Spatial and temporal variation in migrant raptors on Java, Indonesia

Abstract

The migration of northern migrant raptors was studied at three sites in the western half of the island of Java, Indonesia during the northern autumns of 1998 and 1999. During 58 days of observation 4716 migrants were observed, 3087 of which could be identified to species level. The migrants comprised six species, i.e. Chinese Sparrowhawk, Accipiter soloensis (2570 birds identified), Japanese Sparrowhawk, A. gularis (245 birds), Oriental Honey-buzzard, Pernis ptilorhyncus (269 birds), Common Buzzard, Buteo buteo (1 bird), Booted Eagle, Hieraaetus pennatus (1 bird), and Marsh Harrier, Circus aeruginosus (1 bird). Both Chinese Sparrowhawks and Japanese Sparrowhawks were frequently observed in large flocks, whereas Oriental Honey Buzzards were more frequently seen singly or in pairs. Between sites, intra-annual variation in proportions of the three most common species identified remained constant over the two-year study period, whereas within one site (Puncak Pass) the proportion of Oriental Honey Buzzards varied from 6 to 10% between years. Migration starts in the third week of September and passage rates (the number of birds passing per hour during 09.01–12.00 hours) of the raptors peaked at the end of October and showed little spatial variation. Migration was observed until the second week of November but it probably continues until mid- or late November. The findings confirm the existence of a large east-south-eastern passage over Java similar to that over the Thai–Malay Peninsula and Bali. Populations migrating through Java originate from northern Asia and most likely have passed over the Thai–Malay Peninsula and Sumatra before reaching Java. An unknown proportion continues on to Bali and/or the Lesser Sunda Islands, but little is known about their final wintering grounds.