Migration

Common Cranes on Migration
Photo © by semenov
Lac du Der France, 12 February, 2023

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 takes place, some species, particularly long distance migrants, have a biological pre-migratory response. Many long distance migrants moult before migration to ensure maximum efficiency of their plumage, as worn and abraided feathers lose efficiency over time. Another part of the preparation is a process known as hyperphagia It enables them to overeat, thus gaining the fat reserves (hyperlipogenesis) 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 influenced 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, such as Eleonora's Falcon time their migration to follow swarms of migrating insects . These food sources are not available on return migration, so birds use other factors to decide the route used.

Altitude Migration

Where birds move from one altitude to a lower one and back. Usually, this takes place before winter when food becomes hard to find at higher altitudes, forcing them to move to lower levels. The birds then return when the snows melt. It is normally a mid range movement from one geographical area to another, but it can also occur over short distances when a birds moves down a valley to escape the worst of the snow.

Leapfrog Migration

Some species including Canada Goose move to the equivalent latitude in the Southern hemisphere on their winter migration, thus leapfrogging shorter range migrants who travel a shorter distance to the equivalent in latitude in the opposite hemisphere.

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. Yellow-breasted Bunting (further examples 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 has 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.
  

Scientists 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 are known as flyways. Their boundaries are not fixed and different authors recognise slightly different numbers of routes. They also vary somewhat according to the types of birds that are migrating: passerines tend to be more catholic in their choices resulting in them following fewer distinct paths. For example, in North America authorities often distinguish 4 flyways (Pacific, Central, Mississippi, Atlantic) based on movements of raptors and waterfowl. However, these might be better treated as only 3 for passerines [19],[20].

World Flyways

There are five flyways. some of them are split further.

Central Pacific Flyway

(Map needed)
This flyway goes from the Arctic ocean to the Tasman Sea between Australia and New Zealand and beyond. Breeding sites occur mainly on route including the Arctic and Sub-Arctic regions of Alaska, Canada and eastern Asia, the Aleutian Islands. Large numbers of remote Pacific islands are in this this flyway including Midway Atoll, Hawaii, the northern Mariana Islands, Guam, Kiribati's Phoenix Islands, Palmyra Atoll a part of the Pacific Islands NWR complex, (This is a group of about 50 islands in the Line islands), American Samoa, Tonga and New Zealand.

National Wildlife Refuges (NWR] have been set up where migrating birds can utilise the stopover sites in relative safety from humans. These inlude:
Midway Atoll Breeding species include Laysan Albatross, Black-footed Albatross, Short-tailed Albatross and the Laysan Duck. Migatory species inlude Ruddy Turnstone, Wandering Tattler, Pacific Golden Plover, Bristle-thighed Curlew and Long-billed Dowitcher.
Hawaii
Kealia Pond, Maui, the refuge is a wetland along Kealia Beach in Maalaea Bay in the south west of the island. It includes a boardwalk.
James Campbell NWR on O'hau between the heliport at Turtle Bay and Kahuku.
Hanalei NWR on Kaua’i Located south of Princeville on the northern shore, this site includes non-migratory Hawaiian Duck, Hawaiian Coot, Common Gallinule subspecies G. g. sandvicensis, Black-necked Stilt subspecies H. m. knudseni and the Hawaiian Goose. Migratory species inlude Bristle-thighed Curlew, Pacific Golden-Plover, Northern Shoveler, Northern Pintail and Snow Goose

American Flyways

There are 4 Flyways in the Nearctic-Neotropics-Austral system with the latter only recently becoming understood. (map needed)

Atlantic Flyway

Starting at the Arctic's Baffin Island in the north this flyway stretches ever south to the Caribbean and spans more than 3,000 miles or 4,828 km and includes the USA states of Connecticut, Delaware, Florida, Georgia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, and West Virginia. In Canada, it includes the provinces of Newfoundland, New Brunswick, Nova Scotia, Ontario, Prince Edward Island, Quebec and the territory of Nunavut. This flyway also reaches Greenland. It is the eastern most flyway in North America. About 500 species of birds use this flyway.

Birdlife combines the Atlantic with part of the Mississippi Flyway in its "Atlantic Americas" Flyway. This may be appropriate if the focus is passerines or perhaps all bird groups.

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 USA 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, Wisconsin and the Canadian provinces of Saskatchewan, Manitoba and Ontario.

Central Flyway

Starts in the Arctic, moves south through the Great Plains of Canada and the United States of America (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 the Platte and Missouri rivers of central and eastern Nebraska, which gives a high species count there.
The following states lie within the flyway: Montana, Wyoming, Colorado, New Mexico, Texas, Oklahoma, Kansas, [[Nebraska], South Dakota, and North Dakota. In Canada the flyway comprises the provinces of Alberta and Saskatchewan and the Northwest Territories

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: Copper River Delta in Alaska, Canada's Fraser River estuary and the Stikine River Delta
The Stikine River Delta provides food for up to three million migrating birds annually including ducks and geese, swans and sandhill cranes
Point Reyes on the Californian coast, the Upper Bay in Panama and Humedales de Pacoa between Monteverde and San Pablo on the coast of Ecuador.

Austral migration
Austral migrants are species of birds that breed in the temperate areas of South America and move north into areas including the Pantanal, the Amazon basin and more rarely, as far as the Caribbean for the Austral Winter and return in the Spring. They include many species of ducks and geese and passerines such as the largest group, the tyrant flycatchers.

Afro-palearctic Flyway

There are 3 main routes through this Flyway (map needed)

East Atlantic Flyway
(map needed) This route starts at various points in the high Arctic. These include Baffin Island, the Greenland coastline, Iceland and down through the North Sea to the Wadden Sea. Birds starting from Novaya Semlya in Siberia reach the Wadden Sea via the Baltic Sea. The Olonets Plains in Russia host over 500,000 Greater White-fronted Geese and more than 200,000 Bean Geese. Scandinavian birds make the crossing to Denmark at Falsterbo. It is the first major wintering and staging point for onward migration. Birds coming from Greenland also have stop overs in Shetland and along the coast of East Anglia on Englands east coast before they cross the North Sea.

One the most greatest hurdles for migratory birds to overcome are large bodies of water. The Straights of Gibraltar is the shortest way over the Mediterranean Sea that have to be crossed between the continents of Europe and Africa. They are crossed by 250,000 raptors during per season. Following the West African coastline there are major stop overs or end points at the Banc d'Arguin National Park in Mauritania, where 2.25 million Waders overwinter. Next is the Djoudj wetland in Senegal, where 2 million Sand Martins find their wintering quarters. Further down the coast are the Arquipélago dos Bijagós mudflats in Guinea-Bissau which support of 500,000 waders. Little is understood about migration further south along central and Southern Africa, but hopefully this will change.

Mediterranean - Black Sea Flyway

East Asia - East Africa Flyway

Central Asian Flyways

(map needed)

Migration hazards

Predation
Human hunters pose a serious hazard to migratoory birds. Those channelled over the Mediterranean Islands of Malta and Cyprus, along the North African coastline and in the Sahel south of the Sahara at at risk from mist net traps. In the USA, hunting is having a major negative effect on populations during migration. Along the East Asian -Australasian Flyway illegal hunting is having an adverse affect to populations of migrating birds.

Man-made obstacles
Power lines and wind farms take a large toll of migrating birds along the flyways each migration period. In North America alone, power lines kill up to 64 million birds a year. Another 7 million are thought to die in collisions with telecommunications towers. Approximately 234,000 bird deaths a year are caused by wind turbines. These deaths are for all birds. Not just those on migration. However, the majority of these deaths are from migratory birds, especially song birds which migrate at night. Most deaths occur during take off and landing where these turbines are next to stop-over sites.

Desertification
An increase in desertification in the Sahel region south of the Sahara makes it more difficult for birds to find food on route. Recent climate change effects show increased periods of drought throughout many area of the world, including along migration flyways.

Change of farming methods
Increased use of and change in type of pesticides used have had a massive negative impact of insect population and intensification of farming methods have caused severe threats to migrating birds.

Habitat change at stop over sites
Reduction of and change of usage to stop-over sites also cause loss of life to migratory birds.

Barriers to Migration

Bodies of Water

Mediterranean, Gulf of Mexico

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 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.

Ural Mountains
The Urals present a major obstacle for up to 2 billion birds heading south from the Russian Arctic that go on to use the Afro-Palearctic Flyway and returning in the Spring of the following year. Many are juveniles making the crossing without adult guidance. Some species migrate in different groups; these are defined by age and sex due to feather moult needs, or child rearing methods.

Further Reading

• 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.

References

1. Beat about the Bush Trevor Carnaby Jacana Media ISBN 978-1-77009-241-9
2. Bird Life International (2010) Spotlight on flyways. Presented as part of the BirdLife State of the world's birds website. Available from: http://www.birdlife.org/datazone
3. Bird orientation: compensation for wind drift in migrating raptors is age dependent. Thorup et al, The Royal Society, Biology Letters
4. CMS_Flyways_Reviews_Web.pdf
5. Der Falke Journal für Vogelbeobachter Bird mgration - 23.2014.pdf
6. Drift migration - Wikipedia, the free encyclopedia, Kasper Thorup, Thomas Alerstam, Mikael Hake, Nils Kjellén (2003).
7. Flyways.us
8. Gilroy, James J.; Lees, Alexander C. (September 2003). "Vagrancy theories: are autumn vagrants really reverse migrants?" (PDF). British Birds 96: 427–438.
9. Lehigh Earth Observatory
10. Migration Hotspots Tim Harris Bloomsbury, ISBN: 9781408171172
11. Migration in South America: an overview of the austral system Published online by Cambridge University Press: 11 May 2010 R. Terry Chesser
12. Peter Weaver's Birdwatcher's Dictionary Copyright © 1981 by Peter Weaver
13. Rare birds in Britain and Ireland a photographic record. London: Collins. p. 192. ISBN 0002199769.
14. Proc. Royal Soc. London B 270 (Suppl 1): S8–S11. doi:10.1098/rsbl.2003.0014. PMC 1698035. PMID 12952622.
15. Spring Predictability and Leap-Frog Migration Alerstam et al Ornis Scandinavica (Scandinavian Journal of Ornithology) Vol. 11, No. 3 (Dec., 1980), pp. 196-200 (5 pages) Published By: Wiley
16. Reverse migration (birds) - Wikipedia, the free encyclopedia, • #Vinicombe, Keith; David Cottridge (1996).
17. The Cornell Lab of Ornithology All About Birds : Migration
18. US Fish and Wildlife Service
19. See https://ebird.org/news/flywaypaper/
20. F. A. La Sorte, D. Fink, W. M. Hochachka, A. Farnsworth, A. D. Rodewald, K. V. Rosenberg, B. L. Sullivan, D. W. Winkler, C. Wood, and Kelling, S. (2014). The role of atmospheric conditions in the seasonal dynamics of North American migration flyways. Journal of Biogeography 41(9): 1685-1696. https://doi.org/10.1111/jbi.12328
21. Midway's albatross population stable, Honolulu Advertiser, December 27, 2016
22. https://en.wikipedia.org/wiki/Midway_Atoll Wikipedia Midway Atoll, Environment
23. https://friendsofmidway.org/explore/wildlife-plants/birds/migratory-birds/ Friends of Midway Atoll, migratory species
24. https://migration.pwnet.org/pdf/Flyways.pdf
25. https://www.fws.gov/refuge/hanalei Hanalei National Wildlife Refuge
26. https://www.eaaflyway.net/task-force-on-illegal-hunting-taking-and-trade-of-migratory-waterbirds/

• List of references

Recommended Citation

• BirdForum Opus contributors. (2024) Migration. In: BirdForum, the forum for wild birds and birding. Retrieved 19 May 2024 from https://www.birdforum.net/opus/Migration