Dependency of Fan drying of Darters Anhinga Rufa and Little Cormorants Phalacrocorax Niger on Incident Sunlight

3 Pages·2001·2.1 MB·

by  Gerald Dick

MISCELLANEOUS NOTES
DEPENDENCY OF FAN-DRYING OF DARTERSANHINGA RUFA AND
7.
LITTLE CORMORANTS PHALACROCORAXNIGER ON INCIDENT SUNLIGHT
(Withonetext-figure)
The spread-wing posture of cormorants measuredwith ablackthermometer(bimetal) in
anddartershaslongbeenthesubjectofscientific direct sunlight. Statistical analyses ofthe birds’
investigation (Kortlandt 1940, Clark 1969, orientationtothesunfollowedstandardmethods
Curry-Lindahl 1970). The main hypotheses in (Schmidt-Koenig 1975,FowlerandCohen 1986,
connection with this behaviour are that it is Brown and Downhower 1988).
i. balancing posture (Stabler 1957); Spread-wingbehaviourwasshownbyboth
ii. intraspecificsignalofsuccessfulfishing(Jones species during daylight, mainly in the available
1978); iii. aid to thermoregulation (Curry- direct solar radiation, which facilitates wing
Lindahl 1970, Hennemann 1982) or iv. wing drying. The orientation of birds sitting or
drying strategy(Kortlandt 1940, Winkler 1983). perching was not random, as the alignment of
Strong support for the now generally their backs and wing surfaces showed a highly
accepted theory ofwing-drying is provided by significant correlation towards the sun
the fact that the plumage ofboth the cormorant (Rayleigh’s test (Schmidt-Koenig 1975): a =
and the darter is water absorbent for efficient 0.67;p<0.01; n= 70 forthe little cormorant; a =
underwaterswimmingthroughreducedbuoyancy 0.84; p<0.01; n= 38 forthe darter). In the weak
(Ruke 1968, Siegfried et al. 1975, Hennemann morning light before 0700hrs, and in the flat
1984). On the assumption that the spread-wing evening sun after 1700 hrs (Fig.l), only one
posture, which enlarges the area for absorption cormorant and twenty-five darters (21.3% of’
of solar radiation, does serve to dry the wings, 122 observations) showed the spread-wmg
the question arises: How do the birds dry their posture.
wings in the shade, or in the absence ofdirect
sunlight? This study analyzes behavioural 30 October 1990
adaptation in these circumstances.
The studywas carried outintheKeoladeo
National Park, Bharatpur, Rajasthan, India. The
area has a typical dry tropical monsoon climate
(Ewans 1989, Scott 1989). Observations of
spread-wing behaviour of little cormorants
Phalacrocorax niger, and darters Anhinga rufa,
were made from October 26 to November 2,
1990, during calm weather. Binoculars (10x40)
and a compass were used to determine the
orientationofthebirdstoincidentsunlight.Birds
were noted as being exposed to the sun or as
sitting in the shade: periods when there was no
sun (before sunrise and after sunset) were also
noted. Steady spread-wing posture and active
fanning(“fan-drying”)spread-wingposturewere
treated as separate. Ambient temperature was Fig. 1: Ambient temperature
JOURNAL, BOMBAYNATURAL HISTORYSOCIETY, 98(2),AUG. 2001 273
MISCELLANEOUS NOTES
Fan-dryingwas observedmore frequently interpreted as a strategy to relieve the bird of
in the late afternoon than in the rest ofthe day. the necessity of fanning its wings (Kortlandt
There were significant differences (x2 - 10.8; 1940). In the absence of windy conditions,
p<0.01; df= 1; n30)forbothspeciescombined, however, most species orient themselves
between times before and after 1600 hrs. perpendicularly to the incident radiation
Fan-drying was also more frequent in the (Anhinga anhinga Hennemann 1982;
:
absence of incident sunlight (after sunset), as Phalacrocorax niger P. fuscicollis P. carbo
, , ;
well aswhenbirds were inthe shade (x2= 13.4; Winkler 1983;P. harrisi,P. auritus:Hennemann
pta<b0l.e01x;2'dfte=st1;snh=o2w7e)d.Ahitgwholybysitgwnoifciocnatnitng(exn2c=y P1.98a4f;ricP.anucsap: eSniseigfsr,iePd.etnealg.l,e1c9t7u5s),.PT.helurceisduultss,
52.1; p<0.01; df = 1; n = 111) associations of the present study show a significant
between spread-wing posture and direct correlation ofdirectional orientation to the sun
exposure to sun, and fan-drying and no direct incalmweatherandthusconfirmthe importance
sun exposure. of direct sunlight for heat absorption and
Wing flap frequency during fanning was wing-drying.
higher (t = 5.54; p<0.01; df = 64, Student’s In contrast to the relative volume of
t-test) for the little cormorant at 3.3 beats per literatureonthespread-wingposture, fan-drying
second (n = 14) than for the darter at 2.4 beats has been the subject of little study and is
per second (n = 52). mentionedonlyanecdotally(e.g. Portielje 1927,
There is now general agreement that Winkler 1983, Hennemann 1984, Simmons
sunning behaviour and the dark colour of the 1986). Fanning was not observed very often in
plumage ofdarters and cormorants serve to dry thepresentstudy, butwhen itwas, itwas almost
the wings (Simmons 1986). The spread-wing exclusively in the shade or after sunset. This
posture has, indeed, been described as the strongly suggests that lack of solar radiation,
wing-drying posture in response to the “wings which is necessary for drying the wings, is
wet” stimulus (Kortlandt 1940). Darters and compensated for by the active convection
cormorants extract full advantage from the inducedby fanning. It had also beenpostulated
heat absorbing qualities oftheir black plumage elsewhere (Hennemann 1982) thatAnhinga fan
by increasing the area exposed to the sun, wings and tail in preference to holding them
thus facilitating evaporation(Lusticketal.1978, steady under conditions of low solar radia-
1980). Wing-drying has been suggested, in tion. In addition, it was shown by van Rhijn
the American darter (Anhinga anhinga), as a (1977) that herring gulls (Larus argentatus
)
rapid thermoregulatory mechanism for improved evaporation by actively shaking their
re-establishmentofa layerofairnextto the skin feathers.
to conserve metabolic heat (Hennemann 1982). Thehigherfrequencyofwingflaps, ofthe
The drying function ofthe spread-wing posture little cormorant seems to be correlated with its
oflittle cormorants in Sri Lanka as a means of smaller body size compared to the darter (cf.
regaining airworthiness has also been Campbell & Lack 1985).
emphasized (Winkler 1983).
Several studies show that darters and ACKNOWLEDGEMENTS
cormorantsorienttothewindwhenthisis strong
(Siegfried et al 1975, Hennemann 1984, Themanagementofthe Keoladeo Ghana
.,
Winkler 1983). Orientation to the wind can be National Park, and especially Bholu Khan,
274 JOURNAL BOMBAYNATURAL HISTORYSOCIETY, 98(2), AUG. 200/
MISCELLANEOUSNOTES
are thanked for their administrative support July 17, 2001 GERALD DICK
and intellectual guidance. We also Institutfur Oko-Ethologie,
acknowledge the critical revision of a Altenburg 47, A-3573 Rosenburg, Austria
former draft of the manuscript as well IRENE WURDINGER
as the correction of the English language by Universitdt Hildesheim, Marienburger.
Dr. T. Wilson. Platz 22, D- 32 Hildesheim, Germany.
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WHITE STORKS CICONIA CICONIA ON MIGRATION
8.
Migrating white storks Ciconia ciconia lands. A railway line runs parallel to the
stopovertorestnearUdayampattivillage, about irrigationchannelforsomedistance. Therailway
5 km from Kalainzar Karunanidhi (KK) Nagar, crossing, a roadside temple and tile factory
Tiruchirapalli, Tamil Nadu. I have been seeing chimneys far beyond in the southeast are
them here for the last 11 years. The area is unmistakeable landmarks. On the ground, the
roughly bounded by road from KK Nagar to area appears as a loose rectangle, widening in
Pudukkottaiontheeast,bythecurvingirrigation the south, with a cart road running west to
channel in the north and west. It extends to the Vadugapatty. It is about 9 sq. km of mildly
southoverandbeyondthefallowandagricultural undulating, scrubjungle (about 30%) withgrass
JOURNAL, BOMBAYNATURAL HISTORYSOCIETY, 98(2),AUG. 2001 275