<<Taikichiro Mori Memorial Research Fund>>

Urban Green Space Reduction and its Connection to Black Kite (Milvus migrans) Behavior

Dana Galbreath, Keio Gijuku University

 

Introduction

With the dramatic rise in urban sprawl worldwide, humans and wild animals have been forced into increased contact. Certain species of animals known as urban exploiters (pigeons, crows, house sparrows, Black Kites, etc.) have flourished under these new, highly urbanized environments; accordingly, such an extreme change in their environment has been accompanied by adaptive changes in the remaining populations. Mild, timid birds that fear human presence are pushed out, leaving only the risk-taking, aggressive, bold individuals behind. The change in environment does not merely extend to the gradual enveloping and development of rural areas in proximity to cities, however: within the cities themselves, remaining green space is declining rapidly. In the Kanto region particularly, small urban parks and forested areas seem to have little protection from development, and this trend is evoking further novel reactions from urban animals. Attacks from Black Kites, an opportunistic feeder (i.e. scavenger), upon humans have become a noticeable problem, and reports of injuries from Black Kite attacks can be found at several Sagami Bay seaside town tourism centers. When considering typical bird behavior, in which human presence and contact is normally found to be highly disturbing, these attacks become particularly worrisome. The decline of available green space and the associated hiding, nesting, and foraging grounds in these cities may have caused a change in bird community structure that has resulted in only the most risk-taking, most aggressive, and boldest individuals remaining behind. In this study, I hoped to determine the connection between green space and Black Kite aggressiveness in Kanto seaside cities. To do so, I observed Black Kite behavior in a series of locations that served as a gradient of urban green space.

 

Goals

 

1.)    To determine how landscape-usage affects bird behavior, with special focus on aggressive tendencies, risk-taking tendencies, and human tolerance.

 

2.)    To determine how the vegetation coverage in an urban landscape lends itself to buffering these behavioral changes.

 

Methods

Research locations thus far, presented as an urbanization gradient (lowest to highest green space percentages): Enoshima (Fujisawa City), Kamakura Beach (Kamakura City), Zushi Beach (Zushi City), and Iwa Beach/Marina (Manazuru Town).

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Figure 1: Satellite images of (a)Enoshima, Fujisawa; (b)Kamakura Beach, Kamakura; (c)Zushi Beach, Zushi; and (d)Iwa Beach, Manazuru. (Google Images 2011)

 

Fieldwork

·         Field observations of foraging Black Kites are conducted in six-hour intervals on each testing day at the above foraging zones along Sagami Bay.

·         The number of inter- and intraspecific attacks, the number of humans present, the weather conditions, and the percentage of aggressive birds (engaged in aggressive activity) are recorded.

 

GIS Analysis

·         The green space percentages of each location were determined using the ESRI standard pack for Japan, which includes Japan’s nationwide land-use data. Forests, agricultural areas, wetlands, grasslands, and golf courses were considered “green space.” According to Sergio et al. (2003), most black kites forage within 1 kilometer of their nests. Thus, any birds observed at the research location points could have a nest up to one kilometer away and might forage up to another kilometer in the opposite direction. Each research location’s population would therefore have a potential foraging zone of up to 2 km in all directions away from the point of observation. Buffers with two kilometer radii were placed around each research observation location to determine the small-scale green space coverage within the foraging zones of these birds.

·         Field data and GIS data were then compared in order to determine whether a connection exists between green space coverage and Black Kite aggressiveness.

 

Each research location’s population was assessed for its overall level of aggression via their AI.

 

Definitions

Attack: Any intentional physical contact or attempt to physically come into contact with a human/their food or another bird.

Aggressive: Engaging in attacks at any point during the observation period.

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 and types of behavior are as follows: Attacks (Other Birds)=1, Attacks (Black Kites)=2, and Attacks (Human)=3. These are based upon each action’s inherent level of aggression and risk.

 

Results

Figure 2: Land usage map with the plotted research locations and each location’s black kite foraging range.

 

The tabulated green space percentages were as follows: Enoshima=10.5%, Kamakura Beach=28.8%, Zushi Beach =36.0%, and Iwa Beach =70.4%.

The AI of each population was as follows: Enoshima=33.47, Kamakura Beach=11.69, Zushi Beach=2.00, and Iwa Beach/Marina=1.20.

 

Figure 3: The number of attacks per population versus the amount of green space in each population’s foraging zone.

 

Figure 4: The aggression index of each population versus the percentage of green space in each population’s foraging zone.

 

Discussion

The results obtained thus far show a strong inverse linear correlation between the aggression index of each test population and that city’s overall green space percentage (p<0.01). Similarly, there was an inverse correlation between the total number of attacks and the overall green space cover of that population’s foraging range (p<0.01).

As my hypothesis suggested, a correlation between the green space percentage of a black kite population’s foraging zone and that population’s overall aggression was present. The cause could be the stress of urbanization upon the population, their desensitization towards humans, or a lack of resources. Although a linear relationship between the number of human visitors present and the aggression of that black kite population was not found, the evidence thus far hints that a logistical relationship with a threshold may be present. Further tests upon the relationship between green space changes and behavioral changes over time could shed new light on this issue.

Further locations will be added as of March 2011 in order to provide a wider, more complete green space gradient. The overlap of the foraging zones in Kamakura and Zushi did not affect the aggression data due to the fact that the foraging zones did not overlap the actual observation points, and birds in that zone were not included in the final results, but the new locations will be carefully spaced in order to avoid any further issues.

 

Literature

1.)    Cringan, A.T., Horak, G.C., 1989. Proceedings of the Western Raptor Management Symposium and Workshop, National Wildlife Federation, Washington, DC. Effects of urbanization on raptors in the western United States., 219–228.

 

2.)    Marzluff, J.M., 1997. Effects of urbanization and recreation on songbirds. In: Block, W.M., Finch, D.M. (Eds.), Songbird Ecology in Southwestern Ponderosa Pine Forests: A Literature Review. Gen. Tech. Re RM-GTR-292. US Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, pp. 89–102.

 

3.)    Pease, M. L., R. K. Rose, and M. J. Butler. 2005. Effects of human disturbances on the behavior of wintering ducks. Wildlife Society Bulletin 33: 103-112.

 

4.)    Sergio, F., P. Pedrini, and L. Marchesi. 2003. Adaptive selection of foraging and nesting habitat by black kites (Milvus migrans) and its implications for conservation: a multi-scale approach. Biological Conservation 112: 351–362.

 

5.)    Shiraishi, S., K. Koga, and N. Kawaji. 1990. Food habits of the black-eared kite, Milvus migrans lineatus, in Nagasaki Airport and its adjacent areas. J. Fac. Agr., Kyushu Univ. 34: 247-254.