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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.
Please
support the OBC's conservation work by visiting
their website and becoming a member.
INTRODUCTION
The Black Drongo Dicrurus macrocercus
is a mediumsized passerine of temperate and tropical
Asia. It breeds in south-east Iran, Afghanistan,
India, south-east Tibet, and from northern China
discontinuously south through south-west Thailand,
to Bali and Java. Northern populations migrate,
wintering at lower altitudes and latitudes, reaching
as far south as central India in the west, and
Malaysia and Sumatra in the east, where they occur
in tropical savanna, grassland and agricultural
areas (Jeyarajasingham and Pearson 1999).
Little has been published about Black Drongo migration.
Historically, the species was considered an ‘extremely
abundant’ migrant in September at Beidaihe,
China, occurring in ‘huge noisy parties’
(La Touche 1920). In an autumn migration survey
of the same area from 1986–1990,Williams
(2000) counted a maximum of 452 in 1986, but only
196 in 1990. Melville and Fletcher (1982) counted
1,444 flying west in less than two hours of observation
on 14 October 1980 near Bangkok, Thailand. Numbers
migrating through, and wintering in, Thailand
appear to have declined in recent years (P. Round
verbally 2003, D. Wells in litt. 2004).
As
part of a study of raptor migration through southern
Thailand in autumn 2003 (DeCandido et al. 2004),
we also counted the number of migrant Black Drongos
passing the watch site.
METHODS
Chumphon (10o28’N 99o13’E; sea level)
is a town on the coastal plain of southern Thailand.The
north-south Bilauktaung range 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). The study site (10o28.40’N 99o13.26’E)
was in a freshwater marsh at Ban UTapao, Tha Yang
subdistrict, 4.6 km east-north-east of Chumphon,
and approximately 2 km north-east of the
coastal highway. The vegetation comprises primarily
sedges Carex spp., rushes Juncus
spp. and cat-tails Typha spp. with
lone, scattered trees. In clear weather, 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 location of our watch site was approximately
435 km to the south-south-west of the one used
by Melville and Fletcher (1982).
Migrating
drongos were counted using 10x binoculars by RDC,
assisted at times by CN and DA. Count protocols
followed those described in Bildstein and Zalles
(1995) for raptors.We scanned primarily north
for approaching migrants. A bird was counted if
it passed across an imaginary east-west line at
the watch site. Observations typically began at
07h00 local time and usually ended at or before
17h00.Weather conditions (wind speed, barometric
pressure, temperature, humidity) were measured
hourly throughout the day with a hand-held ‘weather
station’, the Kestrel 4000 (Nielsen-Kellerman
Corporation, USA). Wind direction was assessed
with a compass.We counted migrants during late
September to early November because this period
corresponded to the peak of the diurnal bird migration
observed in previous years by CN.
RESULTS
AND DISCUSSION
During 378 hours of observation between 27 September
and 9 November 2003, we counted 11,290 migrating
Black Drongos (29.9 birds/hour). Drongos migrated
throughout the day, primarily between 07h45 and
17h45 (Fig. 1). Between dawn and c.07h30, Black
Drongos made short flights from overnight roosts
to foraging areas. Most birds began migrating
soon after,along with bee-eaters Merops spp.
and swallows Hirundiniae, but before
the onset of most raptor migration. The daily
peak of migration was at 08h00–10h00, when
28% (3,129) of all individuals were counted (37.2
birds/hour). Numbers fell by early afternoon,
but increased again in the late afternoon. There
was often a large movement at 17h00–18h00,
presumably of birds heading to roost. We rarely
saw drongos migrating after 18h00. In contrast,
Crowbilled
Drongo Dicrurus annectans was frequently
mist-netted at night in autumn 1965–1973
at Fraser’s Hill, Malaysia (Medway and Wells
1976, McClure 1998, D.Wells in litt. 2004).
Migration
peaked in late October, with 2,089 birds (97.1
birds/hour) on 17–18 October following two
days of heavy rains throughout much of Thailand,
and 2,766 birds (55.3 birds/hour) on 26–30
October (Fig. 2). By early November, there were
fewer migrating drongos, and we assumed that many
of those we saw had settled on their winter territories.
Further north at Beidaihe, Williams (2000) estimated
that drongo migration peaked on 6–17 September.
Our highest single hourly count (618) on 17 October
was similar to the 825 birds/hour seen in the
late afternoon of 14 October 1980 by Melville
and Fletcher (1982).
There
was no significant difference between the number
of Black Drongos counted when winds were from
the north to north-west compared to the east or
south (P²=0.13, P<0.05). The vast majority
of migrant drongos passed the watch site at a
height of <35 m, and usually <8 m. Early
in the season, drongos occurred as singles, but
formed loose flocks from 5 October onwards. Often,
5–20 birds would pass the watch site, sometimes
across a 30 m front. The species did not appear
to be an obligate flocking species (sensu
Kerlinger 1989) like, for example, the Blue-tailed
Beeeater Merops philippinus. |
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