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Thermal Soaring Compared in Three Dissimilar Tropical Bird Species, Fregata Magnificens, Pelecanus Occidentals and Coragyps Atratus

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Thermal Soaring Compared in Three Dissimilar Tropical Bird Species, Fregata Magnificens, Pelecanus Occidentals and Coragyps Atratus J. exp. Bhl. 102; 307-325, 1983 3Q7 tPrinted in Great Britain © Company of Biologists Limited 1983 THERMAL SOARING COMPARED IN THREE DISSIMILAR TROPICAL BIRD SPECIES, FREGATA MAGNIFICENS, PELECANUS OCCIDENTALS AND CORAGYPS ATRATUS . BY C. J. PENNYCUICK Department of Zoology, University of Bristol, Woodland Road, Bristol BS81UG (Received 12 July 1982—Accepted 3 September 1982) SUMMARY 1. All three species were observed in straight flight, and circling in ther- mals, from Flamenco Island, Panama. Measurements were made by or- nithodolite, an instrument which records a series of timed, three- dimensional position estimates, from which speeds, circle diameters and rates of climb can be calculated. 2. Mean lift coefficients in straight glides ranged from 0*72 to 0-84, except in slope soaring, where a mean of 1-6 was recorded for both the frigatebird and black vulture. 3. Mean circling radii were proportional to wing loading, and varied from 12'Om for the frigatebird to 18-0 m for the pelican. Mean rates of climb ranged from 0-fms"1 for the black vulture to 0-57 ms"1 for the pelican. All species showed mean circling lift coefficients between 1-33 and 1-45, and angles of bank between 22-9° and 24-7°. 4. It is argued that the frigatebird is adapted to stay airborne continuous- ly, day and night, for extended periods, by exploiting thermals over the sea under trade wind cumulus clouds. The low wing loading is seen as an adaptation to circling in narrow thermals, and the low disc loading as an adaptation giving low minimum power, when flapping cannot be avoided. Take-off appears to be possible only by dropping from an elevated perch, not from the ground or water surface. 5. The low aspect-ratio, tip-slotted wings of the black vulture are certain- ly less efficient for soaring, but appear to be better for upward take-off from the ground. The pelican also has tip-slotted wings. Although its aspect ratio is higher, it is still capable of taking off from a level water surface. INTRODUCTION This paper presents comparative observations, made in Panama, on three contrast- ing species of gliding birds. They were the magnificent frigatebird, Fregata mag- nificens, the brown pelican, Pelecanus ocddentalis, and the American black vulture, Coragyps atratus. Although differing in habits and morphology, all three species could be seen soaring in thermals and slope lift, and in straight flight, from the same observation point. A few observations of flapping flight in Bigua cormorants, •Phalacrocorax olivaceus, are also included. Key words: Soaring, tropical birds. 308 C. J. PENNYCUICK Study area and general behaviour Observations were made from the top of Flamenco Island, Panama, on 13 days between 21st May and 6th June 1980. Flamenco is the outermost of a series of three islands which are connected by a causeway, extending southwards from the mainland near Panama City, along the east side of the Pacific entrance of the Panama Canal. The nearby Naos Laboratory of the Smithsonian Tropical Research Institute provided a base for the project. Flamenco is a small, steep island. Its sides, although wooded, gave rise to a small area of slope lift in more or less any wind direction. The observation point was an exposed platform at the highest point on the north side of the island, about 90 m above sea level, giving a clear view to the north, and slightly obstructed views in other directions. Taboga, Taboguilla and Urava Islands, about 14 km away to the south, supported large numbers of pelican nests, and were also used as roosts by frigatebirds. Birds from these roosts flew past Flamenco Island each morning on their way to forage in Panama Bay, with a reverse movement in the evening. While these movements were in progress, a constant stream of pelicans and frigatebirds passed close to, or over, Perico and Flamenco Islands, making use of slope lift and thermals in the vicinity of the islands. Bigua cormorants were also nesting on the outer islands, and participated in these movements. Pelicans flew mostly in flocks, gliding or flap-gliding in vee or echelon formation. These flocks would maintain their coherence for short periods of slope soaring, all the birds in the flock turning together at the end of the slope. In thermals the flock structure gradually degenerated into a milling mass of pelicans circling in either direction. This behaviour differed from that recorded in African white pelicans (P. onocrotalus) (Pennycuick, 1972), and in European cranes (Grus grus) (Pennycuick, Alerstam & Larsson, 1980), which both maintained tight formations when circling. The only species seen behaving in this way at Flamenco Island was the Bigua cor- morant. Flocks of cormorants were seen circling in thermals on a few occasions, although the usual behaviour of this species was to fly straight through thermals or slope lift, maintaining flapping flight. Frigatebirds flew individually, with no apparent flock structure. They would circle in thermals and soar in slope lift, gliding whenever possible. They were also able to proceed in straight, continuous flapping flight when necessary. Black vultures did not follow the same flight lines as the other species. They were abundant over Panama City, about 5 km away, where several hundred of them could often be seen soaring at the same time. They also foraged on the beaches along the causeway, and this brought them to Flamenco Island. They were most often seen gliding, and would soar in thermals or slope lift together with the other species. Like the frigatebirds, they did not show any apparent flock structure in flight. Morphology Body measurements of the three study species are listed in Table 1. 'Wing span' (b) is the distance from tip to tip of the fully outstretched wings. 'Wing area' (S) is the projected area of both wings, together with the portion of the body included between Thermal soaring in tropical birds 309 •iem, according to the usual convention. Aspect ratio (A) and wing loading (Q) are derived from these as follows: A = b2/S (1) Q = mg/S (2) where m is the body mass, and g is the acceleration due to gravity. The sources of the morphological data were heterogeneous, and no great precision can be claimed for them. The black vulture figures are the means from two adult females trapped on nearby Ancon Hill in the course of a project directed by Dr Neal Smith of the Smithsonian Tropical Research Institute, and measured by the author. The brown pelican data are from one adult female found floating in Panama bay, which appeared on dissection to be recently dead and in good condition. Efforts to catch frigatebirds were unsuccessful, so the mass and wing area were taken to be the means for 10 individuals from Florida, recorded by Harrington, Schreiber & Woolfenden (1972). Although there is sexual dimorphism in this species, the sample sizes of flight observations were insufficient to investigate the effects of this on flight. The figures in Table 1 are the means for both sexes. Unfortunately Harrington et al. (1972) did not record wing spans. Aspect ratio was therefore measured directly, from six monochrome photographs of gliding frigatebirds, taken as nearly as possible from directly below. The wing span was then obtained by inverting Eqn. 1. Table 1. Body measurements Wing Wing Aspect Species Mass Span area loading ratio 2 r kg m m Nm" Frigatebird 1-52 2-29 0-408 36-5 12-8 Brown pelican 2-65 2-10 0-450 57-8 9-80 Black vulture 1-82 1-38 0-327 54-7 5-82 The differences of wing shape can be seen in the silhouettes of Fig. 1. These are derived from photographs of birds in flight, and are not exact projections of the wing planform. In particular the black vulture is slightly banked, which makes its aspect ratio appear even lower than it really is. Nevertheless the vulture does indeed have a wing of low aspect ratio and slight taper, in which the distal primaries are emar- ginated, so that a series of slots form at the tip when the wing is fully extended. This type of wing is typical of birds specialized for thermal soaring over land, including the New World vultures (Cathartidae), their Old World equivalents (Aegypiinae), storks (Ciconiidae) and cranes (Gruidae). The brown pelican has essentially the same type of wing, but the aspect ratio is much higher. The slotted wing tip is much less conspicuous in relation to the wing as a whole than in the vulture. In the frigatebird (another pelecaniform bird), the aspect ratio is higher still and the wing tapers to a Afiint, without slots. 310 C. J. PENNYCUICK Fig. 1. Silhouettes made from photographs of birds in flight. From top to bottom: Fregata mag- nificent, Pelecanus occidentals, Coragyps atratus. METHODS Flight measurements Measurements of speed and rate of climb were obtained by ornithodolite. A full description of this instrument has been published elsewhere (Pennycuick, 1982a). The instrument (Fig. 2) consisted of a rangefinder on an alt-azimuth mount, fitted with photoelectric digital encoders for azimuth, elevation and range. In use, it was aimed manually to centre the image of the bird in the rangefinder window. The rangefinder (a coincident image type) was set by means of a large handle, geared to allow the range knob to be turned through its full travel with a quarter-turn of the wrist. On pressing a button adjacent to the aiming handle, the current readings of azimuth, elevation and range, and the time, together constituting an 'observation', were stored in the memory of a battery-operated Nascom 1 microcomputer. Several observations were accumulated before the bird passed out of range of the instrument, and these together constituted a 'run'.
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