Reverse migration (birds)
Reverse migration also called reverse misorientation is a phenomenon in bird migration where a bird will fly in the opposite direction of what is species typical during the migration time.
If a bird sets off in the opposite direction, shown by the orange arrow, it will end up in Western Europe instead of South East Asia. This is a mechanism that leads to birds such as Pallas's warbler turning up thousands of kilometres from where they should be. Keith Vinicombe suggested that birds from east of Lake Baikal in Siberia (circled) could not occur in western Europe because the migration routes were too north-south. Most of these lost young birds perish in unsuitable wintering grounds, but there is some evidence that a few survive, and either re-orient in successive winters, or even return to the same area.
Reverse migration is genetic or learned behaviour?
Some large birds such as swans learn migration routes from their parents. However, in most small species, such as passerines, the route is genetically programmed, and young birds can innately navigate to their wintering area. Sometimes this programming goes wrong, and the young bird, in its first autumn, migrates on a route 180° from the correct route. This is shown in the diagram, where the typical migration route is shown in red but a reverse migration has occurred as seen in orange in the image.
As migration is most often genetically programmed before birth into most birds there can be rare variations and defects that change the migration programming. These variations will account for some of the reverse migration cases but would not account for most of these reverse migrations. After these birds have changed their migration path and are surviving they may breed with others who are also follow the different migration route and reproduce. These offspring from the birds following a different migration route have not learned this new migration route, for them it is genetic. These offspring may also reproduce with others the may pass the new route along to their own offspring continuing this new route though generations.
Methods of examining reverse migration
A single individual bird is tracked using a manually operated tracking radar to understand the targets exact position and trajectory to predict where it will be. As the bird flaps their wings the echo can be recorded and compared to patterns to understand flight patterns and changes in flight patterns. These were primarily used to monitor specific individuals during nocturnal migration through the night.
This is a technique used to track animals with a transmitter and receivers. A miniature transmitter is attached to the subject animal and this transmitter emits a very high frequency (30-300 MHz) which can be picked up with one or more receivers. For studying the moment behaviour of birds in a migration hotspot south-west of Sweden called Falsterbo near Falsterbo bird observatory three receivers were used to triangulate and track the birds.
Ringing birds is when a light weight metal band is attach to the foot to not impair movement but stay on the bird with an identification number. This identification number can provide people catching and these birds movement and history with information such as how old they are and where they have been. At the Falsterbo bird observatory these birds are caught with a mist net measured and banded.
Patterns In Reverse Migration
Reverse migration is the opposite direction or random directions?
Reverse migration is widespread around the world and occurs for many species migrating during the night and also during the day. This irregular migration direction is most often approximately opposite to what is species typical not a random direction. This phenomenon occurs not only with species migrating to a tropical area during the winter months but also with temperate zone migrants, short irruptive food migrants, short distance migrant, and long distance migrants.
An article in British Birds by James Gilroy and Alexander Lees suggests that misorientation primarily occurs approximately opposite direction but can occur in random directions. These random directions could be partly due to genetic variations or abnormalities. These birds that adopt and continue to migrate in this atypical directions have been called Pseudo-vagrancy migrators. Pseudo-vagrancy is the possibility that a bird will migrate in a different area or direction from the normal migration route. Some species have been found to be more likely then others for having the pseudo-vagrancy migration changes. Yellow-breasted Bunting is considered a lower chance of pseudo-vagrancy behaviours as compared to the Yellow-browed Warbler for having higher chance of pseudo-vagrancy behaviours
Solitary reverse migration during the night
It was found that solitary birds migrating during the night are more likely to reverse migrate West, when East is the regular migratory path. This West to East reverse migration was observed more often than a reverse migration to the North rather than the normal South migration. To only examine single species this study examined birds solitarily migrating during the night so that they do not group-migrate and influence other species to follow or follow other species.
Reverse migration due to inadequate fat stores
Using radio tracking thrush songbirds migrating southwards were tracked to examine when and why some during a stopover along the northern coast of Mexico some would not continue south but fly inland northerly. These songbirds that changed their normal seasonal southerly migration were almost all found to be lean and low on fat stores. This inland northerly path may show that the normal stopover location did not have adequate resources to increase fat stores of these birds. So, because they were unable to gain enough fat stores these reverse migrating songbirds moved inland northerly in search of more food.
The reverse migration behaviour has also been seen in the shorebirds red knots who fly in the reverse to typical migration direction. These red knot shorebirds who travel 200 km reverse migration behaviour have been documented over the last 10 years and is a common occurrence. In this study there was not significant difference in body mass fat stores or sex for these reverse migrating birds. It was seen that the birds that made the reverse migration had significantly lower hematocrit which is the percentage of red blood cells. It has been examined before that birds increase their hematocrit before consuming large amounts of food for fat stores in order to have energy for their long migration flight. This would explain why these birds chose to travel in reverse 200 km for high quality soft shell prey to supply fat and increase their hematocrit blood levels before attempting the long migration flight.
- Thorup, Kasper (October 2004). "Reverse migration as a cause of vagrancy". Bird Study. 51 (3): 228–238. doi:10.1080/00063650409461358. ISSN 0006-3657.
- Vinicombe, Keith; David Cottridge (1996). Rare birds in Britain and Ireland a photographic record. London: Collins. p. 192. ISBN 978-0002199766.
- Gilroy, James J.; Lees, Alexander C. (September 2003). "Vagrancy theories: are autumn vagrants really reverse migrants?" (PDF). British Birds. 96: 427–438.
- Rees, Eileen C. (September 1989). "Consistency in the timing of migration for individual Bewick's swans". Animal Behaviour. 38 (3): 384–393. doi:10.1016/s0003-3472(89)80031-4. ISSN 0003-3472.
- Nilsson, Cecilia; Sjöberg, Sissel (2015-11-22). "Causes and characteristics of reverse bird migration: an analysis based on radar, radio tracking and ringing at Falsterbo, Sweden". Journal of Avian Biology. 47 (3): 354–362. doi:10.1111/jav.00707. ISSN 0908-8857.
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- D'Amico, Verónica L.; González, Patricia M.; Morrison, R. I. Guy; Baker, Allan J. (June 2014). "Reverse Movements of Red KnotsCalidris canutusDuring Northward Migration in Argentina". Ardeola. 61 (1): 63–76. doi:10.13157/arla.61.1.2014.63. ISSN 0570-7358.