Species Composition, Timing, and Weather Correlates of Autumn Open-Water Crossings By Raptors Migrating Along the East-Asian Oceanic Flyway

Abstract

Raptor migration rarely involves long-distance movements across open oceans. One exception occurs along the East-Asian Oceanic Flyway. We collected migration data at two terrestrial hawkwatch sites along this flyway to better understand open-ocean movements along this largely overwater corridor. At the northern end of the Philippines, at Basco on the island of Batan, we recorded 7587 migratory raptors in autumn 2014. Near the southern end of the Philippines, at Cape San Agustin on the island of Mindanao, we recorded 27,399 raptors migrating in autumn 2012. Chinese Sparrowhawks (Accipiter soloensis) were the most common raptors observed, making up approximately 89% and 92% of total records for Basco and Cape San Agustin, respectively. The Grey-faced Buzzard (Butastur indicus) was the second most common raptor migrant, accounting for 8% of the total counts at both watch sites. The migration period was about 1-2 wk earlier at Basco, the more northerly site, than at Cape San Agustin. Overwater flights at Basco peaked in both the morning and late afternoon, whereas at Cape San Agustin there was only a morning peak. In general, the rate of migration passage at both sites was highest with clear skies when winds were blowing from the northwest. However, we observed interspecific differences in migration behavior at both sites, with Accipiters more likely to be observed with tailwinds and eastward winds, and Grey-faced Buzzards more likely observed with headwinds. These results help to characterize poorly known aspects of raptor biology and to identify potential migratory bottlenecks or key sites for raptor conservation in little-studied Philippine tropical ecosystems.

The State of Migratory Landbirds in the East Asian Flyway: Distributions, Threats, and Conservation Needs

“…Data from recent tracking studies (Figure 2) further illustrate the use of the 'island route' by passerines and raptors breeding in Japan (Koike et al, 2016;Nourani et al, 2018;Uemura et al, 2020) and species shared with Kamchatka (e.g., barn swallow Hirundo rustica and Siberian rubythroat) (Heim et al, 2020). These results corroborate direct observations in the Philippines and eastern Indonesia (Germi et al, 2009(Germi et al, , 2013Concepcion et al, 2017).…”

“…Two main migratory corridors in the EAF are now well established, (1) the 'island' or 'oceanic' route linking eastern Russia (Kamchatka and Sakhalin) and Japan to the Philippines and eastern Indonesia, and (2) the 'mainland' route, linking eastern Russia, China and continental Southeast Asia (Higuchi, 2005;Yamazaki et al, 2012;Concepcion et al, 2017;Heim et al, 2020). Data from recent tracking studies (Figure 2) further illustrate the use of the 'island route' by passerines and raptors breeding in Japan (Koike et al, 2016;Nourani et al, 2018;Uemura et al, 2020) and species shared with Kamchatka (e.g., barn swallow Hirundo rustica and Siberian rubythroat) (Heim et al, 2020).…”

“…The specific island route(s) used remain poorly studied. Observational data show that two raptors, grey-faced buzzard Butastur indicus and Chinese sparrowhawk Accipiter soloensis, pass south through the Japanese archipelago, Taiwan and the Philippines (Ellis et al, 1990;Yamazaki et al, 2012;Concepcion et al, 2017). Similarly, some passerines, including wagtails, pipits and yellow bunting Emberiza sulphurata, also follow this least energetically costly route (Nourani et al, 2018).…”

Raptor migration in an oceanic flyway: wind and geography shape the migratory route of grey-faced buzzards in East Asia

“…The East Asian oceanic flyway for raptors (hereafter the oceanic flyway) is an exception. This largely overwater flyway stretches from northeastern Siberia to Southeast Asia and is dominated by the Chinese sparrowhawk Accipiter soloensis and the grey-faced buzzard Butastur indicus [ 1 , 8 , 9 ]. It is assumed that the numerous islands that occur along this flyway provide an opportunity for migration by island-hopping.…”

Prospects for monitoring bird migration along the East Asian‐Australasian Flyway using weather radar

“…East Asian migrants often congregate at these narrow land corridors to minimize the lengths of sea crossings during migration, leading to large concentration of birds during the migration season (DeCandido et al, 2004(DeCandido et al, , 2010Nourani et al, 2018;Sugasawa & Higuchi, 2019). A prime example would be raptors that breed in North Asia and migrate via the island chains of Japan, Taiwan, and the Philippines (commonly referred to as the East-Asian Oceanic Flyway), often in tens of thousands of individuals that form large flocks visible on radar screens (Chen et al, 2022;Concepcion et al, 2017;Sun et al, 2010). Weather radars in these islands/peninsulas and narrow land corridors are therefore able to achieve nearly complete coverage of all migrant species with relatively few radars.…”