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Summary Various species of soaring birds were studied by following them in a motor‐glider, mainly over the Serengeti National Park, Tanzania. The characteristics of thermal convection in the study area are described in general terms. The two vulture species of the genus Gyps live by scavenging among the herds of migratory ungulates, especially Wildebeest. They are not territorial, and gather in large numbers on kills. When raising young they may be obliged by game movements to forage at long distances from their nests. Their cross‐country performance is adequate for a foraging radius of over 100 km in dry‐season conditions. Their ability to compete with Spotted Hyaenas is thought to depend partly on this factor and partly on an advantage in arriving early at kills. These two species appear to find food more by watching other vultures than by searching for it directly. The Lappet‐faced and White‐headed Vultures are thought to be sedentary, and to depend on thorough searching of a fixed foraging territory, rather than on following migratory game. They have lower wing loadings than the Gyps vultures, and were not seen cross‐country flying. They never gather in large numbers. The Hooded Vulture is a solitary nester, but it does fly across country, and does gather at kills. Vultures soar individually, and seem to be good at exploiting such phenomena as thermal streets. They do not travel in flocks. Tawny and Martial Eagles react positively to the glider, and are suspected of regarding it as potential prey. White Storks migrate between Europe and Africa, and also travel about within East Africa, by thermal soaring. They soar in flocks, and unlike vultures rely on co‐ordinated social behaviour to locate thermals. In choosing their route, they often fail to react to obvious weather signs. They enter cumulus clouds from the bottom when thermalling, but probably do not climb far above cloudbase. Marabou Storks soar individually, but also sometimes travel in flocks. When doing so, they show less lateral spreading than White Storks, which reduces the effectiveness of the flock as a thermal‐finding unit; on the other hand, they do seem to react to visible weather signs, like vultures or glider pilots. White Pelicans, which travel by thermal soaring between different lakes in the Rift Valley, show the most highly co‐ordinated social soaring behaviour. Unlike White Storks, they fly in formation even when circling. Storks and pelicans showed more signs of alarm when approached by the glider than did the vultures or birds of prey. This could be due to their being preyed upon in flight, for instance by Martial Eagles. The basis of conventional thermal cross‐country flying is outlined, and it is explained why the high wing loadings of the Gyps vultures are appropriate to their peripatetic habits. A method of thermal soaring without circling is discussed, and shown to be more readily feasible for small than for large birds. Some differences in soaring techniques between birds and glider pilots are interpreted in the light of this ca...

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Nine procellariiform species, covering a range of body mass exceeding 200: 1, were studied during a visit to Bird Island, South Georgia, with the British Antarctic Survey, in the 1979-1980 field season. Speed measurements were made by ornithodolite of birds slope-soaring over land, birds flying over the sea but observed from land, and birds observed from a ship. In the second group, which showed the least anomalies, lift coefficients corresponding to mean airspeeds were about 1 for albatrosses, decreasing to about 0.3 for the smallest petrels. All species increased speed when flying against the wind. The small species proceeded by flap-gliding, while the large ones flapped infrequently, and only in light winds. The small species flew lower than the larger ones, but this may be related to the fact that most of the observations were of birds flying into wind. The albatrosses ( Diomedea, Phoebetria ) and giant petrels ( Macronectes ) were found to have a ‘shoulder lock’, consisting of a tendon sheet associated with the pectoralis muscle, which restrained the wing from elevation above the horizontal. This arrangement was not seen in the smaller species, and was interpreted as an adaptation reducing the energy cost of gliding flight. The main soaring method in the large species appeared to be slope-soaring along waves. Windward ‘pullups’ suggestive of the classical ‘dynamic soaring’ technique were seen in large and medium-sized species. However, the calculated strength of the wind gradient would have been insufficient to maintain airspeed to the heights observed, and it was concluded that most of the energy for the pullups must come from kinetic energy, acquired by gliding along a wave in slope lift. Gliding downwind through the wind gradient should significantly increase the glide ratio, but this was not observed. Slope-soaring along moving waves in zero wind was recorded. The data were used to derive estimates of the average speeds that the different species should be able to maintain on foraging expeditions. Estimates of the rate of energy consumption were also made, taking into account the greater dependence on flapping in the smaller species, and on soaring in the larger ones. The distance travelled in consuming fuel equivalent to a given fraction of the body mass would seem to be very strongly dependent on mass. Comparison of the largest species ( Diomedea exulans ) with the smallest ( Oceanites oceanicus ) suggests that ‘range’, defined in this way, varies as the 0.60 power of the mass, although the relation is more complex than a simple power function.

Summary A theory is presented for calculating the relation between mechanical power required to fly and forward speed, for a bird flying horizontally. The significance of this for migration is explained, and quick methods are given (and summarized in the Appendix) for calculating key points on the curve. Speed ranges and effective lift: drag ratios are calculated for a number of different flying animals. Factors affecting migration range are discussed, and the effects of head‐ and tailwinds are considered. Still‐air range depends on effective lift: drag ratio, but not on size or weight as such. The relation of power required to that available from the muscles is considered. Small birds have a greater margin of power available over power required than large ones, and tend to run their flight muscles at a lower stress, or lower specific shortening, or both. There is an upper limit to the mass of practicable flying birds, represented approximately by the Kori Bustard Ardeotit kori. The effect of adding extra weight (food or fuel) is to increase both minimum‐power speed, and maximum‐range speed, in proportion to the square root of the weight, and to increase the corresponding powers in proportion to the three‐halves power of the weight. Birds up to about 750 g (fat‐free) can double their fat‐free mass, and still have sufficient power to fly at the maximum‐range speed. Larger birds are progressively more severely limited in the maximum loads they can carry, and this reduces their range. Many large birds migrate by thermal soaring, thus economizing on fuel at the expense of making slower progress. During a long flight both speed and power have to be progressively reduced as fuel is used up. A formula is given for calculating the still‐air range of a bird which does this in an optimal fashion. The only data required are the effective lift: drag ratio, and the proportion of the take‐off mass devoted to fuel. Increase of height has no effect on the still‐air range, but the optimum cruising speed (and power) is increased. The optimum cruising height is reached when the bird can absorb oxygen just fast enough to maintain the required power. The optimum height increases progressively as fuel is used up. No range is lost as a result of the work done in climbing to the cruising height.

We measured the air speeds of 31 bird species, for which we had body mass and wing measurements, migrating along the east coast of Sweden in autumn, using a Vectronix Vector 21 ornithodolite and a Gill WindSonic anemometer. We expected each species’ average air speed to exceed its calculated minimum-power speed (Vmp), and to fall below its maximum-range speed (Vmr), but found some exceptions to both limits. To resolve these discrepancies, we first reduced the assumed induced power factor for all species from 1.2 to 0.9, attributing this to splayed and up-turned primary feathers, and then assigned body drag coefficients for different species down to 0.060 for small waders, and up to 0.12 for the mute swan, in the Reynolds number range 25 000–250 000. These results will be used to amend the default values in existing software that estimates fuel consumption in migration, energy heights on arrival and other aspects of flight performance, using classical aeronautical theory. The body drag coefficients are central to range calculations. Although they cannot be measured on dead bird bodies, they could be checked against wind tunnel measurements on living birds, using existing methods.

With 1 plate and 3 figures in the text) A method is described of identifying lions from the patterns of vibrissa spots. "Reliability" of identification is discussed in terms of the probability of there being more than one individual with a given spot pattern in the study population, and it is shown how reliability can be. improved, when necessary, by adding together information from more than one source. Means of adapting the reliability calculation to identification in other species are indicated.

Externally mounted transmitters or loggers may adversely affect migration performance for reasons other than the effects of added mass. The added frontal area of a payload box increases drag, and if the box triggers separation of the boundary layer over the posterior body, the drag coefficient could also be increased, possibly by a large amount. Any such effects would lead directly to a decreased migration range and reduced energy reserves on completion of migration. We measured the body drag coefficients of Rose-coloured Starlings in the Seewiesen wind tunnel by the wingbeat-frequency method. The speed at which the wingbeat frequency passed through a minimum was taken to be an estimate of the minimum-power speed (V mp ), from which the body drag coefficient was calculated in turn. Dummy transmitter boxes were mounted on the bird's back by attaching them with Velcro to a sideloop harness pad. The pad alone projected 6 mm above the bird's back, and increased the drag coefficient by nearly 50%, as compared to the ''clean'' configuration with no harness. Adding boxes (square-ended or streamlined) produced no further significant increase in the drag coefficient, but the addition of a sloping antenna increased it to nearly twice the clean value. These increases are attributed to separation of the boundary layer over the posterior upper body, triggered by the payload. We then ran computer simulations of a particular Barnacle Goose, for which detailed information was available from an earlier satellitetracking project, to see how its migration range and reserves on arrival would be affected if its transmitter installation also caused flow separation and affected the body drag coefficient in a similar way. By representing the range calculation in terms of energy height, we separated the effect of the transmitter's mass, which reduces the fat fraction (and hence also energy height) at departure, from that of flow separation, which steepens the energy gradient. The effect of the mass is small, and increases only slightly with increasing distance, whereas a steeper energy gradient not only reduces the range but also reduces the reserves remaining on arrival, to an extent that increases with migration distance. Energy height is related to the fat fraction rather than the fat mass, and is therefore preferable to energy as such, for expressing reserves in birds of different sizes.