Misplaced Pages

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75-530: The area was an important crossing point and oasis town in caravan routes to Oman as early as the third millennium BCE . A large number of burial mounds dating to the late third millennium BCE have been found at Yabrin, with morphologies suggesting a connection to the Dilmun Burial Mounds in Bahrain . The Yabrin oasis was historically noted for its position as the southern-most habitable point from Nejd at

150-471: A "central pool of open water surrounded by a ring of water-dependent shrubs and trees…which are in turn encircled by an outlying transition zone to desert plants." Rain showers provide subterranean water to sustain natural oases, such as the Tuat . Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to

225-1235: A barrier, and detours avoiding such barriers are observed. For example, brent geese Branta bernicla bernicla migrating between the Taymyr Peninsula and the Wadden Sea travel via low-lying coastal feeding-areas on the White Sea and the Baltic Sea rather than directly across the Arctic Ocean and the Scandinavian mainland. Great snipes make non-stop flights of 4,000–7,000 km, lasting 60–90 h, during which they change their average cruising heights from 2,000 m (above sea level) at night to around 4,000 m during daytime. A similar situation occurs with waders (called shorebirds in North America). Many species, such as dunlin Calidris alpina and western sandpiper Calidris mauri , undertake long movements from their Arctic breeding grounds to warmer locations in

300-552: A chalk cliff collapse "while he was a schoolboy at Brighthelmstone", though the man denied being an eyewitness. However, he writes that "as to swallows being found in a torpid state during the winter in the Isle of Wight or any part of this country, I never heard any such account worth attending to", and that if early swallows "happen to find frost and snow they immediately withdraw for a time—a circumstance this much more in favour of hiding than migration", since he doubts they would "return for

375-616: A chick on the Farne Islands in Northumberland off the British east coast, reached Melbourne , Australia in just three months from fledging, a sea journey of over 22,000 km (12,000 nmi), while another also from the Farne Islands with a light level geolocator tag 'G82' covered a staggering 96,000 km (52,000 nmi) in just 10 months from the end of one breeding season to

450-489: A current or past rest stop on a transportation route, or less-than-verdant location that nonetheless provides access to underground water through deep wells created and maintained by humans. Although they depend on a natural condition, such as the presence of water that may be stored in reservoirs and used for irrigation, most oases, as we know them, are artificial. The word oasis came into English from Latin : oasis , from Ancient Greek : ὄασις , óasis , which in turn

525-807: A particular route. For example, the oases of Awjila , Ghadames and Kufra , situated in modern-day Libya , have at various times been vital to both north–south and east–west trade in the Sahara Desert . The location of oases also informed the Darb El Arba'īn trade route from Sudan to Egypt, as well as the caravan route from the Niger River to Tangier , Morocco. The Silk Road "traced its course from water hole to water hole, relying on oasis communities such as Turpan in China and Samarkand in Uzbekistan." According to

600-411: A shovel is needed. Wells may or may not be equipped with pumps. Frequently the pumps are broken and useless, and a rope and bucket are then necessary to obtain water. Most of the wells in this region are less than 100 feet deep, but some are deeper, and 100 feet of rope is not too much to provide. As a rule the rope and bucket at a well, if they were ever provided, soon disappear, and one should never trust

675-464: A timing mechanism, instead moving in response to local weather conditions. Thus mountain and moorland breeders, such as wallcreeper Tichodroma muraria and white-throated dipper Cinclus cinclus , may move only altitudinally to escape the cold higher ground. Other species such as merlin Falco columbarius and Eurasian skylark Alauda arvensis move further, to the coast or towards the south. Species like

750-426: A week or two to warmer latitudes". Only at the end of the eighteenth century was migration accepted as an explanation for the winter disappearance of birds from northern climes. Thomas Bewick 's A History of British Birds (Volume 1, 1797) mentions a report from "a very intelligent master of a vessel" who, "between the islands of Menorca and Majorca , saw great numbers of Swallows flying northward", and states

825-534: Is a direct borrowing from Demotic Egyptian . The word for oasis in the latter-attested Coptic language (the descendant of Demotic Egyptian) is wahe or ouahe which means a "dwelling place". Oasis in Arabic is wāḥa ( Arabic : واحة ). Oases develop in " hydrologically favored" locations that have attributes such as a high water table , seasonal lakes, or blockaded wadis . Oases are made when sources of freshwater, such as underground rivers or aquifers , irrigate

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900-627: Is a much more complex phenomenon that may include both endogenous programs as well as learning. The primary physiological cue for migration is the changes in the day length. These changes are related to hormonal changes in the birds. In the period before migration, many birds display higher activity or Zugunruhe ( German : migratory restlessness ), first described by Johann Friedrich Naumann in 1795, as well as physiological changes such as increased fat deposition. The occurrence of Zugunruhe even in cage-raised birds with no environmental cues (e.g. shortening of day and falling temperature) has pointed to

975-483: Is alterable with selective breeding. Many migration routes of long-distance migratory birds are circuitous due to evolutionary history: the breeding range of Northern wheatears Oenanthe oenanthe has expanded to cover the entire Northern Hemisphere, but the species still migrates up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas. A migration route often does not follow

1050-567: Is an example of leap-frog migration . Many fully migratory species show leap-frog migration (birds that nest at higher latitudes spend the winter at lower latitudes), and many show the alternative, chain migration, where populations 'slide' more evenly north and south without reversing the order. Within a population, it is common for different ages and/or sexes to have different patterns of timing and distance. Female chaffinches Fringilla coelebs in Eastern Fennoscandia migrate earlier in

1125-695: Is clockwise migration, where birds flying North tend to be further West, and flying South tend to shift Eastwards. Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a V formation may conserve 12–20% of the energy they would need to fly alone. Red knots Calidris canutus and dunlins Calidris alpina were found in radar studies to fly 5 km/h (2.5 kn) faster in flocks than when they were flying alone. Birds fly at varying altitudes during migration. An expedition to Mt. Everest found skeletons of northern pintail Anas acuta and black-tailed godwit Limosa limosa at 5,000 m (16,000 ft) on

1200-651: Is evidence that this enables the migrants to obtain more of their preferred foods such as fruits. Altitudinal migration is common on mountains worldwide, such as in the Himalayas and the Andes . Dusky grouse in Colorado migrate less than a kilometer away from their summer grounds to winter sites which may be higher or lower by about 400 m in altitude than the summer sites. Many bird species in arid regions across southern Australia are nomadic; they follow water and food supply around

1275-527: Is inherently risky, due to predation and mortality. The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year. Some species of tubenoses , such as albatrosses , circle the Earth, flying over the southern oceans, while others such as Manx shearwaters migrate 14,000 km (8,700 mi) between their northern breeding grounds and

1350-480: Is integrated into its desert environment through an often close association with nomadic transhumant livestock farming (very often pastoral and sedentary populations are clearly distinguished). The fertility of the oasis soil is restored by "cyclic organic inputs of animal origin." In summary, an oasis palm grove is a highly anthropized and irrigated area that supports a traditionally intensive and polyculture-based agriculture. Responding to environmental constraints,

1425-503: Is known as "partial migration". Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory. In some species, the population at higher latitudes tends to be migratory and will often winter at lower latitude. The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. This

1500-423: Is the date palm ( Phoenix dactylifera L. ), which forms the upper layer . These palm trees provide shade for smaller understory trees like apricots , dates , figs , olives , and peach trees, which form the middle layer. Market-garden vegetables, some cereals (such as sorghum , barley , millet , and wheat ), and/or mixed animal fodder , are grown in the bottom layer where there is more moisture. The oasis

1575-474: The Bay of Fundy and Delaware Bay . Some bar-tailed godwits Limosa lapponica baueri have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas. Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey. Seabird migration is similar in pattern to those of the waders and waterfowl. Some, such as

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1650-688: The Bosphorus at migration times. More common species, such as the European honey buzzard Pernis apivorus , can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can cause funnelling, particularly of large diurnal migrants, as in the Central American migratory bottleneck. The Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth, created when hundreds of thousands of soaring birds avoid flying over

1725-519: The Khumbu Glacier . Bar-headed geese Anser indicus have been recorded by GPS flying at up to 6,540 m (21,460 ft) while crossing the Himalayas, at the same time engaging in the highest rates of climb to altitude for any bird. Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence. Seabirds fly low over water but gain altitude when crossing land, and

1800-689: The Red Sea or Persian Gulf . 23°15′47″N 48°58′52″E  /  23.263°N 48.981°E  / 23.263; 48.981 This article about the geography of Saudi Arabia is a stub . You can help Misplaced Pages by expanding it . Oasis In ecology , an oasis ( / oʊ ˈ eɪ s ɪ s / ; pl. : oases / oʊ ˈ eɪ s iː z / ) is a fertile area of a desert or semi-desert environment that sustains plant life and provides habitat for animals. Surface water may be present, or water may only be accessible from wells or underground channels created by humans. In geography, an oasis may be

1875-561: The Western Desert of Egypt ( Kharga , Dakhla , Farafra , Baharyia , and Siwa ) once had "flowing spring and wells" but due to the decline of groundwater heads because of overuse for land reclamation projects those water sources are no more and the oases suffer as a result. Morocco has lost two-thirds of its oasis habitat over the last 100 years due to heat, drought, and water scarcity . The Ferkla Oases in Morocco once drew on water from

1950-510: The black guillemot Cepphus grylle and some gulls , are quite sedentary; others, such as most terns and auks breeding in the temperate northern hemisphere, move varying distances south in the northern winter. The Arctic tern Sterna paradisaea has the longest-distance migration of any bird, and sees more daylight than any other, moving from its Arctic breeding grounds to the Antarctic non-breeding areas. One Arctic tern, ringed (banded) as

2025-778: The dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon . Some ducks, such as the garganey Anas querquedula , move completely or partially into the tropics. The European pied flycatcher Ficedula hypoleuca follows this migratory trend, breeding in Asia and Europe and wintering in Africa. Migration routes and wintering grounds are both genetically and traditionally determined depending on

2100-498: The turtle dove , the swift and the crane keep the time of their arrival." In the Pacific, traditional land-finding techniques used by Micronesians and Polynesians suggest that bird migration was observed and interpreted for more than 3,000 years. In Samoan tradition, for example, Tagaloa sent his daughter Sina to Earth in the form of a bird, Tuli, to find dry land, the word tuli referring specifically to land-finding waders, often to

2175-523: The 'tubenose' order Procellariiformes , are great wanderers, and the albatrosses of the southern oceans may circle the globe as they ride the " Roaring Forties " outside the breeding season. The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Many are among the longest-distance migrants; sooty shearwaters Puffinus griseus nesting on the Falkland Islands migrate 14,000 km (7,600 nmi) between

2250-840: The Black Sea surface and across high mountains. Birds of prey such as honey buzzards which migrate using thermals lose only 10 to 20% of their weight during migration, which may explain why they forage less during migration than do smaller birds of prey with more active flight such as falcons, hawks and harriers. From observing the migration of eleven soaring bird species over the Strait of Gibraltar, species which did not advance their autumn migration dates were those with declining breeding populations in Europe. Many long-distance migrants appear to be genetically programmed to respond to changing day length. Species that move short distances, however, may not need such

2325-480: The Ferkla, Sat and Tangarfa Rivers but they are now dry but for a few days a year. A 1920 USGS publication about watering holes in the deserts of California and Arizona gave this advice for travelers seeking oases: The usual watering places are springs or wells. Springs are frequently clogged with gravel or rubbish or sometimes even with the bodies of dead animals, and it may be necessary to clean them out. For this work

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2400-417: The Pacific golden plover. Aristotle, however, suggested that swallows and other birds hibernated. This belief persisted as late as 1878 when Elliott Coues listed the titles of no fewer than 182 papers dealing with the hibernation of swallows. Even the "highly observant" Gilbert White , in his posthumously published 1789 The Natural History of Selborne , quoted a man's story about swallows being found in

2475-622: The Southern Hemisphere) to overwinter; In contrast, among (pelagic) seabirds, species of the Southern Hemisphere are more likely to migrate. This is because there is a large area of ocean in the Southern Hemisphere, and more islands suitable for seabirds to nest. The control of migration, its timing and response are genetically controlled and appear to be a primitive trait that is present even in non-migratory species of birds. The ability to navigate and orient themselves during migration

2550-807: The United Nations, "Oases are at the very heart of the overall development of peri-Saharan countries due to their geographical location and the fact they are preferred migration routes in times of famine or insecurity in the region." Oases in Oman , on the Arabian Peninsula near the Persian Gulf , vary somewhat from the Saharan form. While still located in an arid or semi-arid zone with a date palm overstory, these oases are usually located below plateaus and "watered either by springs or by aflaj , tunnel systems dug into

2625-405: The autumn passage of southbound passerine migrants, which it feeds to its young. A similar strategy is adopted by the greater noctule bat , which preys on nocturnal passerine migrants. The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response. Within a species not all populations may be migratory; this

2700-766: The autumn than males do and the European tits of genera Parus and Cyanistes only migrate their first year. Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed flyways . These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different. A common pattern in North America

2775-669: The breeding colony and the North Atlantic Ocean off Norway . Some Manx shearwaters Puffinus puffinus do this same journey in reverse. As they are long-lived birds, they may cover enormous distances during their lives; one record-breaking Manx shearwater is calculated to have flown 8 million kilometres (4.5 million nautical miles) during its over-50-year lifespan. Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey such as vultures , eagles , and buzzards , but also storks . These birds migrate in

2850-483: The chaffinch are much less migratory in Britain than those of continental Europe, mostly not moving more than 5 km in their lives. Short-distance passerine migrants have two evolutionary origins. Those that have long-distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita , are species of southern hemisphere origins that have progressively shortened their return migration to stay in

2925-426: The chance of finding them there. Open wells are sometimes contaminated in the same way as springs and need to be cleaned out, particularly in little-frequented places where they are unused for months at a time. Migrating bird Bird migration is a seasonal movement of birds between breeding and wintering grounds that occurs twice a year. It is typically from north to south or from south to north. Migration

3000-435: The city [ medina ] or village [ ksar ] with its surrounding feeding source, the palm grove, within a relational and circulatory nomadic system." The location of oases has been of critical importance for trade and transportation routes in desert areas; caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on

3075-455: The composition of a migrating flock, and can sometimes encode the sex of a migrating individual, and to avoid collision in the air. Nocturnal migration can be monitored using weather radar data, allowing ornithologists to estimate the number of birds migrating on a given night, and the direction of the migration. Future research includes the automatic detection and identification of nocturnally calling migrant birds. Nocturnal migrants land in

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3150-548: The country in an irregular pattern, unrelated to season but related to rainfall. Several years may pass between visits to an area by a particular species. Sometimes circumstances such as a good breeding season followed by a food source failure the following year lead to irruptions in which large numbers of a species move far beyond the normal range. Bohemian waxwings Bombycilla garrulus well show this unpredictable variation in annual numbers, with five major arrivals in Britain during

3225-495: The daytime. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances. Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as the Strait of Messina , Gibraltar , Falsterbo , and

3300-509: The dull bluish or gray green of the feathery crowns of foliage. It is small wonder that a whole folklore of poetic legends and proverbs has grown up around the date palm in the regions where it flourishes. Many historic oases have struggled with drought and inadequate maintenance. According to a United Nations report on the future of oases in the Sahara and Sahel , "Increasingly   ... oases are subject to various pressures, heavily influenced by

3375-809: The earliest evidence of long-distance stork migration. This bird was referred to as a Pfeilstorch , German for "Arrow stork". Since then, around 25 Pfeilstörche have been documented. Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Migration is marked by its annual seasonality and movement between breeding and non-breeding areas. Nonmigratory bird movements include those made in response to environmental changes including in food availability, habitat, or weather. Sometimes, journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Non-migratory birds are said to be resident or sedentary. Approximately 1,800 of

3450-503: The effects of climate change , decreasing groundwater levels and a gradual loss of cultural heritage due to a fading historical memory concerning traditional water management techniques. These natural pressures are compounded by demographic pressures and the introduction of modern water pumping techniques that can disrupt traditional resource management schemes, particularly in the North Saharan oases." For example, five historic oases in

3525-412: The far south to support long-distance migration. The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. In addition, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in

3600-649: The garden resembles a tropical jungle. Very beautiful are these gardens in the spring, when the apricot and peach trees are in blossom here and there among the palms and the figs and vines are putting forth their leaves. In autumn, when the dates are ripening, the color effects, especially when the tops of the palms are lit up by the last rays of the setting sun, are something that once seen can never be forgotten. The great clusters of fruit, displaying every tint from bright yellow, through orange, vermilion, and maroon, to plum purple and chestnut brown, with their brilliant yellow or rich orange ivory-like stalks, contrast superbly with

3675-607: The ground or carved into the rock to tap underground aquifers." This rainwater harvesting system "never developed a serious salinity problem." In the drylands of southwestern North America, there is a habitat form called Palm Oasis (alternately Palm Series or Oasis Scrub Woodland) that has the native California fan palm as the overstory species. These Palm Oases can be found in California , Arizona , Baja California , and Sonora . People who live in an oasis must manage land and water use carefully. The most important plant in an oasis

3750-418: The local temperature to time their spring migration departure. Notably, departure responses to temperature varied between individuals but were individually repeatable (when tracked over multiple years). This suggests that individual use of temperature is a cue that allows for population-level adaptation to climate change . In other words, in a warming world, many migratory birds are predicted to depart earlier in

3825-508: The loss. The typical image of migration is of northern land birds, such as swallows (Hirundinidae) and birds of prey, making long flights to the tropics. However, many Holarctic wildfowl and finch (Fringillidae) species winters in the North Temperate Zone , in regions with milder winters than their summer breeding grounds. For example, the pink-footed goose migrates from Iceland to Britain and neighbouring countries, whilst

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3900-427: The morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short period between the origin and destination. Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. One cost of nocturnal migration is the loss of sleep. Migrants may be able to alter their quality of sleep to compensate for

3975-502: The most direct line between breeding and wintering grounds. Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land birds, such barriers could consist of large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (important for broad-winged birds). Conversely, in water-birds , large areas of land without wetlands offering suitable feeding sites may present

4050-458: The nineteenth century, but 18 between the years 1937 and 2000. Red crossbills Loxia curvirostra too are irruptive, with widespread invasions across England noted in 1251, 1593, 1757, and 1791. Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. This is because continental landmasses of the northern hemisphere are almost entirely temperate and subject to winter food shortages driving bird populations south (including

4125-536: The northern edge of the Rub' al Khali , and for its date production. The town was considered abandoned by the early modern era. In the 1940s it began to be resettled by Bedouins mostly with a Dawasir background. The economy is agricultural in nature with date orchards and waterwheel irrigation crop circles. The area is located in an area with natural water resources as opposed to the more common, central pivot irrigation projects sustained by desalinated ocean water from

4200-456: The northern hemisphere. Species that have no long-distance migratory relatives, such as the waxwings Bombycilla , are effectively moving in response to winter weather and the loss of their usual winter food, rather than enhanced breeding opportunities. In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for a food supply, but altitudinal migration occurs in some tropical birds. There

4275-688: The potential evapotranspiration of the areas was reduced by 30 to 50 percent within the oasis." The keystone date palm trees are "a main income source and staple food for local populations in many countries in which they are cultivated, and have played significant roles in the economy, society, and environment of those countries." Challenges for date palm oasis polycultures include "low rainfall, high temperatures, water resources often high in salt content, and high incidence of pests." The oases consist of almost unbroken forests of date palms, divided up into many gardens that are separated by mud walls and intersected by innumerable irrigation and drainage ditches… In

4350-515: The result clearly proves, what is in fact now admitted on all hands, that Swallows do not in any material instance differ from other birds in their nature and propensities [for life in the air]; but that they leave us when this country can no longer furnish them with a supply of their proper and natural food ... In 1822, a white stork was found in the German state of Mecklenburg with an arrow made from central African hardwood, which provided some of

4425-953: The reverse pattern is seen in land birds. However most bird migration is in the range of 150 to 600 m (490–2,000 ft). Bird strike Aviation records from the United States show most collisions occur below 600 m (2,000 ft) and almost none above 1,800 m (5,900 ft). Bird migration is not limited to birds that can fly. Most species of penguin (Spheniscidae) migrate by swimming. These routes can cover over 1,000 km (550 nmi). Dusky grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Emus Dromaius novaehollandiae in Australia have been observed to undertake long-distance movements on foot during droughts. During nocturnal migration ("nocmig" ), many birds give nocturnal flight calls, which are short, contact-type calls. These likely serve to maintain

4500-446: The role of circannual endogenous programs in controlling bird migrations. Caged birds display a preferential flight direction that corresponds with the migratory direction they would take in nature, changing their preferential direction at roughly the same time their wild conspecifics change course. Satellite tracking of 48 individual Asian houbaras ( Chlamydotis macqueenii ) across multiple migrations showed that this species uses

4575-428: The same hemisphere, but others such as semipalmated sandpiper C. pusilla travel longer distances to the tropics in the Southern Hemisphere. For some species of waders, migration success depends on the availability of certain key food resources at stopover points along the migration route. This gives the migrants an opportunity to refuel for the next leg of the voyage. Some examples of important stopover locations are

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4650-468: The seasonal comings and goings of birds. Aristotle wrote that birds transmuted into other birds or species like fish and animals, which explained their disappearance and reappearance. Aristotle thought many birds disappeared during cold weather because they were torpid , undetected in unseen environments like tree hollows or burrowing down in mud found at the bottom of ponds, then reemerging months later. Still, Aristotle recorded that cranes traveled from

4725-399: The shade of the palms are grown many other kinds of fruit trees—oranges, olives, figs, apricots, peaches, pomegranates, and jujubes—interlaced with large grape vines that often hang in festoons from the palm trunks. Beneath the trees are small plots of garden vegetables, barley, and alfalfa. Neither date palms nor other trees are planted with any regularity, and the growth is often so dense that

4800-509: The situation in Britain as follows: Swallows frequently roost at night, after they begin to congregate, by the sides of rivers and pools, from which circumstance it has been erroneously supposed that they retire into the water. Bewick then describes an experiment that succeeded in keeping swallows alive in Britain for several years, where they remained warm and dry through the winters. He concludes: These experiments have since been amply confirmed by ... M. Natterer , of Vienna ... and

4875-470: The social system of the species. In long-lived, social species such as white storks (Ciconia ciconia), flocks are often led by the oldest members and young storks learn the route on their first journey. In short-lived species that migrate alone, such as the Eurasian blackcap Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus , first-year migrants follow a genetically determined route that

4950-521: The southern ocean. Shorter migrations are common, while longer ones are not. The shorter migrations include altitudinal migrations on mountains, including the Andes and Himalayas . The timing of migration seems to be controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the Sun and stars, the Earth's magnetic field, and mental maps. Writings of ancient Greeks recognized

5025-689: The start of the next, travelling not just the length of the Atlantic Ocean and the width of the Indian Ocean, but also half way across the South Pacific to the boundary between the Ross and Amundsen Seas before returning back west along the Antarctic coast and back up the Atlantic. Many tubenosed birds breed in the southern hemisphere and migrate north in the southern winter. The most pelagic species, mainly in

5100-560: The steppes of Scythia to marshes at the headwaters of the Nile , an observation repeated by Pliny the Elder in his Historia Naturalis . Two books of the Bible may address avian migration. The Book of Job notes migrations with the inquiry: "Is it by your insight that the hawk hovers, spreads its wings southward?" The Book of Jeremiah comments: "Even the stork in the heavens knows its seasons, and

5175-664: The surface naturally or via man-made wells. The presence of water on the surface or underground is necessary and the local or regional management of this essential resource is strategic, but not sufficient to create such areas: continuous human work and know-how (a technical and social culture) are essential to maintain such ecosystems. Some of the possible human contributions to maintaining an oasis include digging and maintaining wells, digging and maintaining canals, and continuously removing opportunistic plants that threaten to gorge themselves on water and fertility needed to maintain human and animal food supplies. Stereotypically, an oasis has

5250-468: The surface. Any incidence of water is then used by migrating birds , which also pass seeds with their droppings which will grow at the water's edge forming an oasis. It can also be used to plant crops. Oases in the Middle East and North Africa cover about 1,000,000 hectares (10,000 km ), however, they support the livelihood of about 10 million inhabitants. The stark ratio of oasis to desert land in

5325-426: The three strata create what is called the " oasis effect ". The three layers and all their interaction points create a variety of combinations of "horizontal wind speed, relative air temperature and relative air humidity." The plantings—through a virtuous cycle of wind reduction, increased shade and evapotranspiration —create a microclimate favorable to crops; "measurements taken in different oases have showed that

5400-416: The tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. These advantages offset the high stress, physical exertion costs, and other risks of migration. Predation can be heightened during migration: Eleonora's falcon Falco eleonorae , which breeds on Mediterranean islands, has a very late breeding season, coordinated with

5475-624: The water into individual plots, soaking the soil." Oases often have human histories that are measured in millennia. Archeological digs at Ein Gedi in the Dead Sea Valley have found evidence of settlement dating to 6,000 BC. Al-Ahsa on the Arabian Peninsula shows evidence of human residence dating to the Neolithic . Anthropologically , the oasis is "an area of sedentary life, which associates

5550-400: The world means that the oasis ecosystem is "relatively minute, rare and precious." There are 90 “major oases” within the Sahara Desert. Some of their fertility may derive from irrigation systems called foggaras , khettaras , lkhttarts , or a variety of other regional names . In some oases systems, there is "a geometrical system of raised channels that release controlled amounts of

5625-409: The world's 10,000 bird species are long-distance migrants. Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south. Of course, in the southern hemisphere, the directions are reversed, but there is less land area in

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