What is migration
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Bird migration is driven primarily by availability of food and involves seasonal movements between breeding and wintering grounds. While some birds migrate long distances, others undertake shorter altitudinal migrations. Migration patterns include leap-frog migration where higher latitude populations winter at lower latitudes bypassing resident populations, as well as chain migration where populations shift ranges gradually. The timing and routes of migration are often genetically programmed but can also be learned behaviors that take advantage of geographic features and weather patterns to aid efficient travel.
What is migration
- 1. WHAT IS MIGRATION? Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Bird movements include those made in response to changes 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). Migration is marked by its annual seasonality. Non-migratory birds are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants.
- 2. 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 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. Also, 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 the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.
- 3. Bird migration is the regular seasonal movement, often north and south along a flyway between breeding and wintering grounds, undertaken by many species of birds. Migration, which carries high costs in predation and mortality, including from hunting by humans, is driven primarily by availability of food. Migration occurs mainly in the Northern Hemisphere where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea. Historically, migration has been recorded as much as 3,000 years ago by Ancient Greek authors including Homer and Aristotle, and in theBook of Job, for species such as storks, Turtle Doves, and swallows. More recently, Johannes Leche began recording dates of arrivals of spring migrants in Finland in 1749, and scientific studies have used techniques including bird ringing and satellite tracking. Threats to migratory birds have grown with habitat destruction especially of stopover and wintering sites, as well as structures such as power lines and wind farms.
- 4. The timing of migration is controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the sun and stars, the earth's magnetic field, and probably also mental maps. Migration has developed independently in different groups of birds and does not appear to require genetic change; some birds have acquired migratory behaviour since the last ice age.
- 5. REASONS OF MIGRATION Without a reason to migrate, birds would have even more challenging lives than making these excruciating journeys. If no birds migrated, food supplies in breeding areas would be rapidly depleted, and many chicks would starve. Competition for nesting sites would be fierce, and predators would be attracted to the high concentrations of breeding birds and easy meals of nestlings. It is for those two reasons – food and breeding – that birds migrate, but those reasons are far more complicated than they seem.
- 6. 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 (Procellariiformes) 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 the southern ocean. Shorter migrations are common, including altitudinal migrations on mountains such as the Andes and Himalayas.
- 7. MIGRATING FOR A MEAL For all birds, one of the principle driving forces behind migration is food scarcity. If all birds were to stay in the same tropical regions year-round, food would become scarce and breeding would be less successful. But as food sources are regenerating in the north each spring, millions of birds migrate to those areas to take advantage of the abundance. As the food supplies then dwindle in the fall, they return to replenished tropical regions. This pattern of migrating for a meal is true not only for neotropical migrants, but also short-range migratory birds that may move only short distances to pursue a food source. Bird irruptions are also the result of changes in the food supply, with greater irruptions occurring in years when food supplies are low for northern birds, forcing them to seek adequate food further south, well outside their typical range.
- 8. MIGRATING FOR FAMILY Over millennia, birds have evolved different migration patterns, timing and destinations to disperse around the world to breed, taking advantage of a wide variety of suitable conditions to raising their young. Those conditions can vary for every bird species, and may involve specific food sources, habitats that provide adequate shelter and breeding colonies that offer greater protection than a single pair of bird parents. It may seem contradictory to argue that birds migrate to help their offspring survive when many of those same bird parents quickly abandon their young as they mature, leaving the young birds to make their dangerous first migration without adult guidance. It is exactly because the birds have raised their chicks in a relatively rich, safe environment, however, that gives them the advantage of being prepared to make that journey.
- 9. MORE REASONS BIRDS MIGRATE Food may be the key to a regular migration, but birds migrate for other reasons related to helping their offspring survive, including... Climate: Birds have evolved different types of plumage to survive different climates, and changes in those climates can affect migration. Many birds leave the Arctic breeding grounds, for example, when temperatures begin to dip and they need more temperate habitat. Similarly, the hottest tropical regions can be a harsh environment for raising chicks, and it is advantageous to lay eggs further north. Predators: Habitats that have abundant food sources year-round also attract a greater number of predators that can threaten nests. Birds that migrate to different habitats can avoid that onslaught of predators, giving their young a better chance of reaching maturity. Disease: Any large group of birds crammed in one type of habitat is susceptible to parasites and diseases that can decimate thousands of birds in a short period of time. While diseases can and do occasionally devastate breeding colonies, birds that disperse to different locations have less chance of spreading a disease to their entire population, including their new offspring. In the end, the reasons why birds migrate all come down to survival – not of the migrating birds themselves, but of the chicks they will raise. Finding richer food sources, seeking safer habitats and avoiding predators are all migration behaviors designed to ensure breeding success, allowing the birds to survive for another generation and allowing birders the pleasure of witnessing another year's migration.
- 10. HOW DO THEY MIGRATE ? An Arctic Tern calling for it's mate. Birds all over the world are completing their migratory journeys. Sometimes their journeys are as long as 22,000 miles! How Do They Do That?! One bird in particular, the Arctic Tern, is able to migrate back to where it was born even if it has been separated from its parents. Many parents leave the nest before young birds are able to fly. Yet, even without parental guidance, the young birds are able to travel home. When they are alone, they travel in darkness and may even create landmarks when flying over water. Another theory is that the birds are somehow able to access the earth’s magnetic north/south pole lines. The young birds use the sun and these lines to safely guide themselves. However, scientists have not been able to prove that the birds use landmarks, magnetic lines, or some other guidance system. Reaching Their Destination When the bird’s reach their destination they are greeted by many other species of birds and are also able to breed. With large numbers of birds, these migration destinations are great places for predators to attack. The birds must be careful to avoid becoming prey. If the Arctic Tern survives, it can expect to live a long life (anywhere between 20 and 60 years!)
- 11. PATTERNS OF MIGRATION. Within a species not all populations may be migratory; this 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 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 order.
- 12. 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 is clockwise migration, where birds flying North tend to be further West, and flying South tend to shift Eastwards.
- 13. 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 per hour faster in flocks than when they were flying alone.
- 14. Birds fly at varying altitudes during migration. An expedition to Mt. Everest found skeletons of Pintail and Black-tailed Godwit at 5000 m (16,400 ft) on the Khumbu Glacier . Bar-headed Geese have been recorded by GPS flying at up to 6,540 metres 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 the reverse pattern is seen in land birds . However most bird migration is in the range of 150 m (500 ft) to 600 m (2000 ft). Bird-hit aviation records from the United States show most collisions occur below 600 m (2000 ft) and almost none above 1800 m (6000 ft).
- 15. Bird migration is not limited to birds that can fly. Most species of penguin migrate by swimming. These routes can cover over 1000 km. Blue Grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Emus in Australia have been observed to undertake long-distance movements on foot during droughts.
- 16. Aristotle however suggested that swallows and other birds hibernated. This belief persisted as late as 1878, when Elliott Coues listed the titles of no less 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 a chalk cliff collapse "while he was a schoolboy at Brighthelmstone", though the man denied being an eyewitness. However, he also 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 a week or two to warmer latitudes".
- 17. It was not until the end of the eighteenth century that migration as an explanation for the winter disappearance of birds from northern climes was accepted. Thomas Bewick's A History of British (Volume 1, 1797) mentions a report from "a very intelligent master of a vessel" who, "between the islands of Minorca and Majorca, saw great numbers of Swallows flying northward", and states 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.
- 18. ADAPTATION Birds need to alter their metabolism in order to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and require to be molted. The timing of this molt - usually once a year but sometimes two - varies with some species molting prior to moving to their winter grounds and others molting prior to returning to their breeding grounds. Apart from physiological adaptations, migration sometimes requires behavioural changes such as flying in flocks to reduce the energy used in migration or the risk of predation.
- 19. MIGRATION CONDITIONING It has been possible to teach a migration route to a flock of birds, for example in re-introduction schemes. After a trial with Canada Geese, microlight aircraft were used in the US to teach safe migration routes to reintroduced Whooping Cranes.
- 20. IMAGES OF BIRDS
- 27. Made by : Aditya Dudhane 9th class Roll no. – 27 Subject – Migration of birds BCA Activity JBM Global School