2.1 Study area
We conducted our study in the city of Suwon, Gyeonggi Province, South Korea (37° 17’ 28” N; 127° 0’ 32” E). Suwon comprises four administrative divisions: Paldal-gu, Jangan-gu, Yeongtong-gu, and Gwonseon-gu, which include urban areas and surrounding agricultural and natural lands (Fig. 1). Rooks arrive in Suwon—the third most densely populated city in Gyeonggi Province with a population of 1.18 million people—in November, when the average temperature is 7.9°C and annual precipitation is 104.2 mm (Suwon, 2022).
The areas of each district are Yeongtong: 2.75 ha; Paldal: 1.29 ha; Jangan: 3.33 ha; and Gwonseon: 4.71 ha. In the study area, Gwonseon District exhibits the largest expanse of agricultural land, encompassing 0.97 hectares. Residential zones are most prevalent in Yeongtong and Paldal Districts, with areas of 0.64 hectares and 0.31 hectares, respectively. Jangan District, on the other hand, features the most extensive forested land, covering 1.51 hectares (Table S1).
2.2 Study species
The breeding season for rooks primarily spans May through June, with key breeding areas located in the Amur region, northeastern and southern China, and Mongolia. During the winter months, from November to March, rooks migrate to Korea, Japan, Taiwan, and eastern China. They frequently traverse Korea during the spring and autumn, acting as migratory birds in the country's southern regions.
Rooks favor habitats such as flatlands, open terrain, and forests in proximity to their roosting locations. As previously highlighted, rooks are classified as a harmful species under Article 4 of the Enforcement Rules of the Wildlife Protection and Management Act due to their tendency to cause prolonged crop damage in large groups. These birds typically gather in significant numbers, often in the hundreds or thousands (Madge and Burn, 1994), to roost post-sunset, commencing their foraging activities after sunrise (Hubalek, 2017). Consequently, we defined the preferred habitats of rooks based on the distinction between sunset and sunrise, utilizing data from December 2020 to February 2021 provided by the Meteorological Administration in Suwon.
2.3 Acquisition of citizen science data
We collected data on rook presence using the smartphone app CADA v.1.0.15 (Kim, 2020). Opting for a crowdsourcing approach, which is the most basic among the four levels of citizen science (Shum, 2012)—namely, level 1: crowdsourcing, level 2: distributed intelligence, level 3: participatory science, and level 4: extreme citizen science—we incentivized participation. Citizens, acting as sensors, were provided a financial reward of 0.35 USD for recording Corvus frugilegus sightings with their smartphones. Training sessions were not deemed necessary for this method.
The data acquired included the participant's address, age, geolocation (longitude and latitude), photograph of the rook, time of day, and the date of the observation. Once a participant's smartphone was registered, our database logged the location data along with the photos. The collection period spanned from December 2020 to March 2021. To prevent redundancy, we excluded data entries recorded within 20 minutes and a 200 m radius of a prior entry. Additionally, we filtered out irrelevant data, such as images not featuring rooks, both through a machine learning technique and manual visual inspection.
In total, we amassed 6,314 rook location records across Suwon's four districts through crowdsourcing. After refining the dataset, we used 4,522 valid location points (Table 1). Of these, 2,528 points (55%) were from Gwonseon, 1,023 (22%) from Yeongtong, 963 (22%) from Paldal, and 9 (0%) from Jangan.
Table 1
Citizen science data collected every 3 h
Counts of citizen science data
|
Hours
|
24–03
|
03–06
|
06–09
|
09–12
|
12–15
|
15–18
|
18–21
|
21–24
|
Count
|
182
|
47
|
626
|
574
|
496
|
685
|
1162
|
750
|
2.4 Acquisition of environmental variables
We referenced previous studies to select environmental variables favored by rooks appearing in urban areas; these were primarily artificial influences and structures such as food supply, mild temperatures, wind shields, and green spaces (Byrkjedal et al., 2012; Ciah et al., 2017; Clewley et al., 2016; Griffin et al., 2000). Among them, we combined 15 types of land use from the biotope map and five other variables were extracted—elevation, Euclidean distance from buildings with three different classes of floor numbers (1st ~ 5th, 6th ~ 20th, 30th and above), Euclidean distance from agricultural land, Euclidean distance from a utility pole, and Euclidean distance from streetlamp—which we selected as rook-favored variables (Table 2). As interest in landcover and land use maps increases as a practical measure that can consider specific natural environments and ecosystems in various urban development plans, the Ministry of the Environment of South Korea has prepared guidelines for creating “biotope” maps and distributed them to each local government.
The biotope map of Suwon city (Suwon City Government, 2019) was produced using the classification of the following: land use, land cover, green cover ratio, vegetation rate, and landscape data. Within the classifications, in this study, we utilized the land use and the height (number of floors) of the buildings provided by Suwon’s biotope map, which has a native resolution of 5 m2. The distance from the utility pole was extracted by eliminating the areas of poles constructed underground provided by the Korea Electric Power Corporation. After extraction, the area poles were placed on top of transportation facilities at intervals of 30 m. Altitude was extracted with a resolution of 10x10 m using the digital topographic map provided by the National Territory Information Platform. The Euclidean distance from streetlamps was applied after acquiring the location data of streetlights in Suwon provided in 2021 from the public data portal (www.data.go.kr). We weighed these variables over 12-h periods at 3-h intervals according to our MaxENT results.
Table 2
Predictor variables used in the study.
Variable
|
Definition
|
Land use type
|
Land use types
1: Residential area
2: Commercial and business area
3: Mixed residential and business area
4: Land for public use
5: Industrial area
6: Public manufacturing area
7: Transportation facilities area
8: Inaccessible area
9: River
10: Lakes and wetlands
11: Forest
12: Grassland
13: Agricultural land
14: Artificial area
15: Wasteland
|
Building (low, medium, high)
|
Euclidean distance from building floors
low: 1–5 floors
medium: 6–20 floors
high: 21 floors and above
|
Farmland
|
Euclidean distance from agricultural land
|
Streetlamp
|
Euclidean distance from streetlamp
|
Utility pole
|
Euclidean distance from utility pole
|
Elevation
|
Elevation
|
2.5 Processing maximum entropy model
We developed two habitat models to differentiate between daylight and nighttime conditions. Using MaxENT version 3.4.4, we predicted the preferred rook habitats based on their occurrence data. We utilized k-fold cross-validation (k = 10) and designated a random test percentage of 25%. Pearson’s correlation analysis was performed to screen for environmental variables with high multicollinearity; however, none were identified. To adjust for uneven sampling bias, we transformed a bias file—capturing the density of Suwon’s population aged 18 and above within 5 x 5 m pixels—into a normal distribution. This ensured enhanced relative gain contributions and weighted areas with lower population densities appropriately (Stephanie et al., 2013).
The movements of rooks to foraging and roosting zones are influenced by light intensity (Swingland, 1976). Data from the Meteorological Administration in Suwon, spanning December 2020 to February 2021, indicated sunrise and sunset times as 7:35 and 17:41, respectively (Table 3). Considering civil twilight, which persists for 20–30 minutes post-sunset and pre-sunrise, noticeable changes in light intensity commence around 08:00 and 18:00 (Andre and Owens, 2021). After determining the daylight and nighttime weightings of the rook predictor variables, we applied these to the presence records. These records were segmented into eight 3-hour intervals, factoring in sunrise and sunset durations, allowing us to monitor the rook population's spatial distribution throughout the daily cycle.
Table 3
The average sunrise and sunset times in Suwon from December 2020 to February 2021.
Date
|
Sunrise time
|
Sunset time
|
Min
|
Max
|
Mean
|
Min
|
Max
|
Mean
|
Dec/2020
|
07:27
|
07:46
|
07:38
|
17:14
|
17:24
|
17:17
|
Jan/2021
|
07:36
|
07:46
|
07:43
|
17:25
|
17:55
|
17:39
|
Feb/2021
|
07:05
|
07:35
|
07:21
|
17:56
|
18:25
|
18:11
|
Mean
|
07:35
|
17:41
|