The Formosan sika deer was declared extinct in the wild in Taiwan around 1969. In 1994, a series of reintroductions was initiated in Taiwan’s Kenting National Park. Today a wild population is established in the core area of the Park, and even continues to expand. Between October 2016 to March 2018, we used Unmanned Aerial Vehicles (UAV) to survey the herd composition of the sika deer in the Shuiwaku Grassland, a forest-grassland mosaic habitat inside the park. We completed a total of 56 early morning surveys (range: 2–7 per month) during the study period. The photographs and videos collected by UAV were used to identify the age and/or sex of each individuals recorded, and to generate data on deer group size, sex ratio, age-group ratio for each herd, and solitary animals. Overall, we cumulatively detected 394 non-independent deer groups comprised of 4732 individuals, and an additional 130 solitary animals. We found that outside of the rutting season (February–September), adult females remained in the adjacent forest the majority of the time. When females moved into the grassland, they formed small-stable groups with young deer (average size: 6.7 ± 4.5 during the post-rutting season; 6.3 ± 3.2 during the calving season). Males either occupied mixed sex groups (average size: 18.0 ± 9.1 during the post-rutting season; 13.8 ± 7.3 during the calving season), or formed bachelor groups (average size: 5.6 ± 4.3 during the post-rutting season; 5.7 ± 3.8 during the calving season) in the open grassland during this same period. During the territorial rut, the proportion of solitary adult males increased significantly to 24.2 ± 15.4%, whereas females also formed gregarious but unstable mixed sex groups with males (average size: 17.0 ± 12.0). Although we observed the first newborns in the open grassland in August (2017), the ratio of fawns to adult females was highest (56.8%) in October, suggesting that this may be the optimal time to conduct annual birth rate surveys. Overall, we found sexual segregation was present in the Formosan sika deer largely driven by sex-based differences in habitat preference, and the non-random assortment of sexes in response to their reproductive status.