Migratory Daily Departure Times of Chinese Sparrowhawks (Accipiter soloensis) and Gray-Faced Buzzards (Butastur indicus) in Taiwan

Abstract

ABSTRACT The strategies that migratory raptors adopt during migration reveal their long-term adaptations to different environments along their migration routes and of their stopover sites. We recorded the daily departure time of Chinese Sparrowhawks (Accipiter soloensis) and Gray-faced Buzzards (Butastur indicus) during their southward migration in autumn 2019 at Kenting National Park, southern Taiwan, and 2020 in Yushan National Park, central Taiwan. At both watchsites, Gray-faced Buzzards started their migration journey about 1 hr earlier than Chinese Sparrowhawks. After adjusting for the local sunrise time, the first daily appearance time of Gray-faced Buzzards (–2.5 ± 9.3 min) was significantly earlier than that of Chinese Sparrowhawks (47.0 ± 29.3 min) by about 50 min at Kenting National Park. Though they initiated migration later in morning at montane stopover sites in Yushan National Park, the same trend was observed and with even greater difference, about 70 min. These results indicate that these two species migrate in different daily periods and suggest that Chinese Sparrowhawks might choose to exploit stronger thermal uplifts in September before leaving the island, as they started migration later in the morning. In contrast, Gray-faced Buzzards, a species more dependent on soaring-gliding flight, set off earlier in the day, probably to take advantage of stronger northeastern tailwinds that begin in October. We conclude that the prevailing monthly wind conditions, particularly influenced by the northeast monsoons, shape the different diurnal migration times of these two raptors migrating along the East-Asian Oceanic Flyway.