Urbanization and its implications for avian aggression: A case study of urban black kites (Milvus migrans) along Sagami Bay in Japan
Dana Galbreath, Tomohiro Ichinose, Tomoyuki Furutani, Wanglin Yan, and Hiroyoshi Higuchi
Abstract: Urbanization has caused countless changes in the lives, behaviors, and community structures of wild animals. Habitat loss in urban areas has led to the proliferation of certain species over others; in the case of birds, insectivores, frugivores, and certain predators can be found in abundance in cities. These birds, however, occasionally show novel behaviors that can cause stress within human-wildlife interactions. The black kite, Milvus migrans, for example, has displayed a tendency to attack humans for their food in certain urban areas in Japan. In order to determine how habitat availability and land-use types affected these aggressive tendencies, field observations were combined with GIS analysis of five locations along Sagami Bay in Japan. These locations; Enoshima, Fujisawa; Kamakura Beach, Kamakura; Zushi Beach, Zushi; Oiso Beach, Oiso; and Iwa Beach, Manazuru; were assessed according to the amount of each land-use type present and the aggressive tendencies of each location’s black kite population. The aggression of each population, designated by the log of the Aggression Index (AI), was found to be significantly affected by the amount of forest area per black kite (β=-2.704, p<0.001), the amount of non-rice-paddy agricultural area per black kite (β=-9.284, p<0.001), and the season (spring β=0.384, p<0.001). Thus, aggression was higher amongst populations with less forested or agricultural area within their foraging zones, and aggression increased during spring, which is the breeding season.
Keywords: black kite; urbanization; green space; behavior; aggression; land-use types
Urbanization, which can be defined as a concentration of humans, their residences and industry, and their associated anthropogenic influences (Chase and Walsh, 2006), has had dramatic effects upon urban and natural ecosystems. Recent research projects often center upon the potential mitigation of harm done to animals via human developmental projects; due to these studies, urbanization has been widely recognized as one of the primary causes of species endangerment (Blair, 1996; Leider and Haddad, 2011; Walpole and Bowman, 2011), disturbance, and behavioral changes (Anderies et al., 2007; Walpole and Bowman, 2011; Blair, 1996; Chase and Walsh, 2006; Bonier et al., 2007; Tavernia and Reed, 2010). Although many urban animal species have been monitored in order to determine these reactions to human influences, birds are a particularly useful study animal when testing the effects of anthropogenic influences upon resident wildlife due to their sensitivity to habitat changes (Imai and Nakashizuka, 2010).
Studies have shown that changing habitats and land-use types cause bird community structures to change accordingly. As urban development progresses, species richness decreases while abundance increases (Ortega-Alvarez and Macgregor-Fors, 2009; Chase and Walsh, 2006; Imai and Nakashizuka, 2010; Blair, 1996). The species-specific life history traits, such as diet and nesting habits, of urban birds lead to certain species thriving in the cities while other species retreat to more rural areas. Consequently, biodiversity in cities is typically very low, leaving only a handful of urban exploiter species such as pigeons, sparrows, crows, and black kites to compete over abundant resources. Unfortunately, easy access to food has led to higher numbers of like-minded animals competing for the same resources, and abundance levels are often above the carrying capacity of the city, which in turn leads to increased intraspecific competition (Anderies et al., 2007). Predation pressures also change from natural to urban environments (Gering and Blair, 1999), leading to novel interspecific and intraspecific interactions; for example, some small-range raptors may inhabit cities with poor nesting sites simply because in those areas, they are now free from predation (Chase and Walsh, 2006).
Human disturbance upon ecosystems is also a major concern in urban wildlife conservation. Urban environments provide a range of anthropogenic stimuli that can be considered disturbing to birds; noise, physical presence of humans, and motor vehicles, for example, can cause stress in species that are less capable than others of tolerating such activity (Pease et al., 2005; Gonzales et al., 2005; Burton, 2007; Møller, 2008; Baudains and Lloyd, 2007). Since urban bird abundance is high, those birds have a greater tolerance for most human activities than their rural counterparts, although their reactions to human disturbance vary on both the individual level and the species level (Blumstein et al., 2005; Blumstein, 2006). However, tolerance has its limits, and human activities within cities can still negatively impact the birds that live there (Chase and Walsh, 2006; Bonier et al., 2007). Bird populations’ reactions to human disturbance and their tolerance of intensive human presence are closely related to the overall behavioral changes found in urban exploiters. According to various research projects, urban environments tend to select for bold and aggressive birds, most likely as a correlation to the extremely high amounts of anthropogenic stimuli (Scales et al., 2011; Møller, 2008; Evans et al., 2010; Evans, Boudreau, and Hyman, 2010; Searcy et al., 2006). Boldness in birds seems to be a heritable trait, which means that urban habitats may be selecting for increasingly bolder generations that lack fear of humans. Boldness also correlates to aggression, particularly conspecific or territorial aggression (Scales et al., 2011; Bell, 2005). As urban areas grow and new populations take root, these new populations tend towards high initial aggression that eventually lowers over time. Although high aggression is detrimental overall, aggressive birds enjoy higher fitness levels in new populations. Thus, newly developed areas and other urban areas have been subjected to increasing numbers of aggressive, bold birds. However, the influence of habitat availability upon aggression is largely unknown, and the influence of intensive human presence upon aggression is also unclear.
This research project focuses upon these issues in order to determine the effects of urbanization upon aggressiveness in black kites (Milvus migrans). Although black kites are found in numerous countries around the world and are in decline in Europe (Sergio et al., 2011), they have become a nuisance animal in Japan. Black kites found along Sagami Bay in the Kanto region of Japan have been known to attack humans bearing foodstuffs; these occurrences are so common in seaside cities such as Kamakura and Fujisawa that the presiding prefectural and/or city environmental municipal offices have posted signs warning visitors of the danger of black kites. When contacted, tourism centers and health clinics along Sagami Bay cited frequent injuries amongst beachgoers. Due to the aforementioned tendency of wild birds to consider human presence disturbing, this novel behavior of urban black kites is disconcerting. If urbanization has influenced black kite behavior to this degree, future city planning must take these effects into account in order to facilitate mitigation. Unfortunately, urban habitat fragmentation and destruction is a major problem in Japan (Matsushita, 2002), and urban habitat fragments must be carefully managed in order to preserve wildlife (Hedblom and Soderstrom, 2010; Fernandez-Juricic and Jokimaki, 2001; MacGregor-Fors, 2010; Khera et al., 2009).
Black kites were monitored in coastal areas in Japan in order to determine how habitat availability and human visitation affected a population’s overall aggressiveness. The two hypotheses that formed the basis of this study were: (H1) the amount of viable habitat in each population’s foraging zone is negatively correlated to the aggressiveness of black kites, and (H2) the number of human visitors (which is related to urbanization factors such as transportation, convenience, etc.) to a foraging zone is positively correlated to the aggressive tendencies of Black Kites.
The black kite, Milvus migrans, is a medium-sized (630-940g), migratory raptor (Forero et al., 1999; Sergio et al., 2011). The natural prey of the black kite tends to be aquatic (e.g., fish), which supports the tendency of black kites to roost and forage near large bodies of water (Forero et al., 2002); however, they are also known to be opportunistic predators that will take advantage of spatiotemporal fluctuations in prey availability (Koga and Shiraishi, 1994). These birds are well known for their behavioral flexibility with regards to resource availability (Sergio et al., 2011).
Five locations were chosen along Sagami Bay in order to provide a green space gradient: in order from lowest to highest percentages of foraging zone green space, these locations were Enoshima, Fujisawa; Kamakura Beach, Kamakura; Zushi Beach, Zushi; Oiso Beach, Oiso; and Iwa Beach, Manazuru (Fig. 1). For the sake of maximizing homogeneity amongst the different locations, each observation point was located upon a beach in each city or town regardless of whether or not black kites could be found elsewhere.
Fig. 1: Satellite imagery of the research locations: a.) Enoshima, Fujisawa, where research was conducted from the western shore; b.) Kamakura Beach, Kamakura; c.) Zushi Beach, Zushi; d.) Oiso Beach, Oiso, where research was conducted from the eastern shore; and e.) Iwa Beach, Manazuru. (Images ©Google Earth 2012)
Field observations were carried out at specified observation points at each research location, using Nikon Eagleview Zoom binoculars for black kite monitoring and identification. Observations were conducted for a period of six hours on each trial day from 10:00 am to 4:00 pm; this period was chosen due to the fact that the majority of aggressive behaviors and human visitors appeared during this time frame (determined during preliminary observations at each location). During each trial, the amount of intra- and interspecific attacks, the times at which they occurred, the number of humans present, the total number of black kites present, the weather conditions, and the number of aggressive birds (which engaged in aggressive activity at any point during the observation period) were recorded as per typical bird observation study methods (Koga and Shiraishi, 1994; Sergio et al., 2003; Pease et al., 2005; Forero et al., 1999; Baudains and Lloyd, 2007). Food was prohibited during observation hours in order to avoid drawing attention to the researcher.
Observations were recorded during three seasons: early fall (September to mid-October), spring (March through May), and summer (June through August). Winter was not included due to the migration tendencies of the black kites; the birds present during the winter were not the same populations as those present during the spring and the summer.
GIS Analysis of Green Space
In accordance with the goals stated above, the amount of green space, defined as any land-use type that could be considered potential habitat for the black kite or its prey, and the area of each land-use type within each black kite population’s foraging zone were required for analysis. The foraging zone of each population was determined using data from the study conducted by Sergio et al. (2003), in which black kites were found to forage up to one kilometer from their nests. As the nesting locations were not easily accessible in all research locations, the potential foraging zone of each observed population was centered upon the observation point and delineated by a buffer with a two-kilometer radius. Because each bird tends to forage up to one kilometer away from its nest, and because the observation points were within each population’s foraging zone, the nests in question could be up to one kilometer away from the observation point. Thus, the area with the maximum likelihood of foraging black kites included a maximum distance of two kilometers in all directions from the observation point.
Once the foraging zone of each test population had been determined, the green space percentage of each zone was determined using ArcMap 10 for Desktop (Windows version). The observation points were plotted on a vector map of Japan, and then buffers with two-kilometer radii were placed around each point. Land-use data from the ESRI standard pack for Japan was then used to determine the amount of green space within each foraging zone. Any land-use type that could potentially be considered habitat for the black kite or its prey was considered “green space”; thus, wetlands, beaches, grasslands, forests, and agricultural areas were included in the analysis. All land-use types were tabulated within the buffer zones, and the area of each type in square kilometers was determined.
Once the field trials and the GIS analysis had been completed, the data was compared in order to determine if any significant correlations could be distinguished.
Due to the nature of this research, several definitions were required. The definitions used are as follows:
Attack: Any intentional contact with or attempt to contact a human/their food or another bird.
Due to their tendency towards aerial acrobatics, an attack between black kites or between black kites and other birds was recorded if and only if the talons came into use. Attacks upon humans were recorded if the black kite had its talons out to grab food from the human’s hands or from the victim’s immediate physical proximity (within arm’s reach of the person being attacked). Once the food was knocked out of arm’s reach, no further attempts to retrieve it by black kites were considered attacks due to the lowered probability that they would come into physical contact with humans.
Aggressive: Engaging in attacks at any point during the observation period.
In order to avoid bias based solely on the number of attacks committed during the observation period, the number of individuals present who engaged in any aggressive activity were recorded and compared to the total number of individuals observed.
Aggression Index: ,
where n=number of behavior types, j=type of behavior, Bj=score for attack type, Nj=number of that attack type, Pa=number of aggressive birds, and T=total birds. Bj scores are: Attacks (Other Birds)=1, Attacks (Black Kites)=2, and Attacks (Humans)=3.
By weighting different aggressive behaviors according to the inherent level of aggression or risk-taking involved, populations that engaged in riskier behavior would have higher aggression index results than populations that engaged only in low-risk or low-aggression behaviors (Errard and Hefetz, 1997; Tanner et al., 2011). Each population as a whole was tested for its overall AI on each trial day.
The ESRI standard pack land-use data was entered into ArcMap 10, buffers were created around the observation points in order to simulate each black kite population’s foraging zone, and the land-use types were tabulated within each buffer in order to determine the percentage of green space within each foraging zone. Figure 2 shows the resulting map.
Fig. 2: A map showing the land-use types for Japan with the vector map for Kanagawa Prefecture. Foraging zone buffers are delineated by orange circles.
According to the analysis, the green space percentages of each foraging zone were, in order of smallest to largest: Enoshima=12.83%, Kamakura Beach=29.0%, Zushi Beach=36.15%, Oiso Beach=50.43%, and Iwa Beach=70.17%. The area of each land-use type at each location is shown in Table 1. Although the foraging zones for Kamakura Beach and Zushi Beach overlapped, the foraging zones did not overlap at the observation points (and so any birds viewed and recorded at one observation point would be unlikely to be found at or near the other observation point), and birds viewed in that overlapping region were not included in the observational data; consequently, duplicated aggression values are not present.
Table 1: The amount of each land-use type within each population’s foraging zone.
Multiple Regression Analysis and Univariate ANOVA
Many ecological aggression studies use ANOVA tests in order to determine how the tested variables affect the test species’ behavior (Castro and Santiago, 1998; Trubl et al., 2011; Gillies et al., 2006; Fokidis et al., 2011). For the purposes of this study, stepwise multiple regression analysis was first used to determine the shared and unique variability of the different land-use types and the number of humans present upon the aggression index data obtained. The analysis was performed using IBM SPSS Statistics 20. As each independent variable was found to be redundant, it was automatically removed in order to find the best-fit model with the fewest predictors for the response variable, AI (log).
Because the data obtained for the aggression index (AI) were shown to be non-normal and showed outliers, those values were log10 transformed. Due to the presence of zero values, the AI values were transformed using the following equation: AI (log)=log10(AI+AImin/2). This equation was recommended by Carsten Dormann, PhD (Faculty of Forest and Environmental Science, University of Freiburg, Germany). The resulting AI (log) values were found to be normally distributed and could be analyzed using SPSS multiple regression methods (Tabachnick and Fidell, 2007).
Once the stepwise multiple regression analysis was performed upon the data and the most important continuous predictor variables were determined, an ANOVA was conducted in order to create a model equation for the response of AI (log) to the predictors. The model equation took the form of:
Y' = βo + β1X1 + β2X2 + … + βkXk ,
where Y' is the predicted value of the response variable, βo is the value of Y' when all X’s are zero, and β1 through βk refer to the coefficient of each predictor variable, X1 through Xk. Seasonal effects were categorical and thus required a mixed-effects ANOVA in order to determine how the seasons, as well as the other predictor variables, affected AI (log).
The AI (log) values and the various independent variables were tested for correlations amongst each other and for collinearity. AI (log) and the various land-use types, with the exception of other agriculture, were almost all significantly correlated to each other; however, the collinearity diagnostics showed that no multicollinearity was evident once the factors had been reduced to the best models.
Amongst the various land-use types, only two were found to have the highest contribution to predicting the response variable, AI (log). These two values were forest area per black kite and other agricultural (i.e. non-rice paddy) area per black kite. In both the multiple regression and the ANOVA tests, the number of humans per black kite had no statistically significant effect upon the AI (log) values; consequently, H2 was rejected. According to the stepwise multiple regression output (Table 2), forest area per black kite and other agricultural area per black kite were both negatively correlated to AI (log) (p<0.001). Thus, the more forest or other agricultural area (consisting of any agricultural area excluding rice paddies, such as orchards, vegetables, etc.) that was present within the population’s foraging zone, the less aggressive the black kites from that population tended to be, as was implied by the now-supported H1. Each of the two variables had approximately equal effects on AI (log): unique variability for forest area per black kite was 0.346, while the unique variability of other agricultural area per black kite was 0.329, totaling an R2 of 0.675 (adjusted R2=0.660). According to this information, approximately two-thirds of the variability within the recorded AI (log) values can be accounted for by these two independent variables (R2 95% confidence limits from 0.531 to 0.819).
Table 2: Results of the stepwise multiple regression analysis depicting correlations between the significant variables, their unique variability, and their coefficients.
Seasonality, as a random effect, was added to the ANOVA along with the values for forest area per black kite and other agricultural area per black kite. Of the three seasons (fall, spring, and summer) during which observations were recorded, only spring was found to have a significant effect upon AI (log). Fall had no statistically significant effect, and summer was found to be redundant. Spring had a smaller overall effect upon AI (log) (partial η2=0.163) than forest area per black kite (partial η2=0.582) or other agricultural area per black kite (partial η2=0.530). While the two significant green land-use types both inhibited aggressive tendencies, aggressiveness rose in the spring. The best model to predict AI (log) was based upon the aforementioned equation and is displayed as follows:
AI(log) = βo + β1(
The coefficients determined by the ANOVA resulted in the following final model:
AI(log) = 1.466 – 2.208(
Fig. 3: Mean AI (log) and forest area per black kite values by location. The circles represent the AI (log) values, and the triangles represent the forest area per black kite values.
Fig. 4: Mean AI (log) and other agricultural area per black kite values by location. The circles represent the AI (log) values, and the triangles represent the other agricultural area per black kite values.
According to the above data and analysis, H1, which posited that habitat availability has an inverse relationship with black kite aggression, was supported while H2, which suggested that the number of human visitors in a foraging zone promote aggressive tendencies in black kites, was rejected. Black kites in the more rural locations showed a distinct lack of interest in human visitors, and occasional feeding from humans did not appear to induce those populations to consider them a viable food source. These results suggest that variable human presence does not have as much of a sustained effect on bird behavior as large-scale human impacts upon urbanized environments. These results may also have been affected by the circumstances of this study, in which the birds were required to encroach upon human space rather than vice versa, as was the case in most other research regarding physical human-bird interactions. The concept that habitat availability affects black kite behavior, however, was supported by the fact that two major green land-use types, forests and agriculture, significantly lowered the aggressive tendencies of resident black kite populations.
Several implications can be derived from the knowledge of how these land-use types affect black kite aggression. Black kites build nests in trees and have a preference for cliffs, heavily wooded areas with low human traffic, and proximity to large water bodies when doing so (Sergio et al., 2003); consequently, a lack of forested area in a foraging zone, which includes their nesting locations, results in a lack of nesting resources. Black kites that choose to nest in areas with little forested area and high human traffic, such as Enoshima and Kamakura, are forced into more intraspecific competition for these nesting habitats than other populations. They must also deal with more anthropogenic stimuli such as cars and continual physical human presence than the other populations—a situation which lends itself to a tendency towards bold behaviors (Pease et al., 2005; Scales et al., 2011; Bell, 2005). These combined factors may be the cause of the high amounts of intraspecific attacks found in each of these populations; even Manazuru, which had a large amount of total green space but little forested area per black kite, showed this tendency towards intraspecific aggression amongst some of its population’s members. Manazuru, however, had a high amount of other agricultural area, which diminished the effects of nesting habitat reduction upon its population’s aggression.
The amount of other agricultural area also heavily impacted the aggressive tendencies of each black kite population. These areas, which include orchards, farms, non-rice grain fields, vegetable fields, etc., are a major source of food for scavenging black kites (Shiraishi et al., 1990; Koga and Shiraishi, 1994; Blanco, 1994). A lack of the preferred foraging ground for black kites can also cause increased territoriality and aggression amongst conspecifics for the remaining food resources (Anderies et al., 2007; Shochat et al., 2004). As a result, the combination of a lack of the preferred foraging areas and nesting resources appears, as per the above analysis, to lead to an associated rise in the aggressive tendencies of black kite populations. Competition over foraging and nesting resources seems to lead to higher numbers of attacks upon humans, other black kites, and other bird species. The effect of seasonality upon aggression in birds, particularly during breeding season, has been well-documented in previous research (Hamao, 2011; Valeria et al., 2011).
Agricultural areas are important not only to black kites, but to other species of birds as well. In recent research, agricultural areas have been tested for their viability as habitats; for example, High Nature Value agricultural areas, which tend to have a large amount of native flora nearby and environmentally friendly farming practices, have been shown to positively affect bird communities (Doxa et al., 2012; Danhardt et al., 2010; Catry et al., 2012). Unfortunately, agriculture is declining in Japan due to its aging rural population and agricultural land abandonment or conversion to other land-use types (Matsushita, 2002; Matsuki, 2002). While remaining Japanese farmers, with the support of several governmental agencies, are attempting to promote environmentally friendly practices, the lack of young citizens who are interested in farming and the urbanization of rural areas are causing the continued decline and degradation of Japanese agriculture (Matsuki, 2002). Projects such as the Satoyama Initiative, however, are tackling this problem in the hopes of promoting and preserving ecologically friendly agriculture in Japan (Satoyama Initiative, 2012).
Despite the apparent importance of forests to the mitigation of black kite aggression, Japanese forestry laws have difficulty preserving urban forest patches. The various agencies that hold administrative powers over forest development lack coordination and often fail to communicate despite amendments to the National Forest Law that require such efforts during any transitional planning (Matsushita, 2002). As a result, a forest patch that is slated for development under one agency’s plan may continue being considered a forest by another agency throughout the planning period. If the forested area under consideration is smaller than 1 ha, or if the development involves any of the numerous exceptions listed in the Forest Law (including roads, railroads, broadcasting facilities, schools, etc.), permission is not required to develop that land (Matsushita, 2002). These issues create problems when attempting to preserve urban forestland and when trying to promote quality forests nationally. Preserving the amount of total forested area in Japan, as has been the primary goal of many forest conservation agencies (Matsushita, 2002), is inadequate in terms of providing sufficient ecological habitats for urban animals and for the mitigation of problem behaviors that arise with a loss of habitat.
In the face of the loss and degradation of Japanese forests, several mitigating factors have arisen. Several companies, local governments, and citizen-led groups have created programs dedicated to reforestation, forest management, and public education (Iwai, 2002). Animal conservation groups, such as the Japan Wildlife Conservation Society, not only conduct projects dedicated towards preserving biodiversity in Japan, but also teach citizens about the importance of wildlife (Japan Wildlife Conservation Society, 2012). Similarly, efforts based upon educating the public as to the potential of creating nuisance animals and inadvertently encouraging bothersome or dangerous behaviors amongst urban animals should be conducted. Although signs requesting that visitors refrain from feeding black kites are regularly posted in Kamakura and Enoshima, over the course of this study, several instances of visitors feeding black kites and subsequently being attacked by them were witnessed. Public forums addressing the issue of urban habitat degradation and loss and the adverse effects upon wildlife-human interactions should be conducted, especially in areas where these aggressive, dangerous behaviors are common.
Aside from introducing public forums, promoting environmental education, and increasing cooperation between forest management agencies, other methods can also be taken in order to increase the viability of urban habitats. A study by Johan Colding in 2007, for example, described how ecological land-use complementation (ELC) can promote biodiversity within cities. Because urban green spaces are highly fragmented, local habitat quality (and, consequently, its management) is of utmost importance to biodiversity protection. Although it is difficult to create more green space in large cities, Colding hypothesized that habitat enlargement can be completed through the use of different types of green patches; for example, domestic gardens, native tree stands, and ponds can all attract wildlife and act as habitats. Even wooded streets can help group together these green patches, effectively creating a larger habitat for wildlife (Colding, 2007; Fernandez-Juricic and Jokimaki, 2001). If at all possible, clustering of these green areas could promote habitat viability and a great amount of biodiversity, and the principles of High Nature Value evaluations imply that agriculture would benefit from the placement of native tree stands near urban farmlands.
Another recent innovation that increases biodiversity and urban green space is the “green roof.” Urban buildings create a green space by laying down a growing medium such as dirt over a waterproof membrane upon the roof and growing plants within them (Orbendorfer et al., 2007; Fernandez-Canero and Gonzales-Redondo, 2010). These green roofs not only act as an insulator, thereby saving energy, and as aesthetic space, but they also provide shelter, protection, and food to birds, insects, and small mammals. Green roofs with thick substrates can even host trees, thus increasing the potential for creating urban forested areas. By using green roofs, small parks, domestic gardens, and wooded streets, urban planners can effectively increase the available habitat in cities without sacrificing human comfort. Although not all roofs in cities are capable of supporting green roofs, and the initial cost of installing green roofs can be high (Orbendorfer et al., 2007; Fernandez-Canero and Gonzales-Redondo, 2010), the potential benefits towards urban ecosystems should be thoroughly investigated, particularly in light of the inherent difficulties of creating viable habitat space in cities (Henry and Frascaria-Lacoste, 2012; Francis and Lorimer, 2011; Francis, 2010).
By combining these green roof and ELC tactics with the aforementioned changes in administration and education, the negative effects of urbanization upon bird aggressiveness can potentially be mitigated. Previous research has shown that careful city planning, architectural and habitat design, and wildlife management, nuisance behaviors that have developed in urban species over decades of acclimatization can be alleviated (Belant, 1997), resulting in wildlife that is less harmful to itself and to humans. Although black kites were chosen as the focus animal of this study, various other urban birds, such as crows, have become nuisances in countless cities worldwide, and the effects of green space reduction seen here could have implications for other urban animals as well.
When birds become pests or begin displaying troublesome behavior, as the black kite has in Japan, management should take these behavioral changes into account and investigate the potential underlying causes. The injuries suffered by victims of black kite aggression along Sagami Bay serve as an example of potential damages caused by behavioral shifts in wildlife due to urbanization; mitigation or prevention of these behaviors should be a major concern to developers. Through this study, urban green space reduction has been shown to have a significant impact upon black kite aggression. Forests and agricultural areas are particularly important for mitigating aggressive tendencies, and steps should be taken to properly conserve and manage these land-use types. Although the amount of habitat space alone cannot account for all of the behavioral differences witnessed, nesting habitat and preferred foraging habitats are clearly of some importance to black kites, as evidenced from each population’s differences in behavior. Thus, in order to prevent further novel, harmful behaviors from developing amongst urban animals, cities should attempt to determine which types of green space are of vital importance to resident wildlife and consider them accordingly when planning future urban landscapes.
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