A Tern for the Worst: Effects of Urbanization on Migratory Bird Populations in Small Cities
Alexandra Lipinski, Salisbury University
Abstract: Whether directly through infrastructure issues or indirectly through climate change, the human push for urbanization has fractured a community not just of people, but of birds. Specifically, the ever-expanding effects of urbanization have heavily affected migratory birds in cities of all sizes. Migratory birds play vital roles in the ecosystems they reside in, making their rising mortality rates and lower quality of life a pressing issue. Recent scholarly research has shown that issues such as landscape changes are impacting both the survival and the behavior of migratory birds and the communities that they inhabit. However, most peer-reviewed research focuses on large cities; little research focuses on the effects of urbanization in small cities and how these effects apply to migratory bird populations. After analyzing the highest pressing issues for migratory birds, these issues can be combined with large city bird conservation practices to address small city migratory bird concerns. Specifically, exploring the conflict between migratory bird concerns and nonmigratory bird concerns leads to a solution based around addressing structural issues and incorporating human involvement. Furthermore, the extent of this issue calls for more research to further understand the effects of urbanization on migratory birds in small cities. Keywords: urbanization, small cities, migratory birds, city infrastructure
On the second Saturday in May and October each year, World Migratory Bird Day (WMBD) is celebrated by organizations and individuals alike. Many of these celebrations are organized by cities, specifically those designated as Bird Cities. Bird Cities are cities that successfully implement a set of actions designed to promote the conservation of birds (American Bird Conservancy, n.d.). To qualify as a Bird City, each city must officially recognize World Migratory Bird Day and celebrate it each year. According to worldmigratorybirdday.org, “World Migratory Bird Day… is an annual awareness-raising campaign highlighting the need for the conservation of migratory birds and their habitats”. The inclusion of celebrating WMBD in the Bird City qualifications highlights the importance of migratory birds and the issues they face. Migratory birds play important roles in their ecosystems such as being large components of the food chain as well as being economically beneficial to the areas they inhabit (e.g. tourism), making their survival a pressing issue (Egwumah & Inah, 2015).
Urbanization has negative effects on bird populations in cities all around the world. This is especially true for migratory birds. When birds migrate through heavily populated areas, they are likely to face issues that impact their migration. Many passing birds end up injured or dead because of urban infrastructure issues; bird collisions with glass structures such as windows and glass bus shelters can cause a significant amount of bird fatalities for migrating birds (Barton et al., 2017; Bracey et al., 2016). Bird behavior is also affected by urbanization as it limits the migration of some migratory birds (Bonnet-Lebrun et al., 2020). There have been many studies such as Bonnet-Lebrun’s (2020) study that focus on the effects of urbanization on bird populations as well as studies regarding urbanization and migratory birds. However, there are few studies that look at the effects of urbanization on migratory birds in small cities. Similarly, there are few studies that provide solutions to these issues that often affect migratory birds just as much as in large cities. Regardless of budget concerns, small cities should propose a set of solutions that are aimed to reduce threats that harm migratory bird populations. Furthermore, more scholarly research should be conducted to better understand these issues and find new solutions.
While there are many issues in urban areas that affect bird populations, some of these issues are of greater concern towards migratory bird populations either through the number of migratory birds they affect or how they disproportionately affect migratory birds as compared to all bird species. These concerns can be broken down into two main categories: physical concerns and behavioral concerns. Physical concerns as defined by this paper encompass concerns towards migratory bird populations in terms of their physical wellbeing, such as bird-window collisions and domestic cat mortality rates. Behavioral concerns as defined by this paper encompass concerns regarding behavioral changes in migratory birds such as changes in migration patterns. This section highlights some of the prominent concerns in migratory bird conservation efforts.
Physical structures and structure collisions are one of the most prominent issues that migratory birds face, specifically building collisions. Loss et al. (2014) stated that one of the top sources of bird collision mortalities from human causes is building collisions. They found that roughly 365-988 million birds are killed each year due to building collisions, and the largest group of vulnerable species was migratory birds. One of the most prominent causes of building collisions are windows. The amount of glass on the building is positively correlated with bird mortality rates due to window collisions (Loss et al., 2014). While bird-window collisions affect all birds, it particularly affects migratory birds. The largest group of vulnerable species are long distance migrants, likely because their use of different habitat types and encounters with more buildings over their long migrations increases their susceptibility to collisions (Loss et al., 2014). In a study conducted by Bracey et al. (2016), migratory species were killed at higher rates than year-round species. This occurs since migratory bird species are more likely to be attracted to areas with heavier tree cover and water during their migration, and thus are more likely to collide with windows around these areas (Cusa et al., 2015, as cited in Bracey et al., 2016).
While glass and windows on buildings cause many collisions on their own, there are additional factors that affect these collision rates. One of these factors, as introduced above, is the vegetation surrounding buildings. Brown et al. (2020) found that bird collisions were forty times more likely to occur when pear trees were near buildings in winter. This occurs due to the reflections of the vegetation on the glass. A similar principle applies to other glass structures. Glass bus shelters, another type of glass structure often found in cities, face similar issues as window collisions. Vegetation features such as the area of lawn around the glass bus shelters increase rates of bird collisions (Barton et al., 2017). Outside of vegetation, surrounding light sources also increase the rate of bird-window collisions. La Sorte et al. (2017) found that artificial light at night, “may decrease the overall efficiency of migration by increasing time and energy requirements and by increasing overall risk”. Migratory birds are attracted to light in large buildings, specifically during nocturnal migration, which increases their risk for mortality in numerous building types (Evans Ogden, 1996, as cited in Loss et al., 2014).
Although collisions are a major part of increased urban bird mortality rates, there are other physical factors that impact birds’ survival rates. Other physical structures such as communication towers are responsible for the deaths of millions of migratory birds. A study conducted by Longcore et al. (2012) estimates that 6.8 million birds are killed each year by communications towers in the United States and Canada. Most of the birds killed at these towers are neotropical migrants (Longcore et al., 2012). Outside of physical structures, physical factors such as domestic cats also have large impacts on bird mortality rates. Free-ranging domestic cats are estimated to be the single biggest cause of bird mortalities, even more than structure collisions (Loss et al., 2013, as cited in Loss et al., 2014). These cats are estimated to kill between 1.3-4.0 billion birds annually in the United States (Loss et al., 2013).
While many of the physical concerns described above have major impacts on migratory bird populations, there are also many behavioral concerns present in migratory bird communities. One of these concerns is the overall effect that urbanization has on the behavior and composition of migratory bird populations. Urbanization has negative effects on the overall bird diversity within cities. Schneiberg et al. (2020) found that the overall bird richness (the number of bird species in an area) decreased in urban areas. They also found that there were very few specialist species and that many of the species observed were generalists rather than specialists. Urbanization causes these communities to change and the behaviors of each bird changes, as shown by the increase of generalized species. The urban bird communities become homogenized (composed of many of the same species rather than a variety of different species). This homogenization then negatively affects the environment around them through the further homogenization of urban networks, making these communities and networks less resilient to changes within their environment (Schneiberg et al., 2020). It is representative of the ecosystem outside of bird communities; the increase of generalist species over specialists shows that there is little variety within the resources offered in the ecosystem (Schneiberg et al., 2020).
While urbanization has negative effects on the composition and behavior of all types of birds, there are many concerns with how it specifically affects migratory birds. Bonnet-Lebrun et al. (2020) found that their results “supported the hypothesis that urbanization…. affects the propensity of individual birds to migrate”; some birds were more likely to stay in their winter areas during breeding season and others were more likely to stay in their breeding areas during winter seasons. Birds that have set migration patterns are breaking these patterns because of the effects of urbanization. Bonnet-Lebrun et al. (2020) also found that their analysis provided more evidence that urbanization is the cause of some migratory birds’ lack of migration. As upwards of one in five of the world’s bird species migrates each year (Kirby et al., 2008, as cited in Bonnet-Lebrun et al., 2020), this concern is important both to migratory bird populations as well as all bird populations.
Migratory Bird Concerns in Small Cities
Migratory birds often face issues to a higher degree than other bird species. As determined earlier, migratory birds are often the largest group of vulnerable bird species when it comes to the groups of birds most heavily affected by urbanization. Cabrera-Cruz et al. (2018) state that artificial light at night (ALAN) produces light pollution within the ranges of all nocturnally migrating bird species. The risks posed to migratory birds makes them a group of high concern. With the issues mentioned earlier, it is natural to believe that previously mentioned issues such as building collisions and light pollution should be the biggest concerns for migratory bird conservation within small cities since these are issues that heavily affect migratory birds. However, there are other issues that are of high concern. Migratory bird issues do not end with migratory bird specific issues; rather, issues that apply to all birds also affect migratory species.
While these issues are a problem for migratory birds, they often contradict each other. While limiting building collisions is a major aspect of migratory bird conservation, creating green spaces within cities is also very important for urban bird conservation. These issues are major factors in both migratory and nonmigratory bird conservation, yet they often influence each other in negative ways. Migratory birds are likely to be attracted to areas with more water and tree cover which increases their rate of collisions with glass structures around those areas (Cusa et al., 2015, as cited in Bracey et al., 2016). When green spaces are implemented in cities, they may unintentionally cause a higher rate of migratory bird mortality than before. This highlights a major conflict: fixing issues such as a lack of suitable green space creates a rise in collisions in migratory bird species (and all bird species), but not providing more habitats for birds in cities greatly reduces the quality of life and survival rates of all birds. This is applicable for smaller cities since buildings in small cities are often more spread out than in large cities (Cusa et al., 2015, as cited in Bracey et al., 2016).
To prevent unintended effects from addressing these concerns, it is necessary to address both the chosen concern as well as the unintended consequences it may produce; migratory bird specific concerns and general bird concerns should be addressed together. However, issues with budgets often prevent this from occurring. While most cities struggle with funding for various projects, small cities are more likely to have limited budgets for projects such as migratory bird conservation efforts. Small cities, like larger cities, also face issues when it comes to dealing with city stakeholders. There is a larger variety of stakeholders present in urban environments than agricultural areas, making it easy to not consider some stakeholders such as groups involved with city development (Snep et al., 2015). It is vital to create a strategy that allows concerns to be addressed in a budget-friendly manner while incorporating city stakeholders into the process.
Addressing Issues and Concerns
Due to the prevalence of budget concerns in migratory bird conservation efforts, it is important to begin addressing these issues by identifying the largest areas of concern and starting work from there. This includes concerns such as the conflict of building collisions and city green spaces, one of the biggest holes in small city migratory bird conservation. While budget issues are a factor in what strategies can be implemented, it is a common misconception that migratory bird conservation requires a large amount of dedicated funding. There are many strategies that can be implemented that directly address migratory bird issues, but there are also many strategies that indirectly help migratory birds. Many actions taken to protect migratory birds also benefit humans such as implementing city green spaces; people living in cities often prefer living near green spaces and wildlife while having more green spaces can in turn provide more vegetation and habitat for migratory birds (Snep et al., 2015). Similarly, addressing concerns of other wildlife in cities may also benefit migratory birds residing in these cities. Migratory bird conservation is already being achieved through other methods and can consistently be addressed in the future in this way. Migratory bird conservation often serves a purpose for more than just birds.
Regardless of this, there are still concerns with budget. While some issues can be addressed through non-migratory-specific methods, some such as addressing glass collisions cannot be accomplished in this manner. This means that some funds will still have to be allocated towards migratory bird conservation. However, the importance of this issue gives reason as to why addressing this issue is vital and can still be done in a budget-friendly way. As well as focusing on the most pressing concerns first when beginning conservation efforts, starting with small areas within those concerns and increasing from there is a viable strategy. Loss et al. (2014) believe that starting with a smaller number of buildings with a higher mortality rate per building will reduce the overall mortality. Specifically, starting with some high-rises and low-rises can decrease the overall mortality rate over time. Finding a starting point is only one part of creating a solution; it is also important to determine an approach for accomplishing these goals.
Taking Flight into a New Solution
There are multiple factors that influence the creation of a new solution for migratory bird protections in small cities. To begin creating this solution, it is important to first determine what to implement, specifically in the way of physical structures. This includes accounting for the issues that migratory birds face in small cities as well as the conflict between habitat fragmentation and building collisions. Creating a new solution does not end there; rather, it is just the beginning. It is important to determine a method to approach putting these strategies in place for these goals to be accomplished. In the case of migratory bird protections in small cities, the impact of human involvement is the highlight of approaching this issue.
Implementing New Strategies
As supported by Loss et al. (2014), one of the top sources of direct human-caused mortality in birds is building collisions. The easiest way to address this is by limiting the amount of glass (one of the top causes of building collisions) used in city structures. Building windows with less glass surface areas is likely the most effective way to lower migratory bird collision rates (Kahle et al., 2016, as cited in Barton et al., 2017). While limiting the surface area of glass used in buildings is an effective strategy, it is not possible to remove all glass. Since this is not possible, it is important to find additional strategies to mitigate bird mortality rates. A study conducted by Brown et al. (2020) found that bird-friendly windows such as fritted windows (a form of patterned windows) lowered the risk of bird-window collisions. While not all glass can be removed from buildings, mitigation efforts such as changes in glass types can lower the amount of bird collisions.
Determining solutions for other prominent migratory bird issues such as habitat fragmentation and vegetation composition is vital to migratory bird conservation. Increasing the amount of green space within small cities can accomplish this goal. One method of doing this is through green roofs. Partridge and Clark (2018) found that installing green roofs allows for increased green space in urban areas and can be very effective for conserving wildlife. These green roofs also are likely to increase the quality of habitats rather than regular roofs which addresses the issue of vegetation composition in cities. They can also increase the overall connectivity between various habitats within cities (Braaker et al., 2014, as cited in Partridge & Clark, 2018; Braaker et al., 2017, as cited in Partridge & Clark, 2018). In addition, these green roofs can host additional wildlife besides birds and create useful wildlife habitats within city areas (Wang et al., 2017, as cited in Partridge & Clark, 2018).
After determining methods to fix migratory bird issues, the next step is to determine an effective approach to do this. With over 50% of the global population living within or around cities, many people will only be able to experience wildlife within the confines of their cities (United Nations, 2007, as cited in Snep et al., 2016; Snep et al., 2016). With the impact that birds have on humans, it is important for humans to protect birds. Regardless of their impact on humans, people are negatively affecting the migratory birds’ life quality. Bonnet-Lebrun et al. (2020) found that human activities appear to change the environment at rates similar to much longer-term environmental processes. The majority of cities and countries do not have applicable urban bird conservation programs, let alone those that are migratory-bird-specific (Fergus et al., 2013, as cited in Snep et al., 2016). Human activities are the reason for many migratory bird problems, and these problems can only be fixed by humans. To successfully address many migratory bird issues, conservation efforts should involve as many groups of humans as possible.
As determined by Snep et al. (2016), urban environments hold a larger number of stakeholders who participate in conservation efforts than rural areas. It is more complex to create small city conservation strategies than rural areas, and many of the applicable stakeholders are often disregarded and forgotten during the planning process. As local governments can only accomplish so much on their own, it is important to define additional impactful groups. Snep et al. (2016) determined six main stakeholder groups, “urban planners, urban designers and architects, and landscape architects… urban developers and engineers… homeowners and tenants… companies and industries… landscaping and gardening firms… education and communication staff” to incorporate into the conservation process. These groups reach the largest range of topics, but the broad range of topics that they cover means that they are likely to be able to account for the conflicts in urban issues.
Even though each stakeholder group reaches a large range of people, they fail to mention one major group: the individual. Individual people have a large role in migratory bird conservation, and education allows them to be more involved than previously thought. While there is a potential for individual human involvement, the current lack in involvement highlights a need for education on migratory bird issues (Miller, 2005, as cited in Snep et al., 2016). As well as a lack of education, many migratory bird conservation practices may function as blanket statements and not address individuals. Tailoring information to an individual is the most successful way to spread information (Goddard et al., 2013, as cited in Snep et al., 2016).
Without change, migratory birds will continue to face issues in small cities. Physical and behavioral issues pose threats to both the survival and overall well-being of migratory birds. Addressing conflicts between migratory and nonmigratory bird issues while involving humans is vital not just to small cities, but to the greater world that small cities are a fraction of. While combining current research can provide a better look at potential efforts to mitigate the effects of urbanization on migratory birds in small cities, only so much can be accomplished without an increase in research on this subject. To best address this issue, migratory birds in small cities should be studied more in-depth to fully understand the extent of the situation as well as to analyze the complex interrelationship between numerous avian concerns. Furthermore, in order to reach all aspects of migratory bird conservation, additional research on solutions to these issues is required to best solve the numerous issues migratory birds face. Otherwise, migratory birds will continue to struggle finding their footing in an increasingly urbanized world.
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