The magnitude and timing of migration by soaring raptors, pelicans and storks over Israel

The magnitude and timing of the autumn and spring migrations of 35 species of medium-and large-sized raptors, White Pelicans Pelicanus onocrotalus and White Storks Ciconia ciconia were studied in Israel. Observations were carried out from the ground by a line of observers covering most of the width of Israel across the line of migration and by radar. There was a high correlation between the counts obtained by ground observers and by radar. On average, about half a million raptors (mainly Lesser Spotted Eagles Aquila po-marina, Honey Buzzards Pernis apivorus and Levant Sparrowhawks Accipiter brevipes), 250,000 White Storks and 70,000 White Pelicans passed during autumn, and about a million raptors (mainly Honey Buzzards, Steppe Buzzards Buteo vulpinus, Steppe Eagles Aquila nipalensis and Black Kites Milvus migrans) and 450,000 White Storks passed during spring. Peak numbers were higher–over a million raptors and half a million White Storks. There was high interyear variation in the number of migrants recorded during the study, probably caused by weather and counting efforts. For some species, the whole world (Lesser Spotted Eagle and Levant Sparrowhawk) or Palaearctic (White Pelican) population passes over Israel during migration, allowing an estimate of the world populations of these species. Mean dates of arrival of most raptors are highly predictable, with confidence limits ranging between 1.5 and 5.5 days. The migration periods of White Storks and White Pelicans are longer and their mean day of appearance is less predictable (confidence limits range from 4.2 to 13.8 days). During autumn, 90% of the migrating populations of nocking species, such as Levant Sparrowhawk, Lesser Spotted Eagle, Honey Buzzard and Red-footed Falcon Falco vespertinus, pass within 13, 15, 16 and 18 days, respectively, while nonflocking species, such as Egyptian Vulture Neophron percnopterus, Marsh Harrier Circus aeruginosus and Short-toed Eagle Circaetus gallicus, generally take twice as long to pass. Similar passage periods were recorded in spring. For most species, the autumn migration period was longer than the spring migration period, probably because in autumn adults move before the young birds. Three factors affected the timing and spread of the migration wave: age at first breeding, diet and size of the breeding area.     

SOARING BEHAVIOUR AND PERFORMANCE OF SOME EAST AFRICAN BIRDS, OBSERVED FROM A MOTOR-GLIDER

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 calculation. A case in which Black Kites apparently used this technique to soar in random turbulence is described. The cross-country speed attainable by thermal soaring should be similar to the cruising speed under power in both large and small birds. Rough calculations of the energy costs suggest that a large bird (White Stork) should reduce its fuel consumption by a factor of 23 by soaring rather than flying under power, whereas this factor would be only 2–4 for a small bird (Bonelli's Warbler). Other reasons why thermal soaring is an advantageous means of travel for large but not for small birds are also indicated.     

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Thermal soaring birds reduce flight‐energy costs by alternatingly gaining altitude in thermals and gliding across the earth's surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high‐resolution GPS tracking data of migrating honey buzzards Pernis apivorus and wind data from a global numerical atmospheric model. We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time‐optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross‐country air speed over a full climb– glide cycle (V_opt). The second a risk‐averse energy‐efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (V_bgw). Honey buzzards were gliding on average 2.05 ms^{– 1} slower than V_opt and 3.42 ms^{– 1} faster than V_bgw while they increased air speeds with climb rates and reduced air speeds in tailwinds. They adopted flexible flight strategies gliding mostly near V_bgw under poor soaring conditions and closer to V_opt in good soaring conditions. Honey buzzards most adopted a time‐optimal strategy when crossing the Sahara, and at the onset of spring migration, where and when they met with the best soaring conditions. The buzzards nevertheless glided slower than V_opt during most of their journeys, probably taking time to navigate, orientate and locate suitable thermals, especially in areas with poor thermal convection. Linking novel tracking techniques with optimal migration models clarifies the way birds balance different tradeoffs during migration.     

Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics

The comparison of flight styles and flight parameters of migrating raptors in Israel revealed the following. (1) Climbing rate in thermal circling did not differ between species, indicating that chiefly the strength of thermal updrafts determined the climbing rate and that morphological features were less relevant. (2) In interthermal gliding, air speed was positively and gliding angle negatively related to the species' average body mass. Heavier species glided faster and had smaller gliding angles. (3) In soaring and gliding flight, cross-country speed relative to the air was positively related to the species' body mass; it was obviously the result of the gliding ability increasing with body mass. (4) Eagles and buzzards used soaring and gliding flight for more than 95% of the observation time. Additional soaring in a straight line whilst gliding was extensively used by the Steppe Eagle Aquila nipalensis, Lesser Spotted Eagle Aquila pomarina and Booted Eagle Hieraætus pennatus and even more frequently by the resident species, the Griffon Vulture Gyps fulvus and Shorttoed Eagle Circaetus gallicus. Smaller species, such as the Levant Sparrowhawk Accipiter brevipes, harriers (Circus sp.) and small falcons (Falco sp.). showed the highest proportion of flapping and gliding flight (9–33%). (5) In a comparison of the flight parameters and proportions of flight styles, a cluster analysis distinguished two main groups: The first consisted of Montagu's Harrier Circus pygargus, Pallid Harrier Circus macrourus, Levant Sparrowhawk and small falcons; their flight behaviour was characterized by both the high proportion of flapping and the low gliding performance. The second group comprised the typical soaring migrants: Steppe Eagle, Lesser Spotted Eagle, Booted Eagle, Steppe Buzzard Buteo buteo vulpinus, Honey Buzzard Pernis apivorus and Egyptian Vulture Neophron percnopterus, and they had very similar flight behaviour and were closely clustered. The Black Kite Milvus migrans and Marsh Harrier Circus aeruginosus were intermediate between typical soarers and flappers. The two resident species, Griffon Vulture and Short-toed Eagle, were grouped separately from the soaring migrants.     

Testing an emerging paradigm in migration ecology shows surprising differences in efficiency between flight modes

To maximize fitness, flying animals should maximize flight speed while minimizing energetic expenditure. Soaring speeds of large-bodied birds are determined by flight routes and tradeoffs between minimizing time and energetic costs. Large raptors migrating in eastern North America predominantly glide between thermals that provide lift or soar along slopes or ridgelines using orographic lift (slope soaring). It is usually assumed that slope soaring is faster than thermal gliding because forward progress is constant compared to interrupted progress when birds pause to regain altitude in thermals. We tested this slope-soaring hypothesis using high-frequency GPS-GSM telemetry devices to track golden eagles during northbound migration. In contrast to expectations, flight speed was slower when slope soaring and eagles also were diverted from their migratory path, incurring possible energetic costs and reducing speed of progress towards a migratory endpoint. When gliding between thermals, eagles stayed on track and fast gliding speeds compensated for lack of progress during thermal soaring. When thermals were not available, eagles minimized migration time, not energy, by choosing energetically expensive slope soaring instead of waiting for thermals to develop. Sites suited to slope soaring include ridges preferred for wind-energy generation, thus avian risk of collision with wind turbines is associated with evolutionary trade-offs required to maximize fitness of time-minimizing migratory raptors.     

Using a Doppler light detection and ranging (lidar) system to characterize an atmospheric thermal providing lift for soaring raptors

ABSTRACT.   Raptors and other large birds in soaring flight take advantage of upward drafts of air called thermals to maintain altitude with minimal flapping. I used a Doppler light detection and ranging (lidar) system to characterize a thermal in which raptors were soaring. Doppler lidar allows imaging of wind fields to reveal the structure of updrafts and downdrafts in a thermal. The thermal I monitored was in the form of a horizontal convective roll created at a transition from clear sky to partly cloudy sky, and gave both lift and lateral motion to the soaring birds. The thermal was 700 m high with a vertical wind speed that peaked at 3 m/s, so raptors could have soared to and maintained that altitude as the horizontal wind moved the thermal. My results suggest that imaging wind fields with Doppler lidar can be a useful tool for studying thermals and how they are used by soaring birds. An effective combination for further study of bird flight interaction with wind phenomena would be to add lidar measurements to an established means of tracking bird flight by radio or GPS transmitters, aircraft tracking, or radar.     

Wing size but not wing shape is related to migratory behavior in a soaring bird

Both wing size and wing shape affect the flight abilities of birds. Intra and inter‐specific studies have revealed a pattern where high aspect ratio and low wing loading favour migratory behaviour. This, however, have not been studied in soaring migrants. We assessed the relationship between the wing size and shape and the characteristics of the migratory habits of the turkey vulture Cathartes aura, an obligate soaring migrant. We compared wing size and shape with migration strategy among three fully migratory, one partially migratory and one non‐migratory (resident) population distributed across the American continent. We calculated the aspect ratio and wing loading using wing tracings to characterize the wing morphology. We used satellite‐tracking data from the migratory populations to calculate distance, duration, speed and altitude during migration. Wing loading, but not aspect ratio, differed among the populations, segregating the resident population from the completely migratory ones. Unlike what has been reported in species using flapping flight during migration, the migratory flight parameters of turkey vultures were not related to the aspect ratio. By contrast, wing loading was related to most flight parameters. Birds with lower wing loading flew farther, faster, and higher during their longer journeys. Our results suggest that wing morphology in this soaring species enables lower‐cost flight, through low wing‐loading, and that differences in the relative sizes of wings may increase extra savings during migration. The possibility that wing shape is influenced by foraging as well as migratory flight is discussed. We conclude that flight efficiency may be improved through different morphological adaptations in birds with different flight mechanisms.     

THE AUTUMN MIGRATION OF SOARING BIRDS AT THE BOSPHORUS

The migration of soaring birds was observed at Küçük Çamlica at the southern end of the Bosphorus between 14 July and 8 November 1966. Simultaneous watches were also carried out at other points on the Bosphorus on a number of dates. The largest movements of birds of prey occurred on days of light northeasterly winds, the largest movements of storks on days of light winds with a southerly component. On most days the stream of migrants appeared to be concentrated over the southern end of the Bosphorus. Migration frequently occurred right throughout the day, though the peak period was usually not spread over more than three hours. Figures are given for the daily times of migration of the commonest soaring birds. Daily counts of soaring birds (storks, raptors and Cranes) migrating over the Bosphorus at Küçük Çamlica are given. The main species found migrating were (with total number recorded in brackets) White Stork Ciconia ciconia (207,145), Black Stork C. nigra (6,194), Honey Buzzard Pernis upivorus (8,997), Buzzard Buteo buteo (12,949), "Spotted" Eagle Aquila clanga/pomarina (4,309) and sparrowhawk Accipiter nisus / brevipes (5,224). The autumn migration of 1966 is discussed in detail in a systematic list. Buzzards B. buteo were recorded in large numbers for the first time at the Bosphorus, and were the commonest bird of prey. Cranes Grus grus were also recorded for the first time. Comparison is made between our results and those of previous workers, though differences of coverage rule out any firm conclusions.     

SPRING MIGRATION AT THE BOSPHORUS

During observations made at the Bosphorus from 23 March to 6 April 1965, 3473 raptors were seen; the majority were Spotted/Lesser Spotted Eagles and Honey Buzzards. The migration of the main species is considered in some detail and attempts are made to correlate this with the prevailing weather. Comparisons are made with the records of observers both in spring and autumn at the Bosphorus and in spring at Gibraltar. Honey Buzzards pass in spring a month later at Gibraltar than at the Bosphorus. It is suggested that this is related to the timing of spring in the breeding areas to which these birds are returning. Notes on the observations of passerine movements and a night migration of Black Storks are also included.     

Gliding flight in the American Kestrel (Falco sparverius): An electromyographic study

Electromyographic (EMG) activity was studied in American Kestrels (Falco sparverius) gliding in a windtunnel tilted to 8 degrees below the horizontal. Muscle activity was observed in Mm. biceps brachii, triceps humeralis, supracoracoideus, and pectoralis, and was absent in M. deltoideus major and M. thoracobrachialis (region of M. pectoralis). These active muscles are believed to function in holding the wing protracted and extended during gliding flight. Quantification of the EMG signals showed a lower level of activity during gliding than during flapping flight, supporting the idea that gliding is a metabolically less expensive form of locomotion than flapping flight. Comparison with the pectoralis musculature of specialized gliding and soaring birds suggests that the deep layer of the pectoralis is indeed used during gliding flight and that the slow tonic fibers found in soaring birds such as vultures represents a specialization for endurant gliding. It is hypothesized that these slow fibers should be present in the wing muscles that these birds use for wing protraction and extension, in addition to the deep layer of the pectoralis. © 1993 Wiley-Liss, Inc.     

Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales

Given that soaring birds travel faster with supportive winds or in good thermal soaring conditions, we expect weather conditions en route of migration to explain commonly observed regional and seasonal patterns in the performance of soaring migrants. We used GPS‐loggers to track 13 honey buzzards and four Montagu's harriers for two to six migrations each. We determined how tailwinds, crosswinds, boundary layer height (a proxy for thermal convection) and precipitation affected hourly speeds, daily distances and daily mean speeds with linear regression models. Honey buzzards mostly travel by soaring while Montagu's harriers supplement soaring with flapping. Therefore, we expect that performance of harriers will be less affected by weather than for buzzards. Weather conditions explained between 30 and 50% of variation in migration performance of both species. Tailwind had the largest effect on hourly speeds, daily mean speeds and daily travel distances. Honey buzzards travelled significantly faster and farther, and Montagu's harriers non‐significantly faster, under better convective conditions. Honey buzzards travelled at slower speeds and shorter distances in crosswinds, whereas harriers maintained high speeds in crosswinds. Weather conditions varied between regions and seasons, and this variation accounted for nearly all regional and seasonal variation in flight performance. Hourly performance was higher than predicted at times when we suspect birds had switched to intermittent or continuous flapping flight, for example during sea‐crossings. The daily travel distance of Montagu's harriers was determined to a significant extent by their daily travel time, which differed between regions, possibly also due to weather conditions. We conclude with the implications of our work for studies on migration phenology and we suggest an important role for high‐resolution telemetry in understanding migratory behavior across entire migratory journeys.     

Foraging location and range of White-chinned Petrels Procellaria aequinoctialis breeding in the South Atlantic

The foraging range and principal feeding areas of White‐chinned Petrels breeding at South Georgia were determined using satellite telemetry. Foraging trips during incubation lasted 12–15 days and covered 3000–8000 km and 2–11 days and 1100–5900 km during chick‐rearing. Adults covered less distance per day during chick‐rearing (71 km) than during incubation (91 km) but the proportion covered at night (47%) was the same. Mean (31–34 km/h) and maximum (80 km/h) flight velocities were similar during both periods of the breeding season and during day and night. Between incubation shifts, White‐chinned Petrels travelled to the Patagonian shelf; during chick‐rearing they foraged more extensively. Most locations were between 30° to 55°W and 52° to 60°W around South Georgia/Shag Rocks and south to the South Orkney Islands. Diet samples from known foraging locations suggested birds fed mainly on krill and squid. They caught the squid Brachioteuthis? picta and Galiteuthis glacialis around Shag Rocks/South Georgia and also at sites close to the South Orkney Islands; Illex argentinus on the Patagonian shelf. Dispersal of adults after breeding failure was south to the South Orkney Islands then west to the Falkland Islands. This study confirms that breeding White‐chinned Petrels are amongst the widest‐ranging of seabirds; they may minimise competition with other Procellariiformes in the South Atlantic by their more extensive foraging range. The nature and extent of their range also brings substantial risk of high mortality rate in South Atlantic long‐line fisheries.     

Flight mode affects allometry of migration range in birds

Billions of birds migrate to exploit seasonally available resources. The ranges of migration vary greatly among species, but the underlying mechanisms are poorly understood. I hypothesise that flight mode (flapping or soaring) and body mass affect migration range through their influence on flight energetics. Here, I compiled the tracks of migratory birds (196 species, weighing 12–10 350 g) recorded by electronic tags in the last few decades. In flapping birds, migration ranges decreased with body mass, as predicted from rapidly increasing flight cost with increasing body mass. The species with higher aspect ratio and lower wing loading had larger migration ranges. In soaring birds, migration ranges were mass‐independent and larger than those of flapping birds, reflecting their low flight costs irrespective of body mass. This study demonstrates that many animal‐tracking studies are now available to explore the general patterns and the underlying mechanisms of animal migration.     

Cross sectional geometry of the forelimb skeleton and flight mode in pelecaniform birds

Avian wing elements have been shown to experience both dorsoventral bending and torsional loads during flapping flight. However, not all birds use continuous flapping as a primary flight strategy. The pelecaniforms exhibit extraordinary diversity in flight mode, utilizing flapping, flap‐gliding, and soaring. Here we (1) characterize the cross‐sectional geometry of the three main wing bone (humerus, ulna, carpometacarpus), (2) use elements of beam theory to estimate resistance to loading, and (3) examine patterns of variation in hypothesized loading resistance relative to flight and diving mode in 16 species of pelecaniform birds. Patterns emerge that are common to all species, as well as some characteristics that are flight‐ and diving‐mode specific. In all birds examined, the distal most wing segment (carpometacarpus) is the most elliptical (relatively high I_max/I_min) at mid‐shaft, suggesting a shape optimized to resist bending loads in a dorsoventral direction. As primary flight feathers attach at an oblique angle relative to the long axis of the carpometacarpus, they are likely responsible for inducing bending of this element during flight. Moreover, among flight modes examined the flapping group (cormorants) exhibits more elliptical humeri and carpometacarpi than other flight modes, perhaps pertaining to the higher frequency of bending loads in these elements. The soaring birds (pelicans and gannets) exhibit wing elements with near‐circular cross‐sections and higher polar moments of area than in the flap and flap‐gliding birds, suggesting shapes optimized to offer increased resistance to torsional loads. This analysis of cross‐sectional geometry has enhanced our interpretation of how the wing elements are being loaded and ultimately how they are being used during normal activities. J. Morphol., 2011. © 2011 Wiley‐Liss,Inc.     

CRANE GRUS GRUS MIGRATION OVER SEA AND LAND

The detailed process of Crane Grus grus migration over sea and land, respectively, was studied from films of a radar station in Skåne, southernmost Sweden, during the spring migratory periods in 1972 and 1973. The true air speed for travelling over the sea was 67 km h^-1, whereas over land the Cranes made use of thermal air to soar and gain height and the true air speed was 44 km h^-1. Soaring lasted on the average 6.3 min and the distance travelled between the soaring interludes was 13.3 km. True air speed during the flights between thermals was about 70 km h^-1. The Cranes compensated completely for wind drift over land, but only incomplete compensation took place over the sea. The angle between the Cranes' heading and track directions over the sea was composed of 68% compensation and 32% drift.     

THE PROBLEM OF NOMENCLATURE

Whitfield, A. K. &; Blaber, S. J. M. 1978. Feeding ecology of piscivorous birds at Lake St Lucia, Part 3: Swimming birds. Ostrich 50:10-20. The diets, foraging periodicities and feeding behaviour of the Reed Cormorant Phalacrocorax africanus, Whitebreasted Cormorant Phalacrocorax carbo and White Pelican Pelecanus onocrotalus were studied at Lake St Lucia, Natal, South Africa, during 1975 and 1976. The Reed Cormorant fishes in shallow water within 100 m of the shore and mainly caught Sarotherodon mossambicus and Solea bleekeri, while the Whitebreasted Cormorant caught Mugilidae, Rhabdosargus sarba and Thryssa vitrirostris in deeper water. The diet of the White Pelican followed three distinct phases: a pre-incubation phase when the birds followed and preyed heavily on migrating Mugil cephalus shoals; an incubation and post-incubation phase when the adults flew a round trip of 200 km to the north to obtain freshwater fish, mainly cichlids, from the Pongolo pans where fishes were concentrated and densities high; and a post-fledgling phase when both adults and juveniles fed on a variety of marine species of fish in Lake St Lucia for about a month before dispersing to other areas. The feeding and breeding of Reed Cormorants and White-breasted Cormorants is discussed in relation to wind speeds, water turbidity and flooding of backwaters. The diet and long-range foraging behaviour of White Pelicans at St Lucia are compared with data from other African lakes. The breeding season of White Pelicans at St Lucia is related to availability of fish and inaccessibility of the breeding site to predators. The latter is determined by lake levels.     

Weights and measures of Ugandan pelicans with some seasonal variations

Samples of White and Pink-backed Pelicans from the Rwenzori National Park, Uganda were weighed and measured. The results are given below as mean values in the order of male and female White Pelicans and male and female Pink-backed Pelicans. Only a few measurements were taken of female White Pelicans. Further details given in the Tables include sample size, standard deviation and range. Linear measurements (cm): total body length:— 177.3, 136.7, 146.9, 134.0; tail length:—18.1, -, 17.2, 16.6; tarsometatarsus:— 14.2, -, 9.6, 8.7: cu1men:— 41.8, 31.4, 35.7, 30.5; lower mandible:— 49.3, -, 40.3, 34.6; wing span:— 285.8, 246.4, 233.9, 224.2; wing length:— 136.5, -, 112.9, 105.8; standard wing length:— 70.2, -, 60.5, 56.0; chord:— 70.8, -, 35.5, 34.3; aspect ratio:— 4.0, -, 6.6, 6.6; weights (gm unless stated otherwise): total weight (kg);— 11.45, 7.59, 5.97, 4.92; adrenal (mg):— 647.2, 531–0, 401.2, 352.4; heart:— 100.1, -, 49.9, 39.5; kidney:— 40.9, -, 23.6, 23.4; liver:—258.2, 176.5, 128.5, 110.4; lung:— 60.3, –, 29.1, 30.0; pancreas:— 15.2, -, 7.1, 5.3; pectoralis major:— 731.9, -, 389.0, 313.9; pectoralis minor:— 51.8, -, 27.6, 24.3; pituitary (mg):— 40.6, 39.1, 37.4, 37.5; spleen:— 12.7, -, 12.3, 10.1. There was no significant difference between the species in the proportional weights of the organs. Seasonal variations were found in the weights of testes, ovaries, oviducts and fat deposits. Changes in weight of the reproductive organs of the Pink-backed Pelican showed a clear correlation with the breeding cycle. The testes of the migratory White Pelican appeared to be inactive while the birds were in the park. The amount of fat laid down could be directly related to seasonal fluctuations in the food supply. The degree of development of the head crest was greatest during the breeding season in the Pink-backed Pelican and during the second quarter of the year in the White Pelican. 58.6% of the White Pelicans showed traces of colouration on the breast.     

The gliding speed of migrating birds: slow and safe or fast and risky?

Aerodynamic theory postulates that gliding airspeed, a major flight performance component for soaring avian migrants, scales with bird size and wing morphology. We tested this prediction, and the role of gliding altitude and soaring conditions, using atmospheric simulations and radar tracks of 1346 birds from 12 species. Gliding airspeed did not scale with bird size and wing morphology, and unexpectedly converged to a narrow range. To explain this discrepancy, we propose that soaring‐gliding birds adjust their gliding airspeed according to the risk of grounding or switching to costly flapping flight. Introducing the Risk Aversion Flight Index (RAFI, the ratio of actual to theoretical risk‐averse gliding airspeed), we found that inter‐ and intraspecific variation in RAFI positively correlated with wing loading, and negatively correlated with convective thermal conditions and gliding altitude, respectively. We propose that risk‐sensitive behaviour modulates the evolution (morphology) and ecology (response to environmental conditions) of bird soaring flight.     

Exploring bird aerodynamics using radio-controlled models

A series of radio-controlled glider models was constructed by duplicating the aerodynamic shape of soaring birds (raven, turkey vulture, seagull and pelican). Controlled tests were conducted to determine the level of longitudinal and lateral-directional static stability, and to identify the characteristics that allowed flight without a vertical tail. The use of tail-tilt for controlling small bank-angle changes, as observed in soaring birds, was verified. Subsequent tests, using wing-tip ailerons, inferred that birds use a three-dimensional flow pattern around the wing tip (wing tip vortices) to control adverse yaw and to create a small amount of forward thrust in gliding flight.     

Energy requirements of migrating Great White Pelicans Pelecanus onocrotalus

The Great White Pelican Pelecanus anocrotalus is the largest migrating bird in Israel and is an endangered species. The Palearctic populations of the Great White Pelican breed in eastern Europe and Asia and most of them pass through the ‘bottleneck’ of Israel to wintering grounds in Africa. Natural feeding sites for pelicans have diminished during recent decades due to human activities, and sites of extensive aquaculture have become the favourite feeding places for wintering and migrating Great White Pelicans. The fish industry has reported a significant impact on fish yield and the conflict between pelicans and fishermen has escalated so that hundreds of pelicans have died in recent years from shooting or accidental electrocution. We approached this management problem by studying the energy requirements of the Great White Pelican during migration and while wintering in Israel, under different feeding regimes (fish or chicks) and in different seasons, in captivity. The results show that a captive bird consumes 1.1 kg of fish per day. The basal metabolic rate and apparent metabolized energy of the Great White Pelican are both higher than predicted from allometric equations. Energetic demands were quite stable on both diets (fish and chicks) and during both seasons (winter and summer). The fat deposits of migrating pelicans averaged 313.5 g compared with 480 g in wintering birds (3.4% and 5.4% of body mass, respectively). Based on these fat contents and on the measured daily energy consumption, we calculated that birds that do not feed in Israel can fly only up to 1620 km from Israel southward and could not cover the distance to their likely wintering grounds in the Sudd area in southern Sudan. However, birds that replenish their fuel reserves could fly up to 2460 km and hence could reach this area. Therefore, we conclude that Great White Pelicans must feed in Israel during the autumn migration in order to complete their journey to Africa. One solution to the conflict between pelicans and fishermen could be to combine deterrents preventing pelicans from feeding in fish‐ponds with the provision of attractive alternative reservoirs, to ensure regular food supplies during autumn.