Northward Flight Research Articles

Early Cretaceous subduction initiation beneath southern Tibet caused the northward flight of India

Collision between the Indian and Eurasian plates formed the ~2500 ​km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 ​Ma and 130 ​Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arc-related igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.

Broad-Scale Weather Patterns Encountered during Flight Influence Landbird Stopover Distributions

The dynamic weather conditions that migrating birds experience during flight likely influence where they stop to rest and refuel, particularly after navigating inhospitable terrain or large water bodies, but effects of weather on stopover patterns remain poorly studied. We examined the influence of broad-scale weather conditions encountered by nocturnally migrating Nearctic-Neotropical birds during northward flight over the Gulf of Mexico (GOM) on subsequent coastal stopover distributions. We categorized nightly weather patterns using historic maps and quantified region-wide densities of birds in stopover habitat with data collected by 10 weather surveillance radars from 2008 to 2015. We found spring weather patterns over the GOM were most often favorable for migrating birds, with winds assisting northward flight, and document regional stopover patterns in response to specific unfavorable weather conditions. For example, Midwest Continental High is characterized by strong northerly winds over the western GOM, resulting in high-density concentrations of migrants along the immediate coastlines of Texas and Louisiana. We show, for the first time, that broad-scale weather experienced during flight influences when and where birds stop to rest and refuel. Linking synoptic weather patterns encountered during flight with stopover distributions contributes to the emerging macro-ecological understanding of bird migration, which is critical to consider in systems undergoing rapid human-induced changes.

High Migratory Survival and Highly Variable Migratory Behavior in Black-Tailed Godwits

Few studies have been able to directly measure the seasonal survival rates of migratory species or determine how variable the timing of migration is within individuals and across populations over multiple years. As such, it remains unclear how likely migration is to affect the population dynamics of migratory species and how capable migrants may be of responding to changing environmental conditions within their lifetimes. To address these questions, we used three types of tracking devices to track individual black-tailed godwits from the nominate subspecies (Limosa limosa limosa) throughout their annual cycles for up to five consecutive years. We found that godwits exhibit considerable inter- and intra-individual variation in their migratory behaviour across years. We also found that godwits had generally high survival rates during migration, although survival was reduced during northward flights across the Sahara Desert. These patterns differ from those observed in most other migratory species, suggesting that migration may only be truly dangerous when crossing geographic barriers that lack emergency stopover sites and that the levels of phenotypic flexibility exhibited by some populations may enable them to rapidly respond to changing environmental conditions.

Long-distance autumn migration across the Sahara by painted lady butterflies: exploiting resource pulses in the tropical savannah.

The painted lady, Vanessa cardui, is a migratory butterfly that performs an annual multi-generational migration between Europe and North Africa. Its seasonal appearance south of the Sahara in autumn is well known and has led to the suggestion that it results from extremely long migratory flights by European butterflies to seasonally exploit the Sahel and the tropical savannah. However, this possibility has remained unproven. Here, we analyse the isotopic composition of butterflies from seven European and seven African countries to provide new support for this hypothesis. Each individual was assigned a geographical natal origin, based on its wing stable hydrogen isotope (δ2Hw) value and a predicted δ2Hw basemap for Europe and northern Africa. Natal assignments of autumn migrants collected south of the Sahara confirmed long-distance movements (of 4000 km or more) starting in Europe. Samples from Maghreb revealed a mixed origin of migrants, with most individuals with a European origin, but others having originated in the Sahel. Therefore, autumn movements are not only directed to northwestern Africa, but also include southward and northward flights across the Sahara. Through this remarkable behaviour, the productive but highly seasonal region south of the Sahara is incorporated into the migratory circuit of V. cardui.

Coldness Triggers Northward Flight in Remigrant Monarch Butterflies

Each fall, eastern North American monarch butterflies (Danaus plexippus) migrate from their northern range to their overwintering grounds in central Mexico. Fall migrants are in reproductive diapause, and they use a time-compensated sun compass to navigate during the long journey south. Eye-sensed directional cues from the daylight sky (e.g., the horizontal or azimuthal position of the sun) are integrated in the sun compass in the midbrain central complex region. Sun compass output is time compensated by circadian clocks in the antennae so that fall migrants can maintain a fixed flight direction south. In the spring, the same migrants remigrate northward to the southern United States to initiate the northern leg of the migration cycle. Here we show that spring remigrants also use an antenna-dependent time-compensated sun compass to direct their northward flight. Remarkably, fall migrants prematurely exposed to overwintering-like coldness reverse their flight orientation to the north. The temperature microenvironment at the overwintering site is essential for successful completion of the migration cycle, because without cold exposure, aged migrants continue to orient south. Our discovery that coldness triggers the northward flight direction in spring remigrants solves one of the long-standing mysteries of the monarch migration.

Seasonally adaptive migratory headings mediated by a sun compass in the painted lady butterfly, Vanessa cardui

Many insects undertake long-distance migrations to exploit seasonally variable conditions at high latitudes, but the mechanisms used by migrants to select and maintain beneficial flight headings are poorly understood. Using computerized flight simulators, we performed controlled experiments to test the ability of an obligate migrant butterfly (Vanessa cardui) to orient in seasonally advantageous directions (i.e. northwards in spring and southwards in autumn). We also investigated the compass mechanism used to select and maintain these headings. Laboratory-reared autumn-generation butterflies flown in the U.K. displayed a highly significant mean orientation towards the south-southwest, consistent with return migration to winter breeding sites. However, seasonally adaptive flight headings were not observed in wild-caught adults flown at the same time. Spring-generation adults caught in Gibraltar (presumed to be migrating from winter breeding sites in North Africa into Europe) showed no evidence of northward flight headings, but produced a wide scatter of flight headings with a mean direction towards the west. Butterflies flown in the simulators when the sky was not visible produced a random scatter of flight headings and less-directed flight tracks, providing evidence that migrating V. cardui use a sun compass to select and maintain their flight headings. However, when butterflies were subjected to a 6 h clock shift, no change in orientation was observed relative to the control group. Field evidence for a return migration in autumn by V. cardui is surprisingly scarce in the literature, but we conclude that the species does attempt such southward movements and that individuals use a sun compass to select their migratory heading.

Blackcap Warblers Maintain Digestive Efficiency by Increasing Digesta Retention Time on the First Day of Migratory Stopover

Almost all of the internal organs of migrating birds undergo pronounced mass changes, but the digestive tract changes most and fastest. The masses of the small intestine and the liver may be reduced by as much as 50% during migratory flight, indicating extreme phenotypic flexibility. Birds must rebuild these organs during stopovers to facilitate rapid mass gain and fuel deposition for continuation of migration. Laboratory studies indicate that birds may vary mean gut retention time to maintain high digestive efficiency, even while intestine length is substantially reduced. We examined migratory blackcaps (Sylvia atricapilla) after they performed a northward flight across the Sahara Desert in spring, and we subjected them to a 5-d artificial stopover. Body mass (m(b)) changes over the 5-d period resembled the typical mass change pattern of blackcaps stopping over naturally, with a small increase on the first day and a subsequent peak in the rate of m(b) gain on day 2 of the stopover. By day 5 of the stopover, the rate of m(b) gain had decreased to the point that it was not significantly different from that on day 1, presumably because the digestive tract had been rebuilt by this time. The same pattern was observed for food intake rates and amount of excreta produced, while digestive efficiency remained constant throughout the five experimental days. In contrast, mean retention time on stopover day 1 was significantly higher than it was on day 5 (106 min vs. 53 min; P = 0.031). Thus, after a sustained migration period, during a stopover blackcaps apparently compensate for reduced digestive tract size by processing food for twice as long, thereby maintaining high digestive efficiency.

Equatorial convergence of India and early Cenozoic climate trends

India's northward flight and collision with Asia was a major driver of global tectonics in the Cenozoic and, we argue, of atmospheric CO 2 concentration ( p CO 2 ) and thus global climate. Subduction of Tethyan oceanic crust with a carpet of carbonate-rich pelagic sediments deposited during transit beneath the high-productivity equatorial belt resulted in a component flux of CO 2 delivery to the atmosphere capable to maintain high p CO 2 levels and warm climate conditions until the decarbonation factory shut down with the collision of Greater India with Asia at the Early Eocene climatic optimum at ≈50 Ma. At about this time, the India continent and the highly weatherable Deccan Traps drifted into the equatorial humid belt where uptake of CO 2 by efficient silicate weathering further perturbed the delicate equilibrium between CO 2 input to and removal from the atmosphere toward progressively lower p CO 2 levels, thus marking the onset of a cooling trend over the Middle and Late Eocene that some suggest triggered the rapid expansion of Antarctic ice sheets at around the Eocene-Oligocene boundary.

Biogenic features in calcretes developed on mudstone: Examples from Paleogene sequences of the Himalaya, India

Calcrete occurring in the Paleogene sequences of the western Himalayan foreland basin offers very useful information regarding climate and paleogeography of northern part of the Indian subcontinent. These calcretes possess variety of biogenic features (beta fabrics) that include rhizoliths, alveolar septal and vermiform fabric and Microcodium. The Microcodium rosettes show filamentous structures. The rhizoliths and alveolar septal fabric formed by the precipitation of carbonate in soil vadose zone and the vermiform fabric may be a root mat. Perhaps microbes helped in dissolution of carbonate fillings and they were instrumental in precipitation of calcite in the form of rosettes and filaments. Here, the filamentous structure in the Microcodium is significant. We suggest that the Microcodium formed as a result of fungal dissolution and microbial mediation. The lighter values of δ 13C and δ 18O (< − 8.5‰) in the studied calcretes suggest that they developed through meteoric water in the presence of soil organic matter within the dry subtropical climatic zone when the northern part of the Indian subcontinent moved out of wet equatorial climate during its northward flight.

Improving our Understanding of Helicoverpa zea Migration in the Midwest: Assessment of Source Populations

Corn earworm (Helicoverpa zea) (CEW) generally are not able to overwinter annually in the Upper Midwest, thus they must migrate northward to affect the primary vegetable-growing locations in this area. Migration of CEW is highly dependent on the weather. Provided a source region in the southern United States, migration of CEW from south to north appeared to occur when a high-pressure system was located in the eastern United States and a low-pressure system and attendant frontal boundary was observed to the east of the Rocky Mountains and/or northern plains states. Southerly wind flow located west of the high and east of the low forms an “insect pump.” Convection cells allowed CEW to lift and advance from their source region northward in the low-level insect pump. When CEW encountered an area of downward motion during their northward flight, such as a frontal boundary or a light precipitation area, they had a tendency to “drop out” of the sky into fields below, an area referred to as the “Drop Zone.” Favorable weather patterns for CEW migration and subsequent infestation of fields in north-central Illinois were confirmed by high CEW trap counts at Rochelle over the last four decades. Accepted for publication 5 September 2006. Published 19 July 2007.

How Wandering Albatrosses use weather systems to fly long distances. 3. The contributions of Antarctic LOWs to eastward, southward and northward flight

When flying directly southwards, Wandering Albatrosses, Diomedea exulans, used the N—NW winds ahead of cold fronts associated with LOWS, returning northwards in the S—SW winds behind the fronts. Birds flew eastwards in the SW winds between a LOW and a following HIGH, and in NW winds. In westerlies, they flew northeast or south-east and such flights zigzagged across the ocean. Most of these flights were in weather systems in which Antarctic LOWS were a dominant component. Where an extensive stationary HIGH develops over the southern Pacific Ocean, a belt of accelerating winds can develop at the southern interface with circulating southern LOWS, and, in these stronger winds, very rapid eastward transoceanic flights are possible. One albatross flew 1730 km in 1.5 days. A summary is given on how Wandering Albatrosses use the winds of southern oceans to achieve long-distance flights.

Radar Observations of Arctic Bird Migration at the Northwest Passage, Canada

Bird migration was recorded by tracking radar and visual observation at 23 sites in the region of the Northwest Passage, between Baffin Island and Herschel Island in the Beaufort Sea. The studies took place during a ship-based expedition from 29 June to 3 September 1999. A total of 692 tracks (average duration 160 s) of bird flocks on postbreeding migration were recorded. Eastward migration was widespread, with the highest intensities at three sites in the southeastern Beaufort Sea. Mainly shorebirds were responsible for these movements, migrating along routes similar to great circles towards Nova Scotia and other parts of the Atlantic coast of North America (whence they depart on transoceanic flights towards South America). Some of the birds in this migration system probably originated from Siberia, as indicated by high-altitude eastward migration at a site 100 km north of the coast in the Beaufort Sea. Another category of eastward migrants consisted of jaegers, terns, and red phalaropes traveling towards the Davis Strait region and into the Atlantic Ocean. Southward migration was recorded at Baffin Island. A westward migration was pronounced at King William Island (with simultaneous eastward migration) and Amundsen Gulf, while northward movements were important at Banks Island and Melville Island. Apart from westward moult migration of common eiders at Amundsen Gulf, the westward and northward tracks reflected mainly jaegers, terns, gulls, and red phalaropes. These birds were probably making northward flights to exploit pelagic food resources in waters where the ice had recently broken up before their westward migratory exodus from the Arctic region towards the Pacific Ocean. The mean altitude of migration was 793 m, with 27% of all tracks above 1000 m and a maximum height of 3.95 km. The altitude distribution was clearly lower than those of the corresponding migration in Siberia and the shorebird migration at Nova Scotia. The average ground speed (14.9 m/s) was only slightly faster than the mean air speed (13.8 m/s), and migrants gained in speed from the winds (ground speed exceeding air speed) in only 55% of all cases. This means that wind assistance in the study area was much less pronounced than that documented for the migrants in Siberia and Nova Scotia. Bird migration at the Northwest Passage may be characterized by, on average, lower altitudes, less favourable winds, shorter flight steps, and a more widespread accessibility to stopover sites than migration at the Northeast Passage.

Butterfly Migrations in Florida: Seasonal Patterns and Long-Term Changes

Flight traps at Gainesville, in north-central Florida, operating from 1984 to spring 2000, separated butterflies migrating into the Florida peninsula (SSE ± 90°) from those migrating out of the Florida peninsula (NNW ± 90°). Five species flew southward in the fall and northward in the spring: Phoebis sennae (L.), Agraulis vanillae (L.), Junonia coenia (Hübner), Urbanus proteus (L.), and Eurema lisa (Boisduval & LeConte). Five species had significant northward flights in spring but no significant migration in fall: Pieris rapae (L.), Vanessa virginiensis (Drury), Vanessa atalanta (L.), Eurytides marcellus (Cramer), and Libytheana bachmanii (Kirtland). Danaus plexippus (L.) had a southward flight in fall but no significant migration in spring. Eurema daira (Godart) switched from a net movement northward in early fall to a net movement southward in late fall, whereas Eurema nicippe (Cramer) maintained a net movement northward throughout the fall. The major migrants differed significantly in the seasonal timing and duration of peak migration. When the numbers trapped were greatest, the proportion of those flying in the migratory direction was greatest. The numbers of spring migrants of A. vanillae increased during the course of the study, whereas both the spring and fall migrations of J. coenia declined. The fall migrations of P. sennae and U. proteus declined sharply. In 1990–1999, the fall migrations of P. sennae and U. proteus averaged only 37 and 15% of what they had averaged in 1984–1989. Reduced planting of soybeans in source areas for migrants probably contributed to these steep declines.

Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin

The sedimentary history of the Nepal Tethys Himalaya began with deposition of thick carbonates in the Cambro?–Ordovician, followed by a mixed siliciclastic–carbonate epicontinental succession recording two major deepening events in the Early Silurian and Late Devonian. Fossiliferous carbonate ramp deposits in the Tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins (“rift stage”). Break-up in the mid-Early Permian, locally associated with effusion of tholeiitic lava flows, was followed by a transgressive sandy to shaly, locally coal-bearing or bioclastic unit capped by condensed pelagic carbonates in the Middle to Late Permian (“juvenile ocean stage”). Subsidence of the cooling stretched crust led close to bathyal water depths in the Olenekian, but then slowed down in the Middle Triassic to increase again sharply in the Late Triassic owing to renewed extensional tectonic activity and sediment loading during up- and out-building of the Indian continental terrace. Deposition of tropical platform carbonates finally became widespread in the middle Liassic (“mature passive margin stage”). The initial fragmentation of Gondwana in the Middle Jurassic led to rejuvenation of the Indian craton and deposition of quartzo-feldspathic hybrid arenites, capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late Bathonian/middle Callovian. Next, a discontinuous pelagic grey marly limestone unit was followed by the ammonoid-rich offshore Spiti Shale in the Late Jurassic. The final disintegration of Gondwana began in the Berriasian, when quartzose siliciclastics derived again from the rejuvenated Indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the Valanginian? followed in the Hauterivian to Albian by more felsic differentiates such as the trachyandesites exposed in the Lesser Himalaya 120 km to the south. A widespread drowning episode, fostered by waning volcaniclastic supply during a global eustatic rise, is documented by a major glauconitic horizon deposited at middle southern latitudes in the late Albian, overlain by “Scaglia-like” pelagic limestones in the latest Albian. The final part of sedimentary history, during the rapid northward flight of India and its collision with Eurasia, is not documented anywhere in Nepal due to later erosion of Upper Cretaceous to Lower Tertiary strata.

Resources for Long-Distance Migration of Knots Calidris canutus islandica and C. c. canutus: How Broad Is the Temporal Exploitation Window of Benthic Prey in the Western and Eastern Wadden Sea?

In the course of each spring, two subspecies of knots Calidris canutus (islandica wintering in Europe and breeding in the Nearctic, and canutus wintering in west Africa and breeding in Siberia), stage in the international Wadden Sea before their northward flights to the arctic breeding grounds. In March-April islandica-knots predominantly use the intertidal flats in the western part of the Wadden Sea in The Netherlands (canutus is still in west Africa). In May both subspecies use the eastern part of the Wadden Sea in Schleswig-Holstein, Germany only. We studied these knots in core areas at both ends of the Wadden Sea in 1990, to examine whether the absence of knots in the western Wadden Sea in May is related to food resources or to other factors connected with the energy budget of these long-distance migrants. Islandica-knots in March-April in The Netherlands and canutus-knots in May in Germany had similar diets and showed similar size selection of their major bivalve prey Macoma balthica, giving the possibility for indirect competitive interactions. Knots spent more time feeding per day in Germany. The harvestable biomass of Macoma was of comparable magnitude in both parts of the Wadden Sea in May. That knots always leave the western Wadden Sea in late April is, therefore, not a direct response to prey depletion. If Macoma always subsides earlier in the year in the western part of the Wadden Sea than in the east as in 1990, the knots' absence in the west may represent the riding of an eastward wave of good feeding conditions resulting from longitudinal variation in bivalve-prey biology. At least for canutus, the reliance in May on the eastern Wadden Sea may (additionally?) represent early movements - at relatively low body masses - towards the breeding areas to gain the benefits of a small saving on the cost and time of travel, and of being closer to the weather systems that may help their flight into the breeding areas.

Stratigraphic evolution of Mesozoic continental margin and oceanic sequences: Northwest Australia and northern Himalayas

During Mesozoic time the Northwest Australian continental margin and the Thakkhola region of North Central Nepal were “adjacent” passive margins, off Northeast Gondwana. The relatively sediment-starved basins provide excellent opportunities for studying the early structural and depositional evolution of the Indian Ocean, during the Triassic/Jurassic rifting stages (Neo-Tethys), and the Jurassic/Cretaceous transition from rifting to drifting. Nepalese and Australian paleomagnetic studies indicate subtropical paleolatitudes during the Late Triassic to Early Jurassic, but higher latitudes of 35–40°S during the Late Jurassic to Early cretaceous, prior to the sustained northward flight of Australia and Greater India in the Late Cretaceous to Cenozoic. The latitudinal position of the basins and type of sediment are related. Principal continental margin formation along the northern edge of eastern Gondwana started in late Permian to Triassic time. By late Triassic-early Jurassic time platform carbonates with thin, lagoonal shales were laid down in a subtropical climate. The Carnian to “Rhaetian” siliciclastics and carbonates show repeated shallowing-upward sequences. Subsequent southward drift of “Greater India” and Australia during mid-Jurassic time replaced carbonates with more siliciclastic sediment input. Widespread erosion was caused during local uplift of parts of the Northwest Australian continental margin as a result of Jurassic late-rift block faulting. A mid-Callovian-early Oxfordian hiatus in Nepal is a submarine condensed sequence and non-tectonic in origin. In Nepal, the overlying 250 m thick organic-rich dark shales, which are correlated to the Oxfordian/Kimmeridgian clays of circum-Atlantic hydrocarbon bearing basins, can be traced along the northern Himalayan range. These shales probably represent an extensive continental slope deposit formed in a “high-productivity of organics” belt. The diverse foraminiferal microfauna of the belt was previously only known from boreal Laurasia. The Callovian “break up” uncorformity off Northwest Australia is actually a post-rift unconformity and precedes the onset of seafloor spreading by about 5–10 m.y. Seafloor spreading, leading to formation of the present Indian Ocean, started in the Argo Abyssal Plain around 155 Ma ago, in late Jurassic time. Australia and Greater India, including Northern Nepal, separated as early as in the late Valanginian, about 130 Ma ago. Altered ash layers off Northwest Australia record an important volcanic phase in Berriasian-Valanginian time. Mafic volcaniclastics in Nepalese deltaic sediments also testify to continental margin volcanic activity, which may be a precursor to the slightly younger Rajmahal traps in eastern India. An important tectonic event off Northwest Australia took place in Aptian time, 120-115 Ma ago, when hemipelagic sedimentation changed gradually into more pelagic sedimentation, with sharply decreasing rates of deposition. In Nepal, this marks the transition from coarse to fine-grained, organic-rich terrigenous clastics, leading eventually to more carbonate-bearing strata during the mid-Cretaceous global sealevel highstand.

Sedimentary record of the northward flight of India and its collision with Eurasia (Ladakh Himalaya, India)

— Stratigraphic and petrographic analysis of the Cretaceous to Eocene Tibetan sedimentary succession has allowed us to reinterpret in detail the sequence of events which led to closure of Neotethys and continental collision in the NW Himalaya.During the Early Cretaceous, the Indian passive margin recorded basaltic magmaüc activity. Albian volcanic arenites, probably related to a major extensional tectonic event, are unconformably overlain by an Upper Cretaceous to Paleocene carbonate sequence, with a major quartzarenite episode triggered by the global eustatic sea-level fall at the Cretaceous/Tertiary boundary. At the same time, Neotethyan oceanic crust was being subducted beneath Asia, as testified by calc-alkalic volcanism and forearc basin sedimentation in the Transhimalayan belt.Onset of collision and obduction of the Asian accretionary wedge onto the Indian continental rise was recorded by shoaling of the outer shelf at the Paleocene/Eocene boundary, related to flexural uplift of the passive margin. A few My later, foreland basin volcanic arenites derived from the uplifted Asian subduction complex onlapped onto the Indian continental terrace. All along the Himalaya, marine facies were rapidly replaced by continental redbeds in collisional basins on both sides of the ophiolitic suture. Next, foreland basin sedimentation was interrupted by fold-thrust deformation and final ophiolite emplacement.The observed sequence of events compares favourably with theoretical models of rifted margin to overthrust belt transition and shows that initial phases of continental collision and obduction were completed within 10 to 15 My, with formation of a proto-Himalayan chain by the end of the middle Eocene.

The nocturnal migration of the Australian plague locust, Chortoicetes terminifera (Walker) (Orthoptera: Acrididae): quantitative radar observations of a series of northward flights

AbstractNight migrations of insects above the surface boundary layer were studied by direct observations with an entomological radar, and by direct aerial sampling with a kite-borne net, in an area in New South Wales inhabited by large fledging populations of Chortoicetes terminifera (Wlk.). The aerial catches and radar echo-modulation observations suggested that the majority of migrants detected by the radar were locusts, and this conclusion was supported indirectly by light-trap catches and by observations of take–off at dusk. Night flights of locusts began at dusk with a mass take-off and continued on a diminishing scale until about midnight. Displacements were all approximately downwind and to the north, under the influence of a southerly airflow which prevailed throughout the six-day study period. A quantitative radar observation procedure was used to estimate locust aerial densities and migration rates, and to observe the variations of these quantities with height and time; typical and extreme values for all the principal migration quantities are presented. The flight paths of the locusts were estimated from radar observations of target tracks, and probable source areas at distances of up to 200 km to the south were identified; a major overflight which probably originated 150 km away was detected on one occasion. Northward movements of C. terminifera during anticyclonic conditions may help to explain how populations are redistributed in the periods between the long-range southward invasion movements for which this species is well-known.

Spreading history of the eastern Indian Ocean and India's northward flight from Antarctica and Australia: Discussion and reply

Spreading history of the eastern Indian Ocean and India's northward flight from Antarctica and Australia: Discussion and reply

Spreading history of the eastern Indian Ocean and India's northward flight from Antarctica and Australia: Discussion and reply

Spreading history of the eastern Indian Ocean and India's northward flight from Antarctica and Australia: Discussion and reply