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Autumn 2003 raptor migration at Chumphon, Thailand: a globally significant raptor migration watch site.

By Robert DeCandido, Chukiat Nualsri, Deborah Allen and Keith L. Bildstein

Note: This article was originally published in Forktail 20 (2004) the journal of the Oriental Bird Club (OBC) and was kindly submitted by Robert DeCandido.

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METHODS
Chumphon (10o28’N 99o13’E; sea level) is a town (population: 16,000) on the eastern coastal plain of southern Thailand, 460 km south-west of Bangkok, 550 km north of Malaysia, and 55 km east of Myanmar. The north-south Bilauktaung range of mountains 35 km to the west funnels many diurnal bird migrants through this 30-km wide coastal plain
adjacent to the Gulf of Thailand (Wells 1999, Zalles and Bildstein 2000). Migration was observed at a site (10o28.40’N 99o13.26’E) in an unprotected freshwater marsh 4.6 km east-north-east of Chumphon, and approximately 2 km north-east of the coastal highway at Ban U-Tapao, Tha Yang subdistrict, Muang district, Chumphon province. The marsh is composed primarily of emergent aquatic plants, including sedges Carex spp., cat-tails Typha spp., and common reed Phragmites australis.Vegetation averages less than 1.5 m high, and it does not prevent observation of low-flying raptor
migrants. On fair days, it is possible to see 10 km to the north-east, 3 km to the east and west, and about 1 km to the south.

The weather in late summer–early autumn is determined by the interaction between a monsoonal low-pressure system in the Pacific Ocean to the east, and a high pressure system in the Andaman Sea to the west. In September and early October, the high pressure system prevails, and most winds are from the west or north-west. By mid-October, the low pressure system predominates, and winds are easterly. Heavy rain is common in mid-October as low pressure
replaces high pressure in the region. The bulk of the migration occurs during the ‘south-west’ (April–October) rather than ‘north-east’ monsoon (November–January).

We counted migrants from 27 September to 11 November, which correspond to the peak of the raptor movements in the area. Migrating raptors were counted by RDC using 10x binoculars, assisted at times by CN and DA. Count protocols followed those described in Bildstein and Zalles (1995). Most observations began at 07h00 local time and usually ended at 17h00. Most raptors (>95%) were readily identified to species using Porter et al. (1986), Clark (1999), Jeyarajasingham and Pearson (1999),Wells (1999) and Robson (2002). Weather conditions (wind speed, barometric pressure, temperature, humidity) were monitored hourly throughout the day with a hand-held Kestrel 4000 ‘weather station’ (Nielsen-Kellerman corporation, U.S.A.). Wind direction was determined with a compass.

Observers scanned primarily north for approaching migrants. An individual was considered a migrant if it
passed north-to-south across an imaginary east-west line at the watch site, and continued south out of sight. No attempt was made to determine the relative proportion of males versus females, or of adults versus immatures.Totals for large flocks (>300 individuals) of Black Baza Avecida leuphotes, Chinese Sparrowhawk Accipiter soloensis and Grey-faced Buzzard Bustatur indicus were estimated to the nearest 25 individuals.

RESULTS

Table 1 : Raptor species, numbers counted, and seasonal peaks at Chumphon, Thailand, in autumn 2003.

In total, 170,665 raptors of 15 species were counted migrating during 378 hours of observation (averaging 452 birds/hour;Table 1). Black Baza was the commonest migrant (68,219 individuals), representing 40% of the 163,020 individuals identified to species (Table 1). The largest flock of this species, estimated at 1,300 birds, passed on 23 October. Chinese Sparrowhawk (57,667 individuals) made up 34% of all individuals identified, with 550 individuals in one flock on 11 October. We also observed large numbers of Greyfaced Buzzard (14,962 individuals; 9%; largest flock = 500 individuals on 23 October) and Oriental Honey- Buzzard Pernis ptilorhyncus (15,972 individuals; 9%; largest flock = 91 individuals on 7 October).

The highest single day count was 56,101 individuals of ten species on 23 October (Fig. 2). A peak of 11 species was observed on 18 October.There were differences between species in their seasonal pattern of migration (Fig. 3). Of the five commonest species, Chinese Sparrowhawk and Oriental Honey-buzzard migrated through the area primarily before mid- October, Grey-faced Buzzard and Black Baza migrated primarily in late October, and Japanese Sparrowhawk Accipiter gularis migrated through the area throughout the autumn (Fig. 3).

Raptors at Chumphon typically migrated throughout the day, sometimes from shortly after 07h00 until shortly before 18h00 (Fig. 4). On most days, movements of Chinese Sparrowhawk and Grey-faced Buzzard peaked at 09h00–10h00. Oriental Honey-buzzard migration peaked at 10h00–11h00. About half of all raptors (53%) were seen from 09h00 to12h00 (Fig. 4). On most days, counts decreased thereafter until 15h00–16h00, when they again increased. Most of the migrants (72%) were seen when cloud cover exceeded 70%. Between 12h00 and 15h30, raptors frequently flew at heights above 600–800 m, especially if winds were <8km/hour. Entire flocks of certain species (Chinese Sparrowhawk, Grey-faced Buzzard), and individuals of other species (Oriental Honeybuzzard, Japanese Sparrowhawk) sometimes disappeared into the bases of clouds when they soared in thermals. At 19h25 on 1 October 2003, CN observed a flock of about 50 Chinese Sparrowhawks entering a night-time roost in a coconut Cocos nucifera grove near the watch site. Several species, including Black Baza, Chinese Sparrowhawk, and Japanese Sparrowhawk roosted in coastal mangrove forests 10–15 km north of the site.

We considered four species (Black Baza, Oriental Honey-buzzard, Chinese Sparrowhawk and Greyfaced Buzzard) to be obligate flocking migrants at the site (sensu Kerlinger 1989). We also observed mixedspecies flocks of Grey-faced Buzzard and Black Baza, and of Japanese and Chinese Sparrowhawk. Individuals of several other species (e.g. Osprey
Pandion haliaetus, Black Kite Milvus migrans and Greater Spotted Eagle Aquila clanga) were sometimes seen in groups of two or more individuals. Several researchers (Ash 1993, Wells 1999, Nijman 2001b, Robson 2002) have noted flocks of Japanese Sparrowhawk in South-East Asia, but we did not see single-species flocks of this species at Chumphon.

More raptors were counted at the site on winds from the west or north-north-west than the east or south (P² = 41.7, P<0.05). From 27 September to 25 October, winds were typically light to moderate for the entire day. Later in the season, early morning westerly or variable winds shifted to on-shore, easterly winds by about 10h00. Raptors were then seen primarily flying north-to-south-west of the site, and after noon few raptors were observed passing over the site itself.

Three additional raptor species (Black-shouldered Kite Elanus caeruleus, Brahminy Kite Haliastur indus, and White-bellied Sea Eagle Haliaeetus leucogaster) were resident in and around Chumphon and were not counted. Non-raptors seen migrating, or apparently migrating, at the watch site included Little Cormorant Phalacrocorax niger, Grey Heron Ardea cinerea, Purple Heron Ardea purpurea, Oriental Pratincole Glareola maldivarum, Grey-headed Lapwing Vanellus cinereus, Pale-capped Pigeon Columba punicea, House Swift Apus affinis, Asian Palm Swift Cypsiurus balasiensis, Fork-tailed Swift Apus pacificus, Blue-throated Beeeater Merops viridis, Blue-tailed Bee-eater Merops philippinus, Barn Swallow Hirundo rustica, Red-rumped Swallow Hirundo daurica, Sand Martin Riparia riparia, Ashy Minivet Pericrocotus divaricatus and Black Drongo Dicrurus macrocercus (see DeCandido et al. 2004).

Figure 2 : Daily totals of raptors counted on migration at Chumphon,Thailand, autumn 2003.

Figure 3(a) : Number of Black Baza counted at Chumphon,Thailand, in autumn 2003.

Figure 3(b) : Number of Oriental Honeybuzzard counted at Chumphon,Thailand, in autumn 2003.

Figure 3(c) : Number of Chinese Sparrowhawk, counted at Chumphon,Thailand, in autumn 2003.

Figure 3(d) : Number of Japanese Sparrowhawk, counted at Chumphon,Thailand, in autumn 2003.

Figure 3(e) : Number of Grey-faced Buzzard, counted at Chumphon,Thailand, in autumn 2003.

Figure 4 : Pattern of raptor migration through the day (birds/hour) at Chumphon, Thailand, in autumn 2003.

*Unpublished data indicate that a higher count was made at the Miyako islands (Ryukyus), Okinawa, Japan: 53,575 individuals in autumn 1980; data from 1973–2003 give an annual mean of 27,859 individuals (Y. Nitani in litt. 2004).

Small numbers of two other raptors (nine Steppe Eagles Aquila nipalensis and two Imperial Eagles Aquila heliaca) were seen migrating in early November at a location c.8 km west of the site (not included in Table 1). Further research may determine if several other raptor species that are regarded as migrants in Thailand (Hen Harrier Circus cyaneus,Western Marsh Harrier C. aeruginosus aeruginosus, Shikra Accipiter badius and Eurasian Sparrowhawk A. nisus) also regularly pass through the area.

The raptors counted at Chumphon are thought to follow at least two migration routes in East Asia: (a) an Eastern Inland Corridor that extends from southeastern Siberia through eastern China and Indochina (Laos, northern Vietnam, northern Thailand), south through the Thai-Malay Peninsula; and (b) a Coastal Pacific Corridor that extends from north-eastern Siberia, Amurland, and Ussuriland through coastal China, south into Indochina and the Thai-Malay Peninsula (McClure 1998, Zalles and Bildstein 2000). Movements along these corridors are poorly understood, and additional study is needed to determine the routes used by different raptors in the Far East (Ellis et al. 1990, Lane and Parish 1991, Severinghaus 1991, Ash 1993, Chong 2000, Nitani 2000, Williams 2000, Nijman 2001a,Tordoff 2002). Satellite-tracking of two Oriental Honey-buzzards in autumn 2003 showed that they left their breeding grounds in Japan in mid-September, crossed to mainland China and headed south-west across Indochina, then south through the Thai-Malay Peninsula to Sumatra (H. Higuchi in litt. 2004). Further studies may reveal the extent to which outbound raptors including Oriental Honey-buzzard, Chinese Sparrowhawk and Grey-faced Buzzard, follow the Oceanic Pacific Corridor, before heading west to the mainland, and south into Malaysia and Indonesia.

Weather conditions had a major influence on the number of raptors migrating. Significantly more raptors were seen when winds were from the northwest quadrant than when winds were from the south-east quadrant. Raptors seemed to ‘drift’ to the coast with westerly winds when skies were overcast and when strong thermals were lacking. With moderate westerly winds, as many as 10% of the migrants passed approximately 2 km to the east of the site along the Gulf of Thailand. On 23 October, we counted 56,101 migrants during overcast skies and light to moderate west to north-west winds. On that day the area <7 km north and west of the watch site was free of precipitation, but it was raining throughout much of central and northern Thailand. Although steady rain at the watch site ended raptor migration there, we believe that local rains simply diverted migrants around the precipitation cell. Observers in the Neotropics have also noted migration in light or intermittent rain and overcast skies (Bildstein and Saborio 2000).

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Kindly submitted by:

Robert DeCandido, Acopian Center for Conservation Learning,Hawk Mountain Sanctuary, 410 Summer Valley Road, Orwigsburg, Pennsylvania 17961 U.S.A. Correspondence: 1831 Fowler Avenue,The Bronx, New York 10462-3708. Email: rdcny@earthlink.net

Chukiat Nualsri,Thayang Administrative Organization, 135-1 Kromluang Chumphon Road, Muang Chumphon
86000,Thailand. Email: bntern@chaiyo.com

Deborah Allen,The Linnaean Society of New York, P.O. Box 1452 Peter Stuyvesant Station, New York 10009, U.S.A. Email: dallenyc@earthlink.net

Keith L. Bildstein, Acopian Center for Conservation Learning,Hawk Mountain Sanctuary, 410 Summer Valley Road, Orwigsburg, Pennsylvania 17961 U.S.A. Email: bildstein@hawkmtn.org