluctuations (climate change) and pollution. However, there was no statistical influence of annual variations in weather conditions and pollution.  

There are other factors this study did not include in their research. No one answer was given as to why a tremendous decline was observed.  House Sparrow populations varied in areas but overall declined at all sites independently of habitat attributes. This brings me back to my claim that House Sparrow populations are declining because of a combination of pressures.

Why It Matters

It is important to utilize long-term monitoring programs to better understand what effects we can have on the environment. History can provide examples of what can happen if we continue to disrupt nature without sustainable solutions. Knowledge is only part of it, engaging the public in ecological issues is also key to leading us towards conservation initiatives and solutions. This of course is why I have a blog! I hope that I can learn to translate relevant scientific research to my readers so they too can have the knowledge and power to make a change towards sustainability. 

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Reference

Mohring, B., Henry, P., Jiguet, F. et al. Investigating temporal and spatial correlates of the sharp decline of an urban exploiter bird in a large European city. Urban Ecosyst (2020). https://doi.org/10.1007/s11252-020-01052-9