Migration

Migration

The need for migration

Birds migrate for two main reasons

• To move from areas of decreasing food or water supply to areas of increasing or high availability of food or water.
• To find suitable breeding sites.
  

The two are often linked, because areas of increased food supply are often in remote areas such as the Arctic Circle where daylight increases in the Summer months to peak at 24 hours a day, ideal to support vast numbers of insects and plants. These remote areas are not very accessible or desirable for human habitation and so provide good natural nesting sites.
Birds are quite resilient to cold and although migrating to avoid it is a factor, it is mainly the decreasing availability of food that drives the migration.

Migration triggers

Different species and even different populations within a species often have different triggers. This is not yet fully understood, but several factors are thought to trigger migration.

• Biological triggers which include hormonal changes and genetic influences which show as inborn ability.
• Changes in day lengths.
• Changes in temperature, which is strongly linked to
• Weather conditions.
• Availability of food or water.

As changes in day length, temperature and food availability take place, some species, particularly long distance migrants, have a biological pre-migratory response. It enables them to overeat, thus gaining the fat reserves that are needed to survive the journey (example needed). Some species only lay down enough fat to reach the next staging post, where they fatten up for the onward journey. Others lay down enough reserves for a non-stop flight. (examples needed) It is a critical process, because too much weight takes more energy to get there and too little means starvation on route.

Types of migration

Migration may be considered not only in terms of length of distance travelled, Some species migrate between the high Arctic and Antarctic. Others travel much shorter distances between their breeding and wintering quarters, but also the methods used to achieve this. These include:

Loop migration

Birds use a different route for the Spring and Autumn migration. These are influence by several factors.

Geography and weather conditions: Mountain ranges create natural barriers that channel birds naturally along their length, but if the weather conditions are right, certain families of raptors can utilise updrafts and thermals to save energy when travelling in one or both directions. (examples needed) On the opposite leg these conditions aren't there and they use a different route.
Food availability: some bird species time their migration to follow swarms of migrating insects ( more examples needed Eleonora's Falcon). These food sources are not available on return migration, so birds use other factors to decide the route used.

Drift Migration

High winds, mist or fog may cause migrating birds to be blown off course or lose orientation and fly off course. This is more prevalent during the Autumn migration with juveniles birds undergoing their first migration. (examples needed)

Reverse Migration

Some species of passerines have a gene that causes the first migration in juveniles. (example needed) If the gene responsible is faulty, this may cause the birds to fly in completely the wrong direction by 180 degrees. (insert map. example Reverse migration (birds) - Wikipedia, the free encyclopedia)

Delayed migration

The effects of drought can cause delays in the Spring migration from birds leaving their wintering grounds in Africa. Global warming has caused delays in the Autumn migration for birds leaving their breeding grounds in the high Arctic.

Prolonged stopovers

Extremely cold weather in Europe causes a prolonged delay in the Spring migration. The birds have to wait until the snow and ice retreats before they can continue with their migration. Loss of suitably large stop over areas cause food shortages. Birds are forced to turn around and fly back to suitable sites.

Navigation

Birds have several methods by which they can navigate their way across the globe during migration.

The sun's azimuth

It is not the sun's height in the sky that some species use to navigate, but rather it's azimuth, the angle of the sun on the horizon. It has also been discovered that orientation using the sun is learnt early in life imprinting the data in the bird's memory. The circadian rhythm, the bird's inner clock if you like, has also to be synchronised with this

Light polarisation

Light is polarised when daylight hits particles in the atmosphere and line up in a north/south direction particularly as the sun is setting. This alignment is thought to be seen by birds even when the sun is obscured by cloud.

Magnetic sense

The magnetic field of the Earth three components: direction, angle and intensity. The direction of this field points to the magnetic North Pole. The angle is the angle between the magnetic field lines and the surface of the Earth The magnetic field variation around the globe can be used as a map, once the variations are learned by the bird. The intensity of the magnetic field can be used in a similar way to map migration routes.

Scientists believe that there are up to three ways that this works.
Small iron oxide (magnetite) crystals could provide a compass.
The magnetic field is seen by the bird’s eye.
The bird's inner ear, it's sense of balance, enables it to "feel" the magnetic field.
Scientist currently think that birds have compass in their eyes and a magnetic mapping device in their upper mandible.

Migration Routes

The main corridors that birds use to are known as flyways.

World Flyways

There are five flyways. some of them are split further. (for map see CMS_Flyways_Reviews_Web.pdf page 14)

Central Pacific Flyway

(Map needed)
This flyway goes from the Artic ocean to the Tasman Sea between Australia and New Zealand and beyond.

American Flyways

There are 4 Flyways (map needed)

Pacific Flyway
Running from the Siberian tundra through Alaska in the north down to Tierra del Fuego in the south along the eastern Pacific coastline. Over 300 species are known to use this flyway, including millions of wildfowl, passerines and waders.
Stopover points along this route include the Copper River Delta in Alaska, Canada's Fraser River estuary, Point Reyes on the Californian coast, the Upper Bay in Panama and Humedales de Pacoa between Monteverde and San Pablo on the coast of Ecaudor.

Central Flyway
Starts in the Arctic, moves south through the Great Plains of Canada and the USA, the western seaboard of the Gulf of Mexico and southwards to Patagonia. The main endpoints of the flyway are in central Canada and the Gulf coast of Mexico, but some species use it to migrate from the high Arctic to Patagonia. (examples needed). The route narrows along Platte and Missouri rivers of central and eastern Nebraska, which gives a high species count there.

Mississippi Flyway
Over 325 bird species use the Mississippi Flyway, for their round-trip each year from their breeding grounds in Canada and the northern United States to their wintering grounds along the Gulf of Mexico and in Central and South America. It is a corridor comprising the states of Alabama, Arkansas, Indiana, Illinois, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Missouri, Ohio, Tennessee, and Wisconsin,and the Canadian provinces of Saskatchewan, Manitoba and Ontario.

Atlantic Flyway

African/west Eurasian Flyways

There are 3 Flyways (map needed)

Central Asian Flyways

(map needed)

Migration hazards

• weather
• predation
• Human hunters pose a serious hazard to birds channelled over the Mediterranean Islands of Malta and Cyprus and along the North African coastline from mist net traps
• Wind farms (any evidence of birds becoming used to these yet or changing migration route to avoid these?)

Barrier to Migration

Bodies of Water

Mediterranean

Desert

Sahara

Mountain Ranges

Mountain ranges create natural barriers to migration that most species of migrating birds need to fly around. There are some species (examples needed) that have evolved that are able to cross these barriers.
Himalayas
The Bar-headed Goose migrates over this range. Northern Pintail carcasses have been found high in the Himalayas.

References

^[1][2][3]

1. Migration Hotspots Tim Harris Bloomsbury, ISBN: 9781408171172
2. The Cornell Lab of Ornithology All About Birds : Migration
3. Peter Weaver's Birdwatcher's Dictionary Copyright © 1981 by Peter Weaver
4. Reverse migration (birds) - Wikipedia, the free encyclopedia, • #Vinicombe, Keith; David Cottridge (1996). #Rare birds in Britain and Ireland a photographic record. London: Collins. p. 192. ISBN 0002199769.
5. Gilroy, James J.; Lees, Alexander C. (September 2003). "Vagrancy theories: are autumn vagrants really reverse migrants?" (PDF). British Birds 96: 427–438.
6. Drift migration - Wikipedia, the free encyclopedia, Kasper Thorup, Thomas Alerstam, Mikael Hake, Nils Kjellén (2003). # "Bird orientation: compensation for wind drift in migrating raptors is age dependent".
7. Proc. Royal Soc. London B 270 (Suppl 1): S8–S11. doi:10.1098/rsbl.2003.0014. PMC 1698035. PMID 12952622.
8. Der Falke Journal für Vogelbeobachter Bird mgration - 23.2014.pdf
9. CMS_Flyways_Reviews_Web.pdf
10. Lehigh Earth Observatory
11. US Fish and Wildlife Service
12. Flyways.us

==Further Reading== (before or after references?) Alerstam, T. 1982. Bird Migration. Cambridge University Press. Cambridge Berthold, P. and S. B. Terrill. 1991. Recent Advances in Studies of Bird Migration. Annual Review of Ecology and Systematics . 22: 357-78. Berthold, P. 1993. Bird Migration: A General Survey. Oxford University Press. Oxford, New York, and Tokyo. Gwinner, E., ed. 1990. Bird Migration Physiology and Ecophysiology . Springer-Verlag. New York, Berlin, and Heidelberg. William, T. C. and J. M. Williams. 1978. An Oceanic Mass Migration of Land Birds. Scientific American. 239(4): 166-176.