The bird communities observed in rice fields were divided into four assemblages (clusters), according to the similarity of species and population composition, which had clearly different usage patterns. Birds were observed year-round, and the shorebirds, herons, and waterfowl (waterbirds) showed seasonal migration characteristics. Moreover, habitat type and cultivation method affected different bird assemblages differently, due to differences in ecological characteristics and microhabitat use (i.e., paddy, levee, ditch, and road). These characteristics were depended on the time of arrival of the different waterbird assemblages in the rice fields [17, 18, 23]. Shorebirds use the rice field as a stopover for a short period in May; herons obtain food during or after breeding (June–September); and waterfowl stay during the winter in the rice fields, from October to April [17, 18]. Moreover, the rice fields provide different habitat environments for each season, maximizing their usage by waterbird assemblages that arrive in each season [31]. Consequently, it is believed that bird assemblages using rice fields are clearly distinguished because of differences in the season of rice-field use and microhabitats [31]. Farmers prepare to cultivate rice in early May by watering the rice field [23, 32, 33], and shorebirds use rice fields during this period [23, 32, 33]. The amount of animal prey increases rapidly during the growing season of rice (June – September) and herons are continuously observed during this period [23, 32, 33]. From October to April, rice is harvested, and waterfowl feed on the remaining down grain, increasing their use of the rice field [23, 32, 33]. Therefore, the rice field provides a suitable habitat environment for the use of each bird assemblage at different times of the year. Interestingly, land birds use the available resources in the rice field year-round without any particular seasonality.
Rice field microhabitats (paddy, levee, ditch, and road) provide unique habitat qualities, and birds use them differently according to their characteristics [6, 17–19]. All bird assemblages identified in this study used all the rice field microhabitat types, and each microhabitat fulfilled different functions for water and land birds. Waterbirds, such as shorebirds, herons, and waterfowl, mostly used paddies, whereas land birds used more frequently ditches or roads. The paddy, heavily used by waterbirds, accounts for the largest area of the rice field, and showed abrupt changes during the rice cultivation process [20, 21, 23]. Moreover, aquatic and terrestrial ecosystems alternate in the paddy fields [20, 21, 23]. Due to these characteristics, various food sources for waterbirds, such as benthic organisms and down grains, are available repeatedly and periodically, maintaining frequent paddy use by waterbirds that feed on those.
Land birds use roads and ditches more than paddy fields. Roads and ditches have well-developed diverse herbaceous plants, such as farm crops, reeds, and silver grass, which can be used as foraging or resting spaces [17, 18], whereas the paddy is a space for growing rice, a single crop, and has a monotonous vegetation structure [17, 18]. It has been reported that a higher biodiversity of herbaceous species, including crops, attracts more diverse taxa [3], such as bird feeding spiders, butterflies, and surface roaming insects [34, 35]. Therefore, land birds use roads and ditches that offer a more diverse habitat [3].
It is necessary to identify the detailed characteristics of each bird assemblage to understand their use of rice fields and the effects of cultivation methods. Each bird assemblage showed different usage characteristics due to the complex interactions of habitat type and cultivation method. This is closely related to the characteristics of the habitat environment, formed as a result of the interactions between habitat types and cultivation methods. The eco-friendly cultivation paddies, which are used frequently by shorebirds, have a higher diversity of benthic organisms, a potential food source, than the conventional cultivation paddies [36, 37]. Even though these paddies had similar shapes to conventional fields, shorebirds used them considerably more because of the larger amount of available food. In contrast, shorebirds used levees in conventional cultivation fields more than levees in eco-friendly cultivation fields, which could be related to accessibility. Shorebirds have a habit of foraging or resting in an open environment [38, 39]. In the area studied, the levees were managed in different ways according to the cultivation method. In eco-friendly fields, weeds were either left untouched or removed using a weeder, i.e., without a complete removal; whereas in conventional fields, herbicide was used, allowing for complete removal [40]. The levees in the conventional fields presented an open shape, which shorebirds prefer, and thus used more. Herons used the paddy, levee, and road habitats in the eco-friendly fields more than the habitats in the conventional fields. These habitats provided more abundant potential food sources for the herons than the conventional habitats [36, 41]. Moreover, herons rarely used the ditch habitat because it was narrow and low in height, making it difficult for herons to access compared to other habitats [17, 18]. Waterfowl also used habitats in eco-friendly fields more than those in conventional fields. This could be because some paddies in the eco-friendly fields were watered even after harvesting. Watered paddies are an important feeding ground for various waterfowl in winter [42–44], and they were observed concentrated around the watered paddy. Although land birds used all habitat types, they used conventional cultivation areas more than eco-friendly cultivation areas, contrary to waterbirds. Certain farm crops grown in the conventionally cultivated rice fields at the study site (cultivated using pesticides) and were not grown in the eco-friendly fields. These farm crops provided land birds with more diverse habitats in the conventional fields than in the eco-friendly fields, and they allowed land birds to forage or rest more [35].
It has previously been found that rice cultivation techniques, such as chemical substance use and crop cultivation around the paddy, could affect birds using rice fields [21, 26, 38, 42, 45, 46]. The results of this study also support these findings, as it was found that waterbirds preferred eco-friendly fields due to a large amount of potential food. It was also found that land birds preferred to use fields that cultivated a higher number of plant crop species, which provided more diverse habitats, even if chemical substances were used. This increased habitat diversity was possible using pesticides, which allowed various other farm crops to be grown. However, it does not mean that the pesticide use is important. The various habitat structures created by the crops are important, and it would be better to have a variety of habitat structures without the use of pesticides.
In conclusion, this study reported that bird assemblages in rice fields differ based on the agricultural techniques used, as these affect the characteristics of the habitat. As natural wetlands disappear, paddies provide alternative habitats for various birds. Rice paddies are artificial wetlands that can be changed drastically through active human management over a short period of time, compared to natural wetlands. Therefore, it is very important to understand the ecological characteristics and structure of the paddy wetlands. The results of this study will be valuable for establishing management plans for various avian species that use rice fields. Consequently, seeking and implementing clear and systematic management plans for rice field habitats can contribute to the immediate protection and conservation of birds. Future studies are needed to confirm whether the effects of cultivation methods at the community level would also be valid at the species level.