No way to avoid conflict? When both people and birds like urban centers

From June to July in 2019 and 2024, we confirmed the roosting behavior of barn swallows in public habitats using field surveys in all the 18 cities of Hainan Island at the county level and 12 small towns. Hainan Island is geographically located between 108°3′–111°0′ E and 1018°′–20°1′0 N in the southern tropics of China and is 240 km long and 210 km wide, with a total area of 33,900 km². Hainan has a tropical monsoon climate, being warm and hot throughout the year, with abundant rainfall and a summer temperature of 25–29 °C (Zhang et al. 2010).The target species, the barn swallow, is a common summer migratory bird in China that is distributed across all provinces. They start to migrate south each autumn, with only a few remaining in southern Yunnan, Hainan Island, Taiwan, and the Xisha Islands to overwinter (Zheng 2023). Their breeding period is between March and July, with most breeding two nests per year. Most barn swallows on Hainan Island are summer migrants, migrating to more southern parts of Asia to overwinter in autumn (Liang et al. 2013; Tian et al. 2024).At dusk, we observed bird flights over the city from the tops of tall buildings, and when we noticed barn swallows flying in small flocks in a particular direction, we began tracking them; this led us to their communal roosting locations. Once the barn swallow population had settled into roosting in the dark, they were counted (per power line, tree, or building). Because the locations where barn swallows roost are typically brightly lit and not hidden, their numbers can be counted quite accurately. Barn swallows in trees with a suitable underside location can be clearly seen by flashlight at the canopy level, and this observer behavior does not alter their nocturnal roosting behavior (own observations).A handheld GPS device (BHCnav N210, Nanjing, China) was used to pinpoint geographic locations where there were clusters of barn swallows roosting nocturnally, and a bubble map of all public habitats where barn swallows were found across Hainan Island was created to visualize the distribution and sizes of aggregated populations. The location of urban public roosts on Google Maps was used to determine whether they were in city centers and main roads.Because there are different metrics for habitat parameters for birds perched on power lines and those in trees, we measured them separately. A handheld laser rangefinder was used for accurate measurements in the field (UT390B, range: 0–40 m, precision: 0.01 m; Shenzhen, China). The quantitative variables of the trees were tree height, height below branches (to first branch height), crown width (widest part of the canopy), and trunk diameter at breast height. The quantitative variables of the surroundings were distance of the base of the perching trunk from the nearest tree, sidewalk, driveway, and building, light directly underneath, and noise. Researchers took 5 min counts of traffic flow on the perch at night from 20:00 to 22:00. The counts included motor vehicles, non-motorized vehicles, and pedestrians. We also selected non-habitats without overnight stays of barn swallows as a control group to determine the selection preferences of barn swallows. Non-roosting sites are areas of trees that may be suitable for barn swallows but are not used; hence, the control sites had tree species with similar characteristics. The non-roosting sites were approximately 10 min of walking distance from the roosting sites to avoid overlap. Non-roosting and roosting sites were measured with the same parameters, and both were used as independent samples.The reasons for summer nocturnal clustering of birds in cities are unclear, but the characteristics of public habitats in cities substantially contribute to understanding the ecology of urban species. We surveyed all the 18 cities on tropical Hainan Island, south China, to determine the locations and sizes of nocturnal clusters of barn swallows (Hirundo rustica) as well as their preferences for nocturnal roosting habitat. Large aggregations of barn swallows occurred in 88.9% (16/18) of the cities compared to that in 25.0% (3/12) of the small towns, indicating that nocturnal aggregation of barn swallows in cities is a common behavior and has a higher probability of occurrence than that in small towns. Among the barn swallows that roosted nocturnally in the city, 93.8% (15/16) gathered in urban centers, implying that barn swallows prefer to roost nocturnally in areas of the city where human disturbance is most serious and human activities are most active and frequent, which may be related to anti-predation strategies at night. However, at the same time, tens of thousands of nocturnal barn swallows also bring a large amount of bird droppings to the local area, causing public health incidents and affecting the nocturnal commerce of local people. To understand the roosting habits and adaptations of barn swallows in the urban landscape will help mitigate the conflict between large urban bird congregations and human activities and enable local functional management to achieve an optimum balance between urban development and bird conservation.