Caprimulgus affinis is an endemic species commonly known as the Taiwan nighthawk, forest nighthawk, or South Asian nighthawk. In recent years, the C. affinis population has gradually moved outward from river habitats into the metropolitan areas of Taiwan. Because male birds' booming sounds at night can reach up to 90 dB or higher and they can be intermittently tweeting for more than 10 hours, they often disturb the sleep of urban residents and can even cause nightmares. In this study, we analyzed long-term survey data to assess the distribution history of C. affinis in Taiwan. By using 1738 entries of observation data collected from 1999 to 2014, a model of C. affinis distribution was constructed, and a geostatistical method was used to improve the accuracy of the model estimate. In addition, the Mann-Kendall trend test was applied to predict future C. affinis distribution. Based on the results, four variables were selected to construct the C. affinis distribution matrix. Urban population, coastal proximity, and distance to upstream river location represented positive driving forces, whereas city elevation was a negative driving force. C. affinis is currently distributed on the plains of central and southern Taiwan and in eastern Taiwan. The C. affinis emergence trend diagram was plotted using the time-space trend diagram, which showed yearly increases in the C. affinis populations in urban and settled areas including central, western, northeastern, and southeastern Taiwan and yearly decreases in the populations in rural areas including northern and southern Taiwan, especially in fragmented riverine habitats. Regression kriging can correctly describe the distribution of the entire C. affinis population, which leads to the correct understanding of the biological corridor of C. affinis in their migration through the graphical contours in GIS. The analytic model in this study contributes to the establishment of the time-space trend diagram.
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